B13A-0158 INVITED 1340h
Data Management for the Ridge 2000 Program
Since the start of this effort (September 1, 2003) we have developed a data base schema, selected and installed a relational data base management system (PostgreSQL), designed, developed and, deployed a draft set of metadata forms, ingested data from ten Ridge2000 cruises as of September 2004 and deployed a web accessible Ridge2000 data portal: http://www.marine-geo.org/ridge2000/ . At the portal, one can get content using with pre-constructed queries for survey targets and deployed instruments at each of the R2K Integrated Study Sites. Alternatively, our data link allows spatial, temporal and keyword searches to identify and download data. The current metadata forms have been used for 6 cruises and we have received constructive feedback (in addition to the actual metadata) from all three R2K integrated study sites. We are working on incorporating this feedback into an updated set of forms which we expect to release early in 2005. Other recent include substantial improvements to GeoMapApp, links to other data repositories, a major update of our web site, integration with data from Arctic, Antarctic, Margins data sets and the pre-constructed queries on the R2K portal page. Our plans for 2005 include: A second major revision of the metadata forms in early '05, improvements in the metadata ingestion process, enhanced authentication using LDAP, continued active participation in the broader data community developing interoperability as well as implementing direct interoperability with a number of complementary databases including the underway geophysical and multibeam databases at National Geophysical Data Center, the National Deep Submergence facility at Woods Hole, the Geological Data Center of the Scripps Institute of Oceanography, and the databases of the ODP (JANUS at TAMU, and Borehole Geophysics at LDEO)
http://www.marine-geo.org/ridge2000
B13A-0159 INVITED 1340h
Time-Critical Studies: Rapid response to Transient Dynamic Mid-Ocean Ridge Events
The Time-Critical Studies (TCS) Theme of Ridge 2000 focuses on observations of the immediate geochemical and geobiological consequences of magmatic and tectonic events along the global mid-ocean ridge system. To date funding has centered on the Juan de Fuca and Gorda Ridges which are within the range of the U.S. Navy's Northeast Pacific Sound Surveillance System (SOSUS). NOAA's T-Phase Monitoring Program has accessed SOSUS in real-time since 1993, providing the TCS community with detection of seismicity associated with eruptive or tectonic activity along these two ridges. This remote detection of earthquake swarms along the N.E. Pacific mid-ocean ridge coupled to NSF funding for pre-event staging of equipment and supplies has allowed directed and increasingly well-organized field responses to the event site. Major rapid and follow-up response cruises have been successfully mounted to 1993 CoAxial, 1996 and 2001 Gorda Ridge, the 1998 Axial Volcano, and 2001 Middle Valley magmatic episodes. The logistical approach required to study these events has been greatly facilitated by the RIDGE/Ridge 2000 programs and collaboration between university, NOAA and Canadian investigators. The U.S. Navy has recently fully restored the SOSUS hydrophone arrays to essentially fully operational status, assuring long-term remote real-time detection capabilities for the Juan de Fuca and Gorda Ridges. Recent seismic swarms on the West Valley, N. JFR (July, 2003), and on the Cobb Offset (May, 2004) were detected by SOSUS, but were not responded to because of their low intensity and short durations. However, the response team remains poised for the next significant event. Proposals to enhance the event detection and rapid shore-to-event response effort are welcome at any Ridge 2000 target date. The Ridge 2000 program recognizes that even the most rapid ship response will miss the earliest subsurface and water column expressions of magmatic events. Consequently, Ridge 2000 also seeks proposals to develop alternate ultra-rapid response methods including air-droppable monitoring devices and in situ sensors. Coordination with Integrated Study Sites and proposed cabled and moored Ocean Observatories at Ridge 2000 Integrated Study Sites is also encouraged.
B13A-0160 INVITED 1340h
SEAS: Student Experiments At Sea - An Education Outreach Pilot Program Sponsored by the Ridge2000 Program
SEAS is a pilot program for middle and high school students who want to learn science by doing science. SEAS students study the deep sea hydrothermal vent environment and learn to ask questions about this exciting, relatively unexplored world, just as researchers do. SEAS students also learn how to answer their own questions through the process of scientific investigation. With the SEAS program, students have the opportunity to participate in the actual discovery process, along side deep-sea researchers. SEAS builds upon the successes of programs like Dive&Discover and Extreme2000, which demonstrated the capability deep-sea scientists have in engaging students with live research. SEAS extends this concept by inviting students to participate in deep-sea research through formal proposal and report competitions. SEAS challenges students to higher levels of achievement. A curriculum, developed by teachers expert in the translation of scientific inquiry in the classroom, prepares students to participate. SEAS was concept-tested during the 2003-2004 school year, with 14 pilot teachers and approximately 800 students. Twenty Ridge2000 scientists contributed their time and expertise to the SEAS program in its first year. Five student proposals were selected and conducted at sea in April during a Ridge2000 research cruise to the East Pacific Rise. All results were posted to the SEAS website (http://www.ridge2000.org/SEAS/) during the cruise, and students were invited to analyze data for their final reports. Final student reports, along with scientists comments were also posted. During the 2004-2005 school year, SEAS will be evaluated for its impact on student learning and attitudes toward science. The benefits of SEAS to the Ridge2000 scientific community are many. Scientists are invited to contribute in a variety of ways, all of which help satisfy the requirement of NSFs Broader Impacts Criterion. They may contribute time and expertise by answering student questions and reviewing student proposals and reports. They may choose to host the student research on their cruise. By sharing the load, no one scientist is burdened, nor expected to contribute additional funding. The Ridge2000 Program oversees the development, execution and dissemination of SEAS, helping make outreach efficient and easy for scientists.
B13A-0161 1340h
Cross-Correlation of High-Resolution Sonar Imagery and Fine-Scale Seafloor Volcanic and Tectonic Terrains: Applications to Ridge2000 Integrated Study Sites
Sidescan sonar data acquired at low ($\sim$12 kHz) and high ($\sim$120 kHz) frequencies have been used to map seafloor terrains in many tectonic environments over the past $\sim$40 years. However, the degree to which sidescan sonar data can be used to interpret fine-scale seafloor fabrics (for example, lava morphologies, sediment thickness and tectonised regions) has not been examined in detail. Here we examine how representative the variability in the acoustic return (the sonograph) is with respect to seafloor morphology. At mid-ocean ridges, for example, where a diversity of volcanic forms, are often complexly inter-fingered, can we identify the boundaries between them? Can sheet flows be reliably identified as areas of smooth acoustic texture? The regions for this study include the East Pacific Rise (EPR) crest between $1\deg$30'N and $1\deg$52'N, $3\deg$09'N and $3\deg$31'N, the Ridge 2000 EPR ISS site near $9\deg$50'N, and four sites along the Puna Ridge Hawaii, between $19\deg$31'N and $19\deg$51'N. We use 120 kHz DSL-120 sidescan sonar data, with an effective spatial resolution of 2m to produce detailed interpretations of acoustic facies resulting from volcanic, tectonic and variably sediment covered areas. We identify three main and five sub-categories of acoustic facies: regions of lumpy terrain on a scale of hundreds of meters (with differentiated tectonised and sedimented seafloor), smooth terrain, tectonised regions (with differentiation of fault scarps and fissured seafloor) and sedimented regions (with differentiated tectonised regions). From the electronic stills camera (ESC) images we identify a number of seafloor types: brecciated (with differentiated sedimented and tectonised seafloor), volcanic pillow lavas, lobates and sheet flows (with differentiation between high and low bathymetric gradients, tectonised and sedimented seafloor) and sedimented (with differentiation of tectonised regions). A comparison of these two datasets within Geographical Information System (GIS) software allows an assessment of how well these datasets are correlated. The results show a general agreement between interpreted acoustic facies and seafloor type, however, differences between the two datasets highlight the difficulty in extracting fine-scale information from sonar data. A particular disagreement between the datasets was identified with respect to smooth acoustic textures which are observed from regions of different seafloor types including pillows, variably sedimented seafloor and sheet flow. We have found that GIS compilation and analysis is important for the correlation of multi-scalar and high-resolution deep-sea data sets such as those being acquired at the Ridge 2000 ISS, where time-series relationships will be revealed by careful comparison between a wide range of multidisciplinary data.
B13A-0162 1340h
Very High Resolution Bathymetric Mapping at the Ridge 2000 Integrated Study Sites: Acquisition and Processing Protocols Developed During Recent Alvin Field Programs to the East Pacific Rise and Juan de Fuca Ridge
Recent field programs at the East Pacific Rise and Juan de Fuca Ridge have resulted in the refinement of data processing protocols that enable the rapid creation of high-resolution (meter-scale) bathymetric maps from pencil-beam altimetric sonar data that are routinely collected during DSV Alvin dives. With the development of the appropriate processing tools, the Imagenex sonar, a permanent sensor on Alvin, can be used by a broad range of scientists permitting the analysis of various data sets within the context of high-quality bathymetric maps. The data processing protocol integrates depth data recorded with Alvin's Paroscientific pressure sensor with bathymetric soundings collected with an Imagenex 675 kHz articulating (scanning) sonar system, and high-resolution navigational data acquired with DVLNAV, which includes bottom lock Doppler sonar and long baseline (LBL) navigation. Together these data allow us, for the first time, to visualize portions of Ridge 2000 Integrated Study Sites (ISS) at 1-m vertical and horizontal resolution. These maps resolve morphological details of structures within the summit trough at scales that are relevant to biological communities (e.g. hydrothermal vents, lava pillars, trough walls), thus providing the important geologic context necessary to better understand spatial patterns associated with integrated biological-hydrothermal-geological processes. The Imagenex sonar is also a permanent sensor on the Jason2 ROV, which is also equipped with an SM2000 (200 kHz) near-bottom multibeam sonar. In the future, it is envisioned that near-bottom multibeam sonars will be standard sensors on all National Deep Submergence Facility (NDSF) vehicles. Streamlining data processing protocols makes these datasets more accessible to NDSF users and ensures broad compatibility between data formats among NDSF vehicle systems and allied vehicles (e.g. ABE). Establishing data processing protocols and software suites, routinely calibrating sensors (e.g. Paroscientific depth sensors), and ensuring good navigational benchmarks between various cruises to the Ridge 2000 ISS improves the capability and quality of rapidly produced high-resolution bathymetric maps enabling users to optimize their diving programs. This is especially important within the context of augmenting high-resolution bathymetric data collection in ISS areas (several cruises to the same area over multiple years) and investigating possible changes in seafloor topography, hydrothermal vent features and/or biological communities that are related to tectonic or volcanic events.
B13A-0163 1340h
Mosaicking Techniques for Deep Submergence Vehicle Video Imagery - Applications to Ridge2000 Science
Severe attenuation of visible light and limited power capabilities of many submersible vehicles require acquisition of imagery from short ranges, rarely exceeding 8-10 meters. Although modern video- and photo-equipment makes high-resolution video surveying possible, the field of view of each image remains relatively narrow. To compensate for the deficiencies in light and field of view researchers have been developing techniques allowing for combining images into larger composite images i.e., mosaicking. A properly constructed, accurate mosaic has a number of well-known advantages in comparison with the original sequence of images, the most notable being improved situational awareness. We have developed software strategies for PC-based computers that permit conversion of video imagery acquired from any underwater vehicle, operated within both absolute (e.g. LBL or USBL) or relative (e.g. Doppler Velocity Log-DVL) navigation networks, to quickly produce a set of geo-referenced photomosaics which can then be directly incorporated into a Geographic Information System (GIS) data base. The timescale of processing is rapid enough to permit analysis of the resulting mosaics between submersible dives thus enhancing the efficiency of deep-sea research. Commercial imaging processing packages usually handle cases where there is no or little parallax - an unlikely situation for undersea world where terrain has pronounced 3D content and imagery is acquired from moving platforms. The approach we have taken is optimized for situations in which there is significant relief and thus parallax in the imagery (e.g. seafloor fault scarps or constructional volcanic escarpments and flow fronts). The basis of all mosaicking techniques is a pair-wise image registration method that finds a transformation relating pixels of two consecutive image frames. We utilize a "rigid affine model" with four degrees of freedom for image registration that allows for camera translation in all directions and camera rotation about its optical axis. The coefficients of the transformation can be determined robustly using the well-established and powerful "featureless Fourier domain-based technique" (FFDT), which is an extension of the FFT-based correlation approach. While calculation of cross-correlation allows the recovery of only two parameters of the transformation (translation in 2D), FFDT uses the "Phase shift" theorem of the Fourier Transform as well as a log-polar transform of the Fourier magnitude spectrum to recover all four transformation coefficients required for the rigid affine model. Examples of results of our video mosaicking data processing for the East Pacific Rise ISS will be presented.
B13A-0164 INVITED 1340h
The East Pacific Rise $8\deg$$-11\deg$N Integrated Studies Site (ISS); Update and Opportunities
The $8\deg$$-11\deg$N segment of the East Pacific Rise (EPR) represents a dynamic, fast-spreading type of mid-ocean ridge selected for focused interdisciplinary study within the Ridge 2000 program. Diverse fast spreading environments are encompassed by the site including a hierarchy of axial segments and discontinuities. The site "bull's- eye" is at $9\deg$49' - $9\deg$51' N where numerous high temperature vents and diffuse flow communities have been mapped and monitored over the past 10 years. Concentric circles around the bull's eye encompass ridge segments at a range of scales, from the first-order segment bounded by the Siquieros and Clipperton transform faults to the fourth-order segments which include the extent of the 1991 and 1992 volcanic eruptions. Five-year goals for the site include a working model of mantle flow and melt supply; detailed imaging of subseafloor structures and relationships to vent communities and chemistry; quantitative data about microbes and macrofauna and linkages with fluid flow, tectonics, and magmatism; quantification of the heat flux into the water column; and linkages and temporal variation in geological, chemical, and biological parameters. Field programs at the EPR site in 2004 began with a multi-program cruise lead by Schouten and colleagues that included dives for volcanological objectives, the deployment and testing of an array of in situ chemical sensors for use in monitoring vent fluids (Seyfried), and deployment of sample collection plates for a study of the role of microbes in the weathering of ocean crustal rocks (Edwards and Bach). A magnetotelluric and controlled source electromagnetic study (Constable) was carried out from $9\deg$30-50'N targeting mantle and crustal structure. The next cruises at this site were the time series fluid sampling program of Von Damm and biological study of Lutz and colleagues, both programs focused at the site "Bull's eye". OBSs deployed in Fall 2003 for a microseismicity monitoring study (Tolstoy and Waldhauser) were recovered and redeployed during the Lutz et al. cruise, and temperature probes were deployedduring the Von Damm cruise. Programs scheduled for 2005 include ongoing monitoring studies of seismicity and vent fauna. Recently funded studies at the EPR site include an investigation of larval dispersal (Mullineaux) and a geodetic study of seafloor motions using pressure sensors deployed along the ridge crest from roughly $9\deg$-$10\deg$N (Cormier et al.). A three-dimensional multi-channel seismic study of subsurface structure (Mutter et al.) is scheduled for early 2006. An overview of these and other exciting on-going and upcoming studies within the EPR ISS will be presented.
B13A-0165 1340h
Earthquake Monitoring at $9\deg$50'N on the East Pacific Rise RIDGE 2000 Integrated Studies Site
In the fall of 2003 nine ocean bottom seismometers (OBSs) were deployed from the {\it R/V Keldysh} within the `bull's-eye' region of the R2K ISS at $9\deg$49'N - $9\deg$51'N on the East Pacific Rise as part of the Ridge 2000 Integrated Studies Site. These instruments were recovered using the {\it R/V Atlantis in April 2004}, and twelve more were deployed to take their place for a second year of monitoring (with three years total planned). During the turn-around cruise, two short temporary deployments (~4-8 days), of an additional 3 OBSs each, were accomplished to provide very dense instrument spacing (a few 100 m) around specific vents where {\it in situ} chemical monitoring was taking place (Luther et al.). Good data were collected on seven of the nine long deployment and six short deployment OBSs. We will present early results from analysis of these data including an estimate of the level of activity observed through-out the seven month period of the first deployment, and preliminary epicenters. Data will also be shown from the short temporary deployments. Early analysis of these data indicates an event rate of ~8 events per day for events where arrivals are apparent on at least three instruments, and may therefore expect to be located. Also notable in these data are pulses and prolonged periods of what appear to be tremor. This tremor is not generally coherent or synchronous from station to station and is therefore likely a very localized phenomena associated with hydrothermal fluid flow. The exceptionally well characterized and monitored seafloor at this site will allow for unprecedented correlation of observed seismic activity with local biology, geology, geochemical and hydrothermal monitoring. In addition, past and future detailed geophysical imaging of this area will provide an excellent context for observed faulting and fracturing.
B13A-0166 1340h
Off-Axis Lava Transport at the East Pacific Rise 9-10\deg N: Channelized Lava Flows
Extrusive lava flows compose the upper $\sim$500 m of oceanic crust at mid-ocean ridges (MORs). At fast and superfast spreading MORs, this extrusive layer (i.e. seismic layer 2A) doubles in thickness within 2-4 km from the ridge axis. Geophysical measurements and stochastic modeling suggest that the observed thickening of the extrusive lava layer is the product of bimodal lava deposition, both at and away from the axial summit trough (on- and off-axis); however, the mechanism by which lava is transported off-axis is not well-defined. Using high-resolution bathymetry, sonar backscatter intensity maps, and $>$80,000 seafloor digital images contained within a GIS database, we have mapped lava flow features similar to subaerial lava channels that we consider to be the primary mechanism of off-axis lava transport. Channels are distinguished from the surrounding lavas by their low backscatter intensity (they are floored by smooth featureless lava) and by their depressed topography (a result of draining of the liquid lava after the eruption has ceased). Often, only segments of channels are visible along the path of a channel system. The identified channel segments are 50 m to 1.1 km in length. Channel widths are highly variable along single channel systems ranging from 10 to 50 m, but depths are consistently 2-4 m. Channels initiate up to 500 m from the axial summit trough axial summit trough and, when segments composing a channel system are connected, can extend 1-3 km away from the ridge axis. Geochemical analyses indicate that the channelized flows erupted at the AST and are cogenetic with the lobate lava flows in which they occur. We construct detailed maps of the lava surface morphology across the channels from mosaics of down-looking digital images and high-frequency, near-bottom scanning altimetry collected during submersible dives. These maps reveal characteristic, cross-channel variations in surface morphology including brecciated zones of variable width at the channel margins. We interpret the brecciated zones to form from high shear rates at the channel margins and/or by bending of the surface crust as the channel is drained of lava. Using proven results from numerical and physical modeling of submarine lava flows, we are able constrain velocity and shear rate profiles across the channel surface and interpret whether the brecciated region is formed during or after lava transport. In addition, we are able to constrain flux through the channel (i.e. effusion rate) an elusive and important aspect of submarine eruptions with implications for magma supply rate, ridge resurfacing rates, and oceanic crust construction.
B13A-0167 1340h
Burying Tectonic Strain Under Lava Flows at the East Pacific Rise (9\deg N 26'-58'N)
High resolution, deep tow sonar images of the EPR at 9\deg N 26'-58'N in combination with high resolution near-bottom bathymetry provide constraints on the interaction between faulting and magmatism. Areas of the seafloor recently resurfaced by lava flows can be recognized by their high acoustic reflectivity compared to that of older, sedimented seafloor. The off-axis limit of the 'recent' volcanic seafloor is highly variable (between 1.5 and 5 km from the axis) and is controlled primarily by fault scarps that dam lava flows originating at the axial summit trough (AST). We have quantified the deformation intensity (density of fissures and faults) and characterized the fault scarp height both on 'recent' and 'old' (i.e., sedimented) seafloor. For seafloor of the same spreading (calculated from the distance to the AST and the spreading rate), the 'old' seafloor shows more tectonic deformation than the repaved seafloor: fault and fissure density is 5 times greater, fault scarps are 2-4 times higher, and tectonic strain along selected transects is 4 times greater. In all cases inward facing faults account for 70% of the faults. These results demonstrate that almost all the tectonic strain and deformation (80% or more) is systematically buried by volcanic flows in the vicinity of the axis. Faults that develop sufficient scarp heights so as not to be buried by flows dam subsequent lava flows at distances of 1 to 5 km from the AST, and effectively limit the length of lava flows. Near-axis tectonism appears to play a previously overseen role in the build up of the extrusive oceanic crustal later. Burial of 80% or more of the faults and grabens implies an efficient, fault assisted subsidence of volcanics near the AST. The limitation of lava flow length by fault scarps indicates the thickening of Layer 2A away from the aixs may be controlled by the fault nucleation and growth of faults, and not solely by the accummulation or successive lava flows.
B13A-0168 1340h
A look at the data from ``Constraining the Magmatic Budget of the EPR at 9 N Using Broadband Marine MT''
In February 2004 we collected data for the world's largest marine electromagnetic (EM) experiment at the RIDGE2000 integrated study site (ISS) on the East Pacific Rise (EPR) near 9{$^\circ$}N. Electrical conductivity is a strong function of fluid content and temperature, whether magma or seawater, and so the objectives of this RIDGE2000 funded experiment are to map the hydrothermal circulation systems removing heat from the mid-ocean ridge magmatic system, quantify the total amount of melt in the crustal magma chamber, and examine the relationship between mantle melting and crustal melt accumulation. We used two electromagnetic methods to accomplish these goals: a) Marine magnetotelluric (MT) method. A seafloor instrument records natural variations in Earth's electric and magnetic fields for 2 days to 2 weeks. When converted to frequency domain impedance functions and inverted, these data can be used to obtain images of seafloor conductivity up to hundreds of kilometers deep. b) Marine controlled-source EM (CSEM) sounding. An EM transmitter is deeptowed close to the seafloor to provide a man-made source of EM energy. The seafloor recorders monitor the transmitted electric fields, which provide similar information to the MT method except that (i) the CSEM method has better resolution at shallow depths and (ii) the CSEM method is better at measuring resistive (cf.\ conductive) rocks. Forty broadband EM receivers, developed at Scripps Institution of Oceanography with support from the petroleum exploration industry, were deployed a total of 72 times to yield 69 sites of MT/CSEM data. Twenty of the MT deployments were along a 30 km aperture transect across the ISS "bull's eye" focus area at 9{$^\circ50'$}N, in order to image the electrical conductivity structure in the crust and shallow mantle in the vicinity of the ridge axis. Further to the south, 40 MT sites were collected along a 200 km aperture transect at 9{$^\circ30'$}N to target both crustal and upper mantle conductivity structures. This larger aperture transect was positioned to avoid 3D structure associated with the Lamont seamonts and the Clipperton transform. Twelve additional MT sites were collected in a grid along the ridge axis between 9{$^\circ30'$}N and 9{$^\circ36'$}N. For the CSEM portion of the experiment, we deep-towed the newly developed Scripps Undersea EM Source Instrument (SUESI) about 100 m above the seafloor while transmitting a 2 Hz square wave of dipole moment about 20,000 Am. The MT receivers also record the transmitted CSEM fields, and so three CSEM tows were performed while the forty receivers were deployed along the southern line and grid, yielding a total of about 80 km of tows and 1,300 receiver-hours of CSEM data.
http://marineemlab.ucsd.edu/Projects/EPR2004
B13A-0169 1340h
Continuing Evolution of the Hydrothermal System at the RIDGE2000 ISS, $9-10\deg$N EPR: 1991-2004
We have been studying the evolution of the chemical composition of the mid-ocean ridge (MOR) hydrothermal system on the East Pacific Rise from $9\deg$46-51'N since it was impacted by volcanic eruptions in 1991/2. We have been using the chemical and temperature data to infer the processes that are occurring subseafloor in the upper oceanic crust. As of March 2004, the chemical compositions of the vent fluids from this site have not yet stabilized. This observation is helping us to better understand not only the impact of magmatic events on these systems, but also the time scales on which they occur. Centered at the RIDGE2000 ISS "bull's-eye" at 9$\deg$50'N we have noted a striking increase in the number of hydrothermal vents as well as in their measured fluid temperatures beginning after ~2000. In November 2003 we first noted the formation of a black smoker vent at the Tica site (measured T=$342\deg$C). In March 2004 we identified another new area of robust flow near the Bio9 vents at $9\deg$50'N, the 'Alvinellid Mat,' that we anticipate will form an additional black smoker to the three currently active at this site. In March 2004 we measured temperatures of $388\deg$C in fluids from both the Bio9 and Bio9' smokers, putting them essentially on the two phase curve for seawater at this depth. For all of the Bio9 vents, as well as Tica, the fluids contain less than 300 mmoles/kg of Cl, approximately half the local seawater concentration. These high temperature and low Cl concentrations are accompanied by unusually low Si concentrations, $<$9.5 mmoles/kg. These data suggest a relatively shallow depth of reaction for the fluids, within a few hundred meters of the seafloor. These are the hottest temperatures measured in the Bio9 vents since the eruption in 1992. In contrast, the temperatures at P vent, about 60m south have cooled by $\sim$$15\deg$C since 2002. About 400m south, the chlorinity of the fluids from Ty and Io vents have increased, and Tube Worm Pillar, about 400m further south has become inactive. Therefore the changes in the vent fluids vary widely and often in opposite senses, over $\sim$1.5km of very hydrothermally active ridge. A more complete discussion of the changes and our interpretation of their implications for processes occurring subseafloor will be presented.
B13A-0170 1340h
Time-series Deployment of Chemical Sensor Data-logger in a Riftia Patch at Tica Vent (EPR 9\deg50'N)
An in-situ chemical sensor array was deployed in a Riftia pachyptila patch at Tica diffuse flow vent area during a recent submersible investigation with Atlantis/Alvin (At-11-7) to the EPR 9\deg50'N region. Chemical and temperature data were recorded continuously during the 13-day study to quantify short-term time series trends. Biological activity at Tica is unusually robust, which likely relates to a combination of chemical and physical factors in the vent ecosystem. The electrochemical array in the sensor unit consisted of pH, dissolved H$_{2}$ and H$_{2}$S electrodes, which made use of Ir, Pt and Ag base metal components respectively. The sensor measurements were referenced to dissolved Cl in the fluid using a junctioned Ag/AgCl electrode. A Ti sheathed E-type thermocouple was included to provide simultaneous temperature data. Moreover, the sensor array was coupled to a data-logging system through a high pressure conducting cable, which allowed continuous data scanning and recording at an interval of 5 seconds. Owing to the functionality of the inductively coupled communication link (ICL) between the sensor and the data logger, we were able to use real time temperature readings in the submersible to guide deployment. The warmest temperatures were observed at the fractured rock base of the tubeworm colony, which is where the sensor package was deployed. Temperature, however, varied systematically from approximately 10 to 20\deg C throughout the 13-day experiment. Chemical data were generally in phase with temperature with the more extreme departures from ambient conditions associated with temperature maxima. In general, the lowest pH values were approximately 0.5 units less than ambient seawater, while dissolved H$_{2}$S concentrations ranged from 10 and 100 $\mu$ mol/kg. The mean value of dissolved H$_{2}$S was in the range of 20-30 $\mu$ mol/kg in excellent agreement with that actually measured using conventional approaches. Dissolved H$_{2}$ concentrations, in contrast, were orders of magnitude lower and ranged from 10$^{-9}$ to 3x10$^{-8}$ $\mu$ mol/kg. To out knowledge these are the first in-situ time series dissolved H$_{2}$ data and are clearly relevant to the geomicrobiological processes affecting dispersed flow hydrothermal systems at mid-ocean ridges. During the course of our study, other chemical and temperature sensors were deployed and re-deployed at different vent sites by Alvin for variable times and at different (P-T-X) conditions. The chemical sensors proved to be highly mobile, robust and performed very well during all seafloor trials.
B13A-0171 1340h
Phylogenetic Diversity of Young Ocean Crust at the East Pacific Rise 9$\deg$N
Numerous studies show increasing evidence for a significant biosphere in oceanic lithosphere. Geochemical modeling suggests that most biological activity at or below the seafloor occurs in young crust ($<$10 Ma) on mid-ocean ridge flanks where low-temperature fluid circulation is substantial. In this environment, oxygenated seawater reacts with basalt and releases chemical energy that could support the growth of microorganisms. Fluid fluxes rapidly decrease further off-axis in older, more altered crust likely leading to a sharp decline in biological activity. To date, most evidence in support of a deep biosphere relies on anomalous textural features and geochemical signatures in aged basalt glass. In order to unambiguously attribute these alteration features to microbial activity, molecular microbiological data is required to corroborate these morphological and chemical observations. The application of molecular techniques to old ocean crust, however, can be difficult because of issues such as low cell density, contamination, and sluggish activity. Hence, studies on young ocean crust may provide insight and constraints on processes that could also apply to older crust. In this study, we have investigated the initial colonization of very young mid-ocean ridge basalt by endolithic microorganisms, and the changes in microbial diversity as a result of increasing rock alteration. Seafloor basalt samples were collected during RV {\it Atlantis} cruise AT11-7 in February 2004, from the East Pacific Rise (EPR) between 9$\deg$28'N and 9$\deg$50'N. Samples representing various flow morphologies, glass contents, and ages (up to $\sim$20 kyrs) were collected by DSV {\it Alvin} and brought to the surface in bioboxes. All basalts contain glass that ranges from very fresh to slightly altered with Fe-oxidation rims and/or Mn-oxide crusts. Total community DNA was successfully extracted from glass samples representative of a variety of alteration states. Clone libraries were constructed from PCR products of 16S rRNA genes using bacterial primers. Approximately 90 randomly selected clones from each library were sequenced. Phylogenetic analyses will indicate the overall diversity of young ocean crust and will help determine the succession of microorganisms colonizing the rock with increasing alteration. These results may also give us a better indication of the physiology of these microorganisms. Ultimately, this information will provide more accurate estimates of the impact of microbial activity in important geochemical processes such as the evolution of crustal composition.
B13A-0172 1340h
Hydrothermal Mineral Deposits From a Young (0.1Ma) Abyssal Hill on the Flank of the Fast-Spreading East Pacific Rise
It has been estimated from heat flow measurements that at least 40% of the total hydrothermal heat lost from oceanic lithosphere is removed from 0.1-5 Ma abyssal hill terrain on mid-ocean ridge flanks. Despite the large magnitude of estimated hydrothermal heat loss from young abyssal hills, little is known about characteristics of hydrothermal vents and mineral deposits in this setting. This study describes the first abyssal hill hydrothermal samples to be collected on the flank of a fast-spreading ridge. The mineral deposits were discovered at "Tevnia Site" on the axis-facing fault scarp of an abyssal hill, located on $\sim$0.1 Ma lithosphere $\sim$5 km east of the East Pacific Rise (EPR) axis at 10\deg 20'N. Observations of Galatheid crabs, "dandelion" siphonophores, and colonies of dead, yet still intact, Tevnia worm tubes at this site during Alvin dives in 1994 suggests relatively recent hydrothermal activity. The deposits are friable hydrothermal precipitates incorporating volcanic clasts brecciated at both the micro and macro scales. The petrographic sequence of brecciation, alteration, and cementation exhibited by the samples suggests that they formed from many pulses of hydrothermal venting interspersed with, and perhaps triggered by, repeated tectonic events as the abyssal hill was uplifted and moved off-axis (see also Haymon et al., this session). Observed minerals include x-ray amorphous opaline silica and Fe-oxide phases, crystalline Mn-oxides (birnessite and todorokite), an irregularly stratified mixed layer nontronite-celadonite, and residual calcite in sediment-derived microfossils incorporated into the breccia matrix. This mineral assemblage suggests that the deposits precipitated from moderately low-temperature ($<$140\deg C) fluids, enriched in K, Fe, Si, and Mn, with a near-neutral pH. The presence of tubeworm casings at the site is evidence that the hydrothermal fluids carried H$_{2}$S, however no metal sulfide phases were identified in the samples. Although the fluids were actively venting from an abyssal hill distal to the ridge crest, the presence of Fe- and K-rich nontronite-celadonite suggests an axial fluid source. However, the observed textures, minerals, and microfossils, combined with the absence of copper, zinc, and sulfur minerals, clearly distinguishes these near-axis samples from hydrothermal deposits formed at higher temperatures ($>$350\deg C) on the mid-ocean ridge crest.
B13A-0173 1340h
Evidence for Pulsed Hydrothermal Venting from Young Abyssal Hills on the EPR Flank Suggests Frequent Seismic Pumping of Ridge Flank Fluid Flow
Although measured heat flow suggests that 40-50% of oceanic hydrothermal heat and fluid flux is from young (0.1-5 Ma) abyssal hill terrain on MOR flanks, hydrothermal vents in this setting rarely have been found. On the EPR flanks, seafloor evidence of venting from abyssal hills has been discovered recently at two sites: on $\sim$0.1 Ma seafloor at $10\deg$20'N, $103\deg$33.2'W ("Tevnia Site") and on $\sim$0.5 Ma seafloor at $9\deg$27'N, $104\deg$32.3'W ("Macrobes Site"). Manifestations of venting at these sites include: fault scarp hydrothermal mineralization and macrofauna; fault scarp flocculations containing hyperthermophilic microbes; and hilltop sediment mounds and craters possibly created by fluid "blow-outs." Hydrothermal deposits recovered at the $\sim$0.1 Ma "Tevnia Site" are fault breccias that record many episodes of brecciation followed by hydrothermal cementation (Benjamin et al., this session). Tubeworm casings, live crabs, and "dandelions" observed at this site indicate that the most recent episode of venting was active during, or shortly before, this site was visited with Alvin in 1994. To create the 200 m-high axis-facing fault scarp at Tevnia Site in 100,000 years, an average uplift rate of at least 2 cm/y is required. Since off-axis earthquakes located on abyssal hill fault scarps typically are $<$M5, it is likely that each episode of slip has small vertical displacement (a few cm). Therefore fault slippage must repeat on a decadal timescale to match the observed rate of hill uplift, probably breaking open the fault scarp and rejuvenating hydrothermal flow on a very frequent basis. In addition, close proximity to Clipperton Transform may subject Tevnia Site to frequent M5-M6 seismic events with strong ground shaking and hydraulic pressure pulses capable of breaking open subseafloor pathways clogged with fragile minerals. We hypothesize that the multiple brecciation/cementation events recorded in the Tevnia Site samples, and biological evidence for recent venting at the site, are evidence that hydrothermal plumbing systems are maintained semi-continuously over 100,000 years by tectonic shaking and reactivation every few years-to-decades as abyssal hills are uplifted on ridge flanks. Hyperthermophiles identified in microbial floc on the axis-facing fault scarp at the $\sim$0.5 Ma "Macrobes Site" are associated with chalcopyrite particles (Ehrhardt et al., this session). These observations indicate recent, transient venting of high-T (at least $250\deg$C), Cu-rich fluids from the fault scarp. The hilltop here has many meter-scale sediment mounds and craters (macs). These appear to be small mud volcanoes that have formed at different times from fluids expelled episodically through the thin sediments blanketing the hilltop. Formation of macs may be triggered by frequent (decadal) seismic events that also produce transient bursts of high T fluid flow from the adjacent fault scarp. Hyperthermophiles in the subsurface may flush out and bloom on the fault scarp during these events, and then remain dormant until the next event. The combined evidence from the two EPR ridge flank sites points to seismic pumping of EPR abyssal hill hydrothermal pulses every few years-to-decades. These frequent hydrothermal pulses may be important in sustaining ridge flank biota. Monitoring of EPR abyssal hill fault scarps is needed to test these ideas.
B13A-0174 1340h
New Mineral and Microbial Evidence of High Temperature Hydrothermal Venting Along Off-Axis Abyssal Hill Fault Scarp on the East Pacific Rise Flank at 9$\deg$27'N
During Alvin dives in May 2002, orange-brown filamentous flocculations were sampled from an axis-facing abyssal hill fault scarp located on $\sim$0.5 Ma seafloor west of the EPR at 9$\deg$ 27'N. Many sedimentary structures thought to be products of episodic fluid expulsion were found on the top of this hill (see Haymon et al., this session). Recovered samples of flocculations from this abyssal hill have presented an opportunity to study the quantitatively significant yet rarely observed processes of off-axis hydrothermal circulation and the unknown nature of subsurface microbial communities on the ridge flanks. We report here on hydrothermal sulfide minerals and hyperthermophilic microbes identified in the orange-brown flocculations collected from the fault scarp. Powder X-ray diffractometry and Environmental Scanning Electron Microscopy (ESEM) were used to characterize mineral phases in fault scarp materials. Chalcopyrite, pyrrhotite, and pyrite constitute a significant portion of the mineral assemblage .Presence of chalcopyrite suggests that high temperature ($>$250$\deg$C) hydrothermal fluids vented from the fault scarp, and presence of pyrrhotite indicates a low oxidation state of these fluids. Molecular phylogenetic surveys are consistent withthese findings. An archaeal 16srRNA clone library reveals that a significant portion of phylotypes (54% or 39 total clones) cluster within hyperthermophilic, chemoautotrophic groups of Archaea including {\it Thermoproteales}, {\it Desulfurococcales}, {\it Thermococcales}, {\it Korarchaeota}, and {\it Methanopyrales}. The physiology and common metabolites of these groups indicate that hydrothermal fluids along the fault scarp likely exceeded 80$\deg$C and contained a variety of reduced species such as H$_{2}$S, H$_{2}$, CO$_{2}$, and CH$_{4}$. Because hydrothermal fluids emerging along off-axis fault scarps tap crustal fluid reservoirs, these scarps appear to be accessible windows into the potentially vast subsurface biosphere on ridge flanks Some of the archaeal groups identified in this study ({\it Thermoproteales}, {\it Korarchaeota}, {\it Methanopyrales}) are rarely observed at ridge crest vents or in other marine environments. This suggests that the unexplored subsurface biosphere of ridge flanks is distinct from any known marine habitat.
B13A-0175 1340h
Preliminary Hydrothermal Heat Flow Measurements at the 9-10° N East Pacific Rise
The March 2004 expedition at 9-10° N East Pacific Rise, part of the RIDGE 2000 program, allowed amongst other things to pursue the monitoring and sampling of a number of vents after the volcanic eruptions that occurred in 1991/2. Earlier observations have shown that the chemical composition and the temperature have not yet stabilized at many of the vents. This expedition also gave us the opportunity to test an experimental device to estimate the velocity of diffuse flow and heat transport at a couple of vents. These experiments represent the first such attempts to measure these parameters for the 9-10° N hydrothermal system. The idea was to focus the fluid flow through an opening and videotape the motion of particles composing the fluid. Though crude, this method should provide an upper estimate of the fluid velocity and the mass flux of the underlying upflow zone. This operation was also done at the same vents just with a scale maintained close to the flow. This approach yields a lower estimate of the velocity. These tests were run on three dives achieved with the deep submergence vehicle Alvin (dives # 3987, 3990 and 3992, respectively at M-vent, TICA-vent, Bio9, Bio9' and Bio9''). Typical results obtained for the total heat flux ranged from Q = 10^5 W to Q = 10^7 W. This experience has provided insight into a new design based on the temperature measurement at different regions in the stream. We hope to develop this new device in the coming months and deploy it in 2005.
B13A-0176 1340h
Multibeam Studies of Abyssal Hills on the Flanks of the East Pacific Rise ISS, 10\deg18'N to 10\deg50'N.
We have acquired multibeam data for the East Pacific Rise (EPR) and its flanks from 10\deg18'N to 10\deg50'N, a segment of the EPR ISS zone. We are exploiting that data to characterize abyssal hill topography generated over a segment of ridge whose morphology changes rapidly along its length. Combined sets of bathymetric data from two cruises, in 1994 and 2003, cover an area of approximately 7000 km$^{2}$ to the north of the Clipperton Fracture Zone (CFZ). Adjacent the CFZ, the ridge axis is nearly indistinguishable from the surrounding seafloor, and lacks the axial high characteristic of mid-ocean ridges with a steady state magma chamber. As the ridge continues north, it becomes shallower, broader and more defined. The subdued topography to the south, along with the lack of an apparent axial magma chamber reflection (Detrick et al., 1987), has led many to argue that the ridge axis from 10\deg18'N to 10\deg55'N has a depleted magma supply. This reasoning implies that the magma supply increases northward along the ridge axis, corresponding to a change in the thermal structure. This variation in thermal structure may result in changes in the physical characteristics of abyssal hill topography. For example, at a ridge segment with an abundant magma supply, the warmer (thinner) lithosphere is likely to result in more closely spaced abyssal hills. These bathymetric data allow us to examine the influence of magmatic budget and thermal structure on the formation and distribution of abyssal hills along a fast-spreading ridge. We investigate how the varying ridge morphology affects the off-axis topography by measuring the spacing, distribution, rms height, and length of abyssal hills. Preliminary analysis of three transects across the field area at 10\deg25'N, 10\deg30'N and 10\deg40'N show that there is no obvious or consistent trend to the spacing of abyssal hill faults, particularly east of the ridge axis. There is a region of generally higher bathymetry at $\sim$104\deg00'W that is not mirrored on the east side of the ridge. This area of high bathymetry corresponds to larger and less regular spacing between scarps. Fault spacing averages 3.31 km over all transects, and varies between 2.36 and 4.80 km.
B13A-0177 1340h
A Recent Volcanic Eruption on a "Magma Starved" Segment of the East Pacific Rise ISS, "10\deg44'N
In November 2003, three Alvin dives were made on the East Pacific Rise at 10\deg44'N, (funded by NSF DEB-0072695). We discovered evidence for extremely young lava flows, indicating a recent volcanic eruption. Most of the flow is relatively thin, perhaps only 1 to 2 meters thick, but locally thicker with voids beneath of over 10 meters. The basalts were very glassy, with little devitrification. The pillows were lined with abundant bacterial deposits. We found a number of collapse pits and fissures. Several of these were emitting warm water with bacterial floc, features often referred to as snow blowers. The most prominent collapse feature is a central sinuous feature that extends for several hundred meters, and has a depth of about 10 meters. Initial dating of the basalts is described in an accompanying abstract, and is consistent with a very recent eruption. Global seismicity data do not reveal any events on the EPR at this latitude, and as of this writing, the NOAA (PMEL) hydrophone data for the equatorial Pacific have not yet been recovered. Even the most magma deficient segments of the world's mid-ocean ridge system must undergo magmatism and volcanic activity to produce the basalts that are nearly ubiquitous on the seafloor. Most of the fast spreading East Pacific Rise displays evidence of an abundant magma supply, including the presence of an axial magma chamber (AMC) reflector and an elevated axial high. An exception is the segment of the EPR north of the Clipperton Transform between 10\deg18'N and 10\deg55'N. Along that 70 kilometer length of ridge no AMC reflector was detected in the original survey by Detrick and others in 1985. In contrast, later seismic refraction work indicated anomalously low velocities and high attenuation in the lower crust beneath the axis in this area. Near the Transform, the ridge is narrow and relatively deep ~2800 m). It shoals and widens to the north, suggesting a more abundant magma supply. The 10\deg44'N site lies at the northern end of this magma depleted ridge segment. This suggests either the axial magma chamber has changed over the intervening years, or an AMC reflector is not a necessary condition for magmatic activity on a ridge axis.
B13A-0178 1340h
$^{210}$Po-$^{210}$Pb Dating and U-Series Disequilibria of a Young Basaltic Flow at $10\deg$44'N EPR
Divers in the submersible Alvin observed and sampled a very young appearing lava flow at $10\deg$44'N on the East Pacific Rise (EPR) during a November 2003 biodiversity-funded dive series (DEB-0072695 to J.R. Voight) to the area. The occurrence of the glassy, unsedimented, low-lying graben filling lava flow was unexpected on this magma-starved ridge segment just north of the Clipperton Transform Fault and so 3 dives were used to investigate a 4 km long segment of the flow. Measurements (in progress) of $^{210}$Po-$^{210}$Pb disequilibrium in 4 samples of this lava flow confirm that it is likely extremely young (0-2 months old when the samples were collected). Such age dating is critical here because although this area of the EPR is covered by the SOSUS hydrophone array, data are not available in real time and will not be downloaded until November 2004. The $^{210}$Po-$^{210}$Pb geochronometer (T$_{1/2}$ =138.4 d) involves a time series of 3 or 4 $^{210}$Po analyses over the course of 1 year or more, so that our analyses are still ongoing (just 2 data per sample so far) and our results quite preliminary. By meeting time we will have better constrained eruption ages with the addition of more data to the ingrowth curves. Results of other chemical analyses on these same samples are being used to compositionally characterize the lavas and to constrain magmatic process timescales. All 13 of the lavas sampled in Nov 2003 are low MgO (6.5-6.7 wt%) basalts, displaying inter-sample compositional variations (lava flow heterogeneity) at the higher end of the known MORB flow catalogue, in contrast to mafic (8 wt% MgO) and compositionally homogeneous lava flows erupted at the nearby and well-studied $9\deg$50'N EPR site. $^{230}$Th-$^{238}$U disequilibria in the young $10\deg$44'N flow clusters around 14-15% $^{230}$Th-excesses, which overlaps the high end of the data range from "zero-age" MORB flows at $9\deg$50'N EPR and the Juan De Fuca Ridge (JDF). Slightly higher excesses, lower ($^{230}$Th/$^{232}$Th ), and higher Th/U ratios (2.5-2.6) distinguish our samples from $9\deg$50'N, suggesting a slightly different source composition (and perhaps melting rate) between the locales. The young flow is variably covered by thick, mucus-rich bacterial mats, sparsely populated by small animals, and had copious bacterial floc issuing from multiple diffuse flow vents ("snowblowers"). Focused high temperature flow vents were not observed (maximum measured fluid temperatures were just $10\deg$C), yet many of the active hydrothermal fluid flows were within lava collapses and were thus difficult to sample.Two hydrothermal vent fluid samples had elevated H$_{2}$S and Si above background levels, in spite of the near seawater Mg content of the fluids.
B13A-0179 INVITED 1340h
Current Research at the Endeavour Ridge 2000 Integrated Studies Site
Integrated geophysical, geological, chemical, and biological studies are being conducted on the Endeavour segment with primary support from NSF, the W.M. Keck Foundation, and NSERC (Canada). The research includes a seismic network, physical and chemical sensors, high-precision mapping and time-series sampling. Several research expeditions have taken place at the Endeavour ISS in the past year. In June 2003, an NSF-sponsored cruise with R.V. {\ital T.G.Thompson}/ROV {\ital Jason2} installed microbial incubators in drill-holes in the sides of active sulfide chimneys and sampled rocks, fluids, and microbes in the Mothra and Main Endeavour Field (MEF). In July 2003, with {\ital Thompson/Jason2}, an NSF-LEXEN project at Baby Bare on Endeavour east flank conducted sampling through seafloor-penetrating probes, plus time-series sampling of fluids, microbes, and rocks at the MEF. In September 2003, with {\ital Thompson}/ROV {\ital ROPOS}, the Keck Proto-Neptune project installed a seismic network consisting of 1 broadband and 7 short-period seismometers, installation of chemical/physical sensors and time-series samplers for chemistry and microbiology in the MEF and Clam Bed sites, collection of rocks, fluids, animals, and microbes. In May/June 2004, an NSF-sponsored {\ital Atlantis/Alvin} cruise recovered sulfide incubators installed in 2003, redeployed a sulfide incubator, mapped MEF and Mothra vent fields with high-resolution Imagenix sonar, sampled fluids from MEF, Mothra, and Clam Bed, recovered year-long time-series fluid and microbial samplers from MEF and Clam Bed, recovered and installed hot vent temperature-resistivity monitors, cleaned up the MEF and deployed new markers at major sulfide structures. In August 2004, there were two MBARI/Keck-sponsored cruises with R.V. {\ital Western Flyer}/ROV {\ital Tiburon}. The first cruise completed the seismic network with addition of two more broadband seismometers and serviced all 7 short-period seismometers. {\ital Tiburon} then performed microbial and chemical investigations at MEF, Mothra, Sasquatch, and Middle Valley, collecting fluid, particle, and animal samples for culture and phylogenetic analysis. {\ital Tiburon} continued in late August/September with detailed petrological sampling. A Keck-sponsored {\ital Thompson/ROPOS} cruise in September continued work on chemical/physical sensor deployments and time-series chemical and microbial sampling. A graduate student workshop at Friday Harbor beginning October 2004 will analyze the first year of data from the seismic network and begin to correlate seismic activity with hydrothermal activity. The Endeavour ISS is still in a phase of data collection and sensor development, but moving toward data integration.
http://www.ridge2000.org
B13A-0180 1340h
Local Earthquakes on the Endeavour Segment of the Juan de Fuca Ridge: First Seismic Results from the Keck Seismic/Hydrothermal Observatory
The W.M. Keck Foundation is supporting a five-year program to conduct prototype seafloor observatory experiments to monitor the relationships between episodic deformation, fluid venting and microbial productivity on the Endeavour segment of the Juan de Fuca Ridge and at the intersection of the Nootka fault and the Cascadia subduction zone. At the Endeavour, the experiment is sited near the central portion of the segment in a region where the spreading axis is characterized by a 100-m-deep, 500-m-wide axial valley that hosts five high-temperature hydrothermal vent fields spaced 2-3 km apart. The objectives of the experiment are to monitor local and regional seismicity around the vent fields in conjunction with the deployment of sensors and samplers to monitor temporal variations in the physical, chemical and ultimately microbial characteristics of the hydrothermal fluids. The Endeavour seismic network was installed in the summer of 2003 with the ROV ROPOS and comprises seven GEOSense three-component short-period corehole seismometers and one buried Guralp CMG-1T broadband seismometer. Five of the seven short-period seismometers were inserted in horizontal coreholes drilled into seafloor basalts; two were deployed in concrete monuments on the ridge flanks. It is the first seismic network on a mid-ocean ridge in which the sensors are deployed with an ROV beneath the seafloor in order to ensure good coupling and minimize the effects of current-generated noise. In August 2004, we used the ROV Tiburon to service the Endeavour seismic network and recover the first year of data. In addition, we installed a second broadband and three short period seismometers on the Nootka fault and a third broadband seismometer on the Explorer plate. The Endeavour seismic network performed well with all eight instruments recording high-quality data. A preliminary inspection of the data reveals many examples of local, regional and teleseismic earthquakes. One striking characteristic of the local earthquake records is the high fidelity of the shear wave records. The preliminary analysis of the local earthquake data will be performed as part of an undergraduate research apprenticeship class taught this fall at the University of Washington's Friday Harbor Laboratories.
B13A-0181 1340h
Seismic Characterization of Crustal Magma Bodies at the Endeavour Segment, Juan de Fuca Ridge
Multichannel seismic reflection data collected in July 2002 at the RIDGE2000 Integrated Studies Site at the Endeavour segment, Juan de Fuca Ridge show a high-amplitude, mid-crustal reflector underlying all of the known hydrothermal vent fields at this segment. Based on the amplitude-offset behavior of this reflector, we identify it as a crustal magma body rather than a cracking front interface. The Endeavour Segment magma chamber reflector is found at two-way travel times of 0.85-1.5 s (~1.9-4.0 km) below the seafloor and extends approximately 25 km along axis although it is only 1-2 km wide on the cross-axis lines. The reflector is shallowest (at 2.5 km depth on the along-axis line) beneath the central, elevated part of the Endeavour segment and deepens toward the segment ends, with a maximum depth of 4 km. The cross axis lines show the magma chamber reflector dipping to the east with the shallowest depths under the ridge axis and greater depths under the eastern flank of the ridge. The amplitude-offset behavior of this mid-crustal axial reflector is consistent with a negative impedance contrast, indicating the presence of melt or a crystallizing mush. We have constructed tau-p transforms and partial offset stacks to examine the variation of melt-mush content of the axial magma chamber along axis. In addition, we have forward modeled the upper crustal velocity structure both on and off-axis. These velocity profiles are used to project hypocenters of well-located microseismicity in this region [Wilcock et al., 2002] onto the along-axis and cross-axis stacked seismic sections. Most axial earthquakes are concentrated in a depth range of 1.5 - 2.7 km, just above the axial magma chamber. In general, seismicity is distributed diffusely within this zone indicating thermal-related cracking above the magma chamber. However, the cross-axis line at the Salty Dawg vent field shows seismicity localized along a steeply dipping fault-like plane that terminates just above the magma chamber.
B13A-0182 1340h
Seismic Structure of the Axial Magma Chamber Along the Southern Juan de Fuca Ridge From Full-Waveform Inversion and Partial S-Wave Stacking
Multichannel seismic data collected along the Cleft segment on the southern Juan de Fuca Ridge shows that this intermediate-spreading center is underlain by a mid-crustal reflector interpreted as the top of an axial magma chamber (AMC). The AMC reflection is present along most of the segment, and deepens gently from 2.1 km near the southern end of the segment beneath the RIDGE Cleft Observatory Site, to 2.4 km at the northern end beneath the site of the mid-1980's submarine eruption. {\it F-k} filtering of super-CDP gathers allows the identification of a weak, coherent seismic phase interpreted as the {\it P}- to {\it S}-wave conversion at the AMC ({\it P$_{AMC}$S}). Stacking of this event shows that the {\it P$_{AMC}$S} is only detectable along the northern part of the segment. In this area, one-dimensional waveform modeling in the time intercept-slowness ({\it $\tau$-p}) domain indicates that the AMC is ~100 m thick, and it is characterized by a decrease in {\it P}-wave velocity from 6 km/s to 3.7 km/s. In contrast, the {\it P}-wave velocity within the southern, shallower AMC is larger (4.5 km/s). Our results suggest along-axis variations in the crystallinity of the AMC. Assuming that the AMC represents spherical crystals suspended in a melt matrix (mush), the AMC along Cleft varies from a high crystal content ($\sim$10% melt) magma chamber at the southern end of Cleft, to a mush zone with $\sim$60% melt in the source of the 1980's eruption at the northern end.
B13A-0183 1340h
Evidence for Along-Strike Hydrothermal Circulation Within Young Oceanic Crust on the Eastern Flank of the Endeavour Axis, Juan de Fuca Ridge
Many geological, geophysical, and geochemical indicators at spreading centers suggest that the dominant direction of fluid circulation may be along-strike, subparallel to the primary orientation of neovolcanic and near-ridge faulting. Researchers are beginning to identify additional evidence from young ridge flanks for along-strike fluid (and heat and solute) flow. We present newly-compiled thermal, swath-map, and seismic data collected on young crust east of the Endeavour Segment of the Juan de Fuca Ridge. Hydrosweep data reveal several prominent basaltic outcrops just north of the ODP Leg 168 drilling transect, within an area experiencing a hydrothermal transition (HT) from exposed to sediment-covered basement. Seafloor heat flow is strongly suppressed 20 km east of the spreading center, and the pattern of thermal rebound with increasing distance from the ridge has been interpreted as evidence for dominantly across-strike hydrothermal circulation. We show that within one part of the hydrothermal transition, located on 1.2-1.3 Ma seafloor, the data are at least as well explained by along-strike fluid circulation, with cold seawater recharging through outcrops to the north of the HT area. Application of a simple analytical model for coupled heat and fluid flow suggests specific discharge in basement on the order of 1 m/yr, a rate consistent with earlier estimates of across-strike circulation, and with independent estimates of driving forces and crustal permeability. Dominantly along-strike circulation has also been inferred on 3.5 Ma crust to the east of the HT area, suggesting that this mode of fluid transport may be the rule, rather than the exception, on this young ridge flank. Since primary flow paths are likely associated with tectonic processes that occur during and soon after the crust is formed, it seems probable that along-strike fluid flow is also important today at the Endevaour Segment.
B13A-0184 1340h
Inferences of Particle Size and Composition From Video-like Images Based on Acoustic Data: Grotto Plume, Main Endeavor Field
Optical and acoustic scattering from particles in a seafloor hydrothermal plume can be related if the particle properties and scattering mechanisms are known. We assume Rayleigh backscattering of sound and Mie forward scattering of light. We then use the particle concentrations implicit in the observed acoustic backscatter intensity to recreate the optical image a camera would see given a particular lighting level. The motivation for this study is to discover what information on particle size and composition in the buoyant plume can be inferred from a comparison of the calculated optical images (based on acoustic data) with actual video images from the acoustic acquisition cruise and the IMAX film "Volcanoes of the Deep Sea" (Stephen Low Productions, Inc.). Because the geologists, biologists and oceanographers involved in the study of seafloor hydrothermal plumes all "see" plumes in different ways, an additional motivation is to create more realistic plume images from the acoustic data. By using visualization techniques, with realistic lighting models, we can convert the plume image from mechanical waves (sound) to electromagnetic waves (light). The resulting image depends on assumptions about the particle size distribution and composition. Conversion of the volume scattering coefficients from Rayleigh to Mie scattering is accomplished by an extinction scale factor that depends on the wavelengths of light and sound and on the average particle size. We also make an adjustment to the scattered light based on the particles reflectivity (albedo) and color. We present a series of images of acoustic data for Grotto Plume, Main Endeavour Field (within the Endeavour ISS Site) using both realistic lighting models and traditional visualization techniques to investigate the dependence of the images on assumptions about particle composition and size. Sensitivity analysis suggests that the visibility of the buoyant plume increases as the intensity of supplied light increases, the particle size decreases, and the particle reflectivity increases. However, decreasing the particle size (and thus increasing the extinction scale factor) results in a wider, less defined plume and increases the relative importance of the acoustic background noise; the best fit of our calculated optical images to the character of actual video images of the bottom few meters of the plumes (the acoustic data volume is 55 m tall) suggests that average particle size is fairly large ($\sim$1000 $\mu$m) in the buoyant plume. This suggests that existing data on particle size distributions underestimates the average particle size; the best explanation is the breakup of aggregates of particles during collection and filtering of water samples (no in situ measurements exist). We also investigate the effects of particle color on plume color by using models based on data collected by Feely et al (1987), Walker and Baker (1988), and Mottl and McConachy (1990). Highly reflective particles result in result in sharper-edged plumes suggesting that pyrite (albedo $\sim$0.6) and chalcopyrite (albedo $\sim$0.3) are the dominant particle compositions. This study shows that plume particles in the buoyant plume are probably larger than previously suspected and a predominance of pyrite and chalcopyrite is necessary to explain the high reflectance of black smoker plumes.
http://www.caip.rutgers.edu/~kls93/realplume/index.html
B13A-0185 1340h
Distribution of Particulates in Hydrothermal Plumes of the Endeavour Axial Valley: Preliminary Results from the Sea Breeze Project
Hydrothermal vent plumes provide zones for chemical reactions between vent fluids and seawater, potential habitats for anaerobic bacteria and zooplankton, and a probable mechanism for the dispersal of vent larvae. Within the Endeavour Integrated Study Site are five known vent fields situated along the axial valley of the Endeavour Segment of the Juan de Fuca Ridge (N.E. Pacific Ocean). Each of these fields has a particle rich neutrally buoyant plume above it almost constantly, a common characteristic of vent systems worldwide. The purpose of this study was to determine 1) how plume particle distribution varies along the Endeavour segment axial valley; 2) whether a correlation exists between vent activity and particle density in the surrounding water, and 3) if the peak signals in backscatter and light transmission fall within a consistent range of potential density values along the axial valley. Light transmission and backscatter data were collected from vertically oscillating CTD casts at 21 stations along the axial valley covering the fields of Mothra, Main Endeavour, High Rise, Salty Dawg, and Sasquatch during the Sea Breeze - REVEL 2004 seagoing program. Plume particle density within ocean water was measured using a Wetlabs transmissometer and a Seapoint turbidity sensor. Preliminary results indicate a positive correlation between "black smoker" activity and signal strength in backscatter and light transmission. Main Endeavour and High Rise, known to exhibit the most rigorous hydrothermal activity, show correspondingly high amplitude signals in both backscatter and light transmission. Predicted diurnal currents seem to effect lateral plume particle movement away from vent sources, greatly impacting the particle density in surrounding areas. Peak signals in backscatter and light transmission occur in less dense water moving northward from Mothra to Salty Dawg.
http://www2.ocean.washington.edu/seabreeze/
B13A-0186 1340h
The Fate of Ammonium in the Endeavour Hydrothermal Plume: Microbial Ammonia Oxidation and Ammonium Assimilation
Highly elevated ammonium concentrations (up to 177 $±$ 4.5 nM) were observed within the axial valley and the hydrothermal plume along the Endeavour Segment, and were quickly dissipated due to both autotrophic ammonia oxidation and assimilation. Autotrophic ammonia oxidation rates accounted for at least 93 % of the total net ammonium removal rates, and reached a maximum of 53 nM d$^{-1}$ in the neutrally buoyant plume. Ammonia oxidation rates showed a sigmoidal relationship with ammonium concentrations based on data collected over three years. The resulting empirical half-saturation constant (153 nM) was at least an order of magnitude lower than those measured in cultured studies. This high substrate affinity implies that natural oceanic ammonia-oxidizing bacteria are highly adapted to the generally limited ammonium in the deep sea. Ammonia oxidation in this plume potentially added 26-127 mg N m$^{-2}$ d$^{-1}$ of nitrate into the deep-sea water column. This oxidation process was heavily influenced by the presence of organic-rich particles, with which ammonia-oxidizing bacteria were often associated (40-68 %). Ammonia-oxidizing bacteria, including those in both the $\beta$- and $\gamma$- {\it Proteobacteria} subgroups, contributed up to 10.8 % of total microbial abundance within the plume. Ammonium assimilation rates were also substantially enhanced within the neutrally buoyant plume (up to 20 nM d$^{-1}$), and accounted for at least 42 % of total net ammonium removal rates. The two ammonium uptake rates together always exceeded the total net removal rates, suggesting an active {\it in situ} regeneration of ammonium (1.0-23.5 nM d$^{-1}$ or 4.2-99 mg N m$^{-2}$ d$^{-1}$ integrated over plume depths) that was equivalent to 3- to 130-fold of what could be provided by the particulate flux from the euphotic zone reaching these depths. Ammonia oxidation is responsible for ammonium turnover in 0.7-13 days, and is an important {\it in situ} organic carbon production process (3.9-1.8 mg C m$^{-2}$ d$^{-1}$) especially at an early stage of the Endeavour neutrally buoyant plume.
B13A-0187 INVITED 1340h
The Eastern Lau Basin Integrated Studies Site (ISS): Recent Progress and Future Plans
Rapid progress is being made in understanding the Eastern Lau Spreading Center (ELSC) Integrated Studies Site, which is a new focus area in the Ridge 2000 Program. The ELSC, located in the western Pacific near Tonga, is a 390 km-long first-order ridge segment that displays a broad range of effects of the back-arc environment. Its southern end, at only 40 km from the Tonga arc volcanic front, is propagating southward into a back-arc rift. Its northern end is 100 km from the volcanic front and terminates at a large, nontransform offset. The ELSC undergoes substantial and systematic changes in primary parameters affecting crustal accretion including spreading rate and mantle source composition. As a consequence it displays large changes along its length in lava chemistry, axial depth and morphology, melt lens characteristics, and crustal thickness and structure. A focus of the work at the ELSC is to understand how changes in these forcing functions affect crustal accretion, hydrothermal venting, and faunal composition and abundance. A geophysical/hydrothermal study of the entire ELSC during an initial R2K cruise in April-May 2004 (PI: Martinez) disclosed a surprisingly high level of hydrothermal activity along the ELSC. This survey involved shipboard multibeam, two deep-towed sonars (DSL120A and IMI30), and concurrent MAPR/Chemical Scanner, CTD/rosette tow-yos, and vertical casts. Hydrothermal activity as indicated by water column plumes increases toward the north, even though magmatic robustness decreases. An ancillary study (PI: Thurnherr) also deployed autonomous floats during this cruise to investigate deep circulation patterns that affect hydrothermal plumes and faunal dispersal. A second cruise in September 2004 (PI: Langmuir) focused on locating vent sources using ABE, petrological sampling, and determining water column properties. Three additional cruises are scheduled during 2005. The first (PI: Tivey) will provide an initial characterization of vent fields, fluid chemistry, mineralogy, and biodiversity using Jason II and net tows; a following cruise (PI: Childress) will investigate community ecology using Jason II; and a final cruise (PI: Vrijenhoek) will study genetics and species diversity in the hydrothermal vent communities.
http://www.ridge2000.org/
B13A-0188 1340h
Geophysical and Hydrothermal Survey of the Lau Basin Integrated Studies Site
In April-May 2004 we conducted the first combined geophysical and hydrothermal survey of the entire backarc Eastern Lau Spreading Center (ELSC) and Valu Fa Ridge (VFR) on R/V Kilo Moana as part of the Ridge 2000 Lau Integrated Studies. Backarc crustal accretion differs in systematic ways from that at mid-ocean ridges (MORs) because of mantle wedge compositional variations including hydration and depletion effects with distance from the arc volcanic front that respectively increase and decrease melt production relative to MORs. This effect decouples melt production from spreading rate and allows crustal accretion and hydrothermal activity to be investigated under different parameters than commonly prevail at MORs. To study these effects the length of the ELSC/VFR was surveyed with nested shipboard multibeam and two deep-towed sonars (DSL120A and IMI30) and concurrent MAPR/Chemical Scanner and CTD/rosette tow-yos and vertical casts. The spreading center can be divided into three geophysically and morphologically distinct segments each with smaller overlapping sub-segments. The southernmost segment and closest to the volcanic front is the VFR ($22\deg45'-21\deg26'$S). It is volcanically robust, spreads at 39-61 mm/yr, forms a narrow peaked ridge capped by multiple small volcanic cones and is underlain by a deep axial magma lens reflector. Sonar backscatter on the VFR suggests that volcaniclastics debris mantles the ridge flanks in places, probably resulting from high volatile contents of erupted lavas and shallow (1600-2000 m) axial depths. Areas of numerous but small relief faults can be found in this area. The next segment to the north ($21\deg26'-20\deg32'$S), referred to here as the southern ELSC (S-ELSC) increases in spreading rate to 61-76 mm/yr, but abruptly deepens to 2000-2500 m and looses its axial high morphology and axial volcanic cones although it is also underlain by a magma lens reflector. Backscatter intensities increase somewhat and longer and larger faults develop. The northern ELSC (N-ELSC) ($20\deg32'-19\deg20'$S) spreads at 76-96 mm/yr but deepens to 2500-3000 m and has no magma lens reflector. It has higher near-axis backscatter than the S-ELSC with greater coverage by volcanic flows and pillow mounds and fewer large faults evident within the 1-2 km near-axis deep-towed sonar coverage. However, overall along the length of the ELSC/VFR broader coverage from the shipboard multibeam system shows greater faulting and larger flanking abyssal hills from south to north, indicating greater tectonism northward as spreading rates increase but magmatic robustness decreases. Hydrothermal activity was measured by the percentage of ridge length covered by all distinct plume signatures identified. This activity increases to the north with spreading rate as at MORs but is counter to the trend with magmatic robustness found at MORs, as these indicators decrease northward along the ELSC/VFR. We hypothesize that total mantle heat input, as parameterized by spreading rate, and permeability of crustal structure, as indicated by faulting, are stronger controls on hydrothermal activity than magmatic budget.
B13A-0189 1340h
Hydrothermal Prospecting and Petrological Sampling in the Lau Basin: Background Data for the Integrated Study Site
In the spring of 2004, Martinez and others (this meeting) carried out an extensive mapping program of the southern Lau Basin spreading centers with high resolution bathymetry and sidescan data. They were also defined the overall distribution of mantle plumes along the ridges using miniature autonomous plume recorders and CTD tow-yos. These important new data were the first step of an integrated RIDGE 2000 program of four cruises to identify the overall tectonic, petrological, hydrothermal and biological characteristics of the spreading centers to permit a well informed selection of targets for more focussed studies in coming years. During September and October of 2004, on board R/V Kilo Moana, we are carrying out the second phase of investigations, with the aim of identifying specific hydrothermal sites and providing regional sampling of lavas from the region. The primary tools of investigation for hydrothermal site discovery are the autonomous benthic explorer (ABE) as well as CTD's and bottom towed cameras. Making use of new CTD data to verify the plumes identified in the Martinez et al cruise, ABE is being used in three different modes. Phase I focusses on the water column at the level of the neutrally buoyant plume. These results enable planning of the more focussed Phase II, closer to the bottom, to provide high resolution bathymetry and characterization of the buoyant plume. Phase III provides photographic coverage of even more limited regions focussed on the vent fields defined in Phase II. Camera tows using TOWCAM are planned to provide the highest resolution images of new hydrothermal sites. This cruise represents the first attempt to use ABE in such a staged and systematic program of regional vent site discovery. Since the ship is free to carry out other investigations during the ABE dives, which can be 24 hours or more in length, high density sampling of the various Lau Spreading Centers will be carried out at the same time as the hydrothermal investigations. A newly designed percussion core and free-fall winch provide the potential of very rapid sample turn around and the best sampling coverage of any back-arc spreading center. Shipboard analyses of rocks collected by dredging and percussion core and of fluids collected during CTD casts in conjunction with a double pass along the ridge system will permit hypothesis testing in real time. Preliminary results based on the multi-disciplinary sea-going program will be reported.
B13A-0190 1340h
New Techniques for Hydrothermal Exploration: In Situ Chemical Sensors on AUVs - Preliminary Results From the Lau Basin
Less than one quarter of the global ridge-crest has yet received even cursory investigation for the presence or absence of hydrothermal activity. To improve exploration efficiency, particularly at high latitudes, new methodologies independent of tethered vehicles are required. To that end, we have begun the use of in situ chemical sensors allied to the increasing capabilities of autonomous underwater vehicles. Here, we present first results from our most recent efforts aboard the second R2K cruise to the Lau Basin (C.Langmuir, PI; Autumn 2004) to (a) map non-buoyant hydrothermal plumes, (b) intercept buoyant hydrothermal plumes and (c) locate and image novel hydrothermal fields on the seafloor. The AUV used for this work is ABE and the sensors deployed are direct extensions of the in situ Fe/Mn sensor deployed previously on SOC's AUTOSUB to investigate seasonally-reducing waters in Loch Etive, NW Scotland. Each in situ instrument comprises an electronics package that contains a tattletale control system with a flash memory card for on-board logging and a chemical manifold, consisting of a series of valves, pumps and a colorimetric cell. Analysis of iron is enabled by the determination of the coloured complex formed between iron II and ferrozine, manganese uses the colour change of PAN in the presence of reduced manganese. The system includes capacity for switching between sample, blank and two on-board samples for "in flight" calibrations with blanks and standards held in medical bags, outside of the pressure-balanced manifold, to attain in situ water-column temperatures. An in-line filter prevents large particle clogging and detection limits for both iron II and manganese II are ca.2nM.
B13A-0191 1340h
Plume mapping and shipboard chemical data used to locate new vent sites in the Lau Basin
A central goal of the second Ridge2000 cruise (September-October 2004) to the Lau backarc basin in the southwest Pacific is to locate, map, and image new vent sites on the East Lau Spreading Center and the northern portion of the Valu Fa Ridge. Our primary tool for plume mapping and vent location is the Autonomous Benthic Explorer (ABE), including novel in situ chemical sensors (see abstracts by Yoerger et al. and German et al. in this session). In addition, we are using MAPRs (Miniature Autonomous Plume Recorders) to measure profiles of temperature and optical backscatter on dredge and rock core lowerings, and shipboard analysis of methane, hydrogen, pH, iron, and manganese from CTD casts, to locate and characterize the plumes. This presentation will focus on the profile data and chemical analyses, which we use to provide a preliminary comparison of chemical characteristics between vent fields.
B13A-0192 1340h
Formation of black and white smokers in the North Fiji Basin: Sulfur and lead isotope constraints
The hydrothermal chimneys were recovered from 16$^{o}$50-_S triple junction area in the North Fiji Basin. The chimney samples are divided into three groups according to their mineralogy and metal contents; 1) Black smoker, 2) White smoker, 3) Transitional type. Black smoker chimneys are mainly composed of chalcopyrite and pyrite, and are enriched in high temperature elements such as Cu, Co, Mo, and Se. White smoker chimneys consist of sphalerite and marcasite with trace of pyrite and chalcopyrite, and are enriched in low temperature elements (Zn, Cd, Pb, As, and Ga). Transitional chimneys show intermediate characteristics in mineralogy and composition between black and white smokers. Basaltic rocks sampled from the triple junction show wide variation in geochemistry. Trace elements composition of basaltic rocks indicates that the magma genesis in the triple junction area was affected by mixing between N-MORB and E-MORB sources. The sulfur and lead isotope compositions of hydrothermal chimneys show distinct differences between the black and white smokers. Black smokers are depleted in $^{34}$S {ä$^{34}$S = +0.4 to +4.8) and are low in lead isotope composition ($^{206}$Pb/$^{204}$Pb = 18.082 to 18.132; $^{207}$Pb/$^{204}$Pb = 15.440 to 15.481; $^{208}$Pb/$^{204}$Pb = 37.764 to 37.916) compared to white smoker and transitional chimneys (ä$^{34}$S = +2.4 to +5.6; $^{206}$Pb/$^{204}$Pb = 18.122 to 18.193; $^{207}$Pb/$^{204}$Pb = 15.475 to 15.554; $^{208}$Pb/$^{204}$Pb = 37.882 to 38.150). The heavier sulfur isotopic fractionation in white smoker can be explained by boiling of hydrothermal fluids and mixing with ambient seawater. The lead isotope compositions of the hydrothermal chimneys indicate that the metal in black and white smokers come from hydrothermal reaction with N-MORB and E-MORB, respectively. Regarding both black and white smoker are located in the same site, the condition of phase separation of hydrothermal fluid that formed white smokers might result from P-T condition of high temperature reaction zone below the hydrothermal venting site. Our results suggest that white smokers were formed by hydrothermal circulation closely related to E-MORB magma intrusion at shallower depth. Meanwhile, Black smoker probably formed by deeper intrusion of N-MORB magma before the formation of white smoker.
B13A-0193 1340h
RIDGE 2000 Integrated Studies: Patterns of Hydrothermal Ecosystem Variation Within Lau Basin
The Lau Basin represents a contrast to other mid-ocean ridge systems because of its relative youth (<6 Ma), unique autotrophic ecology of (often gastropod-dominated) ecosystems, and potential for heterogeneous geochemical habitats. These settings provide fertile ground for numerous alternative hypotheses to understand larval transport, species interactions, community structure and succession, endemism, faunal speciation, and biogeographic convergences. These hypotheses require knowledge of the relational distribution of actively venting sites (for larval transport, gene flow, evolution), and individual communities/species (regional species pool for community succession and species interaction), and in some cases geochemical conditions and circulation patterns. While Lau Basin fauna have been taxonomically assessed and trophic (i.e., symbiotic) character of the dominant species revealed virtually nothing is known about the ecological settings and distribution of communities within the Basin - information critical to initiate detailed and efficient biological studies. Western Pacific sites like Manus, North Fiji, and Lau share some taxonomic similarities at specific, genetic and familial levels, but it is known that each site also supports what so far appear to be endemic species with dynamic evolutionary histories. In September 2004, we characterized patterns of species occurrence among Lau Basin vent sites via digital imaging and mosaic mapping using the Autonomous Benthic Explorer (ABE) AUV and the WHOI TOWCAM imaging system. These data were used to create microhabitat maps and quantitatively assess the distribution, abundance, spatial relationships, and variation in microhabitat structure of both sessile and mobile fauna, and to relate them to hydrothermal activity and geological features. Our studies provide a detailed photo-characterization of constituent communities to elucidate the distribution of venting activity, faunal habitats, and provide a comparison to other Western Pacific back-arc basins for future RIDGE 2000 integrated studies.
B13A-0194 1340h
Variability of Southern Valu Fa Ridge Magmatic Systems
Valu Fa Ridge (VFR), which encompasses the southernmost segments of the East Lau Spreading Center (ELSC), is an important end-member in the spectrum of back-arc spreading centers because it is strongly affected in all aspects of the spreading process by inputs from the nearby active Tofua (Tonga) volcanic arc, and because its magma systems are rapidly evolving as the VFR propagates to the south. New lava samples collected by the TELVE Expedition of the R/V Southern Surveyor (Australia) from the four southernmost VFR segments have greatly increased the number and spatial distribution of fresh volcanic glass samples, quadrupling the availability of "primitive" (MgO $>$ 6 wt.%) glass samples and encompassing significant along- and across-axis geochemical variability. These new data provide an opportunity to evaluate both the evolution of crustal magmatic processes relative to southward rift propagation and variability in mantle source inputs relative to the active volcanic arc. The four sampled segments of VFR are separated by left-stepping overlapping offsets that differ from their mid-ocean ridge counterparts in their longer, more parallel overlapping limbs and in the absence of an overlap basin. The TELVE glasses display considerable major element variability and the VFR is unusual among well-developed spreading centers in its strongly bimodal volcanism, in the abundance of evolved lavas and in the coexistence of two distinct liquid lines of descent. Dacites and rhyolites (SiO2 ~67-75 wt.%) are relatively abundant close to segment ends or discontinuities along the southernmost three ridge segments, but rare from the northernmost sampled segment and from off-axis seamounts. A "Daly Gap" from ~60-67 wt.% SiO2 and 1.75-0.75 wt.% MgO is present along the northern segments but absent near the southern propagating rift tip. Primitive (MgO $>$ 6wt.%) glasses were recovered from all VFR segments and on 5 of the 9 sampled seamounts. Both high-silica glasses and FeTi basalt glasses appear to represent the culmination of simple crystal fractionation trends derived from similar tholeiitic parental compositions that bifurcate between 5 and 6 wt. % MgO. Lavas from the off-axis seamounts, both east and west of VFR, are systematically more mafic than the on-axis samples and most have higher K2O contents. A single boninitic glass recovered from seamount K-11 is similar in composition to melts reported in a xenolith from the same seamount by Kamenetsky et al. (EPSL, 151, 205, 1997). At its southern end, the VFR is propagating into old arc crust of unknown age. If the high silica lavas are excluded, along-axis major element trends define a typical rift propagation pattern. High-silica lavas occur sporadically with no apparent relationship to the petrologic trends defined by the more mafic lavas, suggesting that magmas only fractionate to high SiO2 compositions under ideal, localized conditions. Of particular interest is the possibility that the high-Si trend is initiated and/or facilitated by changes in oxygen fugacity, perhaps initiated by assimilation of hydrated pre-existing crustal material. Preliminary trace element data for the southern VFR segments show the characteristic Nb and Ta depletions associated with volcanic arc sources, confirming the strong input from the volcanic arc to the back-arc mantle source region. More complete spatial coverage is expected to help define the extent and, perhaps the pathways, of this mantle input.
B13A-0195 1340h
Petrology and Geochemistry of the Eastern Lau Spreading Center
The spreading centers of the southern Lau Basin lie at varying distances from the volcanic front of the Tongan arc. The Valu Fa spreading center in the south is only some tens of km from the arc volcanoes, while the Central Lau Spreading Center (CLSC) further north is much further from the arc. The Eastern Lau Spreading Center (ELSC) that links the two is at progressively varying distances from the arc. Previous studies have shown that lavas from the CLSC are similar to basalts from ordinary mid-ocean ridges, while Valu Fa lavas have higher water contents and distinctly higher SiO2, reflecting a greater influence of subduction components. Modeling of existing data from the ridges also shows a significant change in the background composition of the mantle wedge within the Lau Basin. Lavas from the south appear to be sampling a more depleted mantle, with lower Na2O and FeO contents than those further to the north. Making use of the new mapping results of Martinez and others (this meeting), we have undertaken an extensive sampling campaign focussed primarily on the Eastern Lau Spreading Center to investigate the transition from arc-like to MORB-like lavas. Sampling at steadily varying distances from the volcanic front should permit a clear definition of the subduction components influencing the mantle wedge, offer insight into the mechanisms by which subduction components affect the melting process, and possibly constrain the progressive evolution of slab components with increasing distance from the trench. All these issues are of importance to a broad range of convergent margin studies. The existence of this chemical transition also permits a better understanding of the relationship between ridge segmentation and mantle composition: are there abrupt changes across offsets, suggesting efficient focussing and mixing mechanisms within ridge segments, or are the transitions gradual along the ridge, indicating a more distributed delivery of magma to the crust? The major elements will also give indications of lava temperatures and the distribution of magma bodies in the crust, making it possible to investigate possible relationships with the distribution and intensity of the new hydrothermal activity discovered in this region. Samples will be obtained during the LauII cruise in September and October of this year. Planned shipboard analyses should permit a first-order investigation of these various questions.
B13A-0196 1340h
LAUB-FLEX: Circulation in the Lau Backarc Basin Determined by Floats
During early RIDGE2000 cruises to the Lau backarc basin integrated study site, a number of neutrally buoyant floats have been deployed. The floats are programmed to drift at 1700m, near the depth of the hydrothermal plumes known at the time of deployment. Every 3--4 weeks the floats return to the surface in order to transmit their position as well as auxiliary data back to shore using a network of satellites. The data are made available in near-real time on the web. In contrast to current-meter data, which become available only after recovery of the instruments and which, because of their Eulerian nature, are not very well suited for studying dispersal near topography, the float data yield timely, cost-effective and directly usable information about dispersal in the deep ocean. The available float data after 3 months reveal an intriguing and consistent circulation pattern in the Lau Basin, with predominantly northward flow along the East-Lau Spreading Center. The float-derived circulation is qualitatively consistent with a hydrographic analysis that was carried out during the project proposal, but the available velocities are 2--3 times larger than expected. Assuming that the currently available data are representative and taking typical deep-ocean values for the horizontal eddy diffusivity, implies that dispersal in the Lau Basin is dominated by advection. This inference has important consequences for the biogeography as it implies that dispersal is primarily unidirectional. This is different at other deep-ocean sites, such as the EPR crest and the ridge flank of the MAR, where dispersal is eddy-diffusion dominated and, therefore, inherently omnidirectional.
http://www.ldeo.columbia.edu/~ant/LAUB-FLEX
B13A-0197 1340h
Fault-hosted hydrothermal breccia at 22\deg40' N on the Mid-Atlantic Ridge
The SMARK region is approximately 100 km south of the Kane Transform on the Mid-Atlantic ridge (MAR). The region is noteworthy for its west-facing Eastern Median Valley wall (EMVW), a 2300 m-high seismically active fault scarp that exposes the upper 1-2 km of $<$1Ma slow-spread oceanic crust. The oceanic crust at SMARK is pervasively faulted and fractured basaltic pillow lavas and dikes. The setting of the SMARK area and the nature of the samples are ideal for testing hypotheses about the relationship between faulting, hydrothermal systems, and microbial communities. Select samples collected in 1995 by dredging and with Alvin from the EMVW fault zone include fault gouges and breccias with a range of cataclastic and granular textures. Clasts are angular (but in many places with smooth grain boundaries) and consist primarily of fine-grained to glassy basalt. The matrix grains are primarily silt-sized with subordinate clay. The composition of the matrix minerals is equally magnesium and iron rich owing to a combination of orange-brown clay (palagonite) minerals, chlorite, and amphiboles. Preliminary XRD of one sample shows evidence for barite. In contrast, samples with no identified sulfur-bearing phases have concentric alteration rims of iron surrounding clasts. Iron-rich samples also contain disseminated grains of magnetite (and, in some samples, titano-magnetite). The data are interpreted in terms of a model wherein axial faults and fractures initially function as conduits rather than seals to (hydrothermal) fluid flow. Although the fault rocks would be predicted to seal the conduits as they move off-axis and cool, variation in the oxidation of iron (expressed as alteration rims, for example) within the matrix minerals is testament to continuing, and possibly off-axis fluid flow. In a setting of evolving permeability, decreasing temperature, and changing oxidation potential of fluids, we expect biological communities thriving in the sub-surface environment to have several windows of stability over time. Further clarification and tests of this model are planned using both structural and geochemical analyses of the fault rocks.
http://www.duke.edu/~mwb8/GMT.html
B13A-0198 1340h
Fluid-Rock Interaction in the Basement of the Lost City Vent Field: Insights from Stable and Radiogenic Isotopes
The Lost City Hydrothermal Vent Field (LCVF), with its characteristic carbonate-brucite chimneys, is located on a terrace, at a water depth of 750 to 850m on the southern escarpment of the Atlantis Massif (Mid-Atlantic Ridge $30^{\circ}$N). The Atlantis Massif, an oceanic core complex, consists of peridotites and gabbroic rocks that have undergone several phases of serpentinization, talc-metasomatism and carbonate precipitation related to progressive deformation and interaction with seawater during a long-lived exhumation history. We present stable and radiogenic isotope data from the serpentinized peridotites and gabbros that provide constraints on the history of seawater-rock interaction and the role of serpentinization in methane-production and sulfide mineral precipitation. Early phases of serpentinization and metasomatism occurred at temperatures up to $\sim$250$^{\circ}$C, as indicated by depleted bulk-rock O-isotope compositions of the serpentinites and gabbros. Sr- and Nd-isotope data allow modelling and quantification of seawater-rock interaction. The isotopic compositions of the gabbros show heterogeneity, likely related to variable interaction/exchange with hydrothermal fluids. The serpentinites have Sr- and Nd-isotope compositions close to seawater values and correspond to high water/rock ratios (from 1.53 x 10$^{3}$ to 3.65 x 10$^{6}$), indicating large volumes of seawater circulating through the massif during serpentinization. In contrast, the serpentine-talc schists, resulting from high strain and focused fluid flow of Si-rich fluids during detachment faulting and exhumation, exhibit variable but low fluid/rock ratios (from 10 to 150). Analyses of bulk-rock carbon contents and carbon isotope compositions show total non-carbonate carbon contents of $<$600 ppm with C-isotope compositions of -29 to -22$\permil$ (VPDB). The negative $\delta$$^{13}$C values are likely linked to processes of fluid-rock interaction during serpentinization. C- and O-isotope compositions of carbonate-rich serpentinites and carbonate veins reflect mixing of seawater and deep serpentinization-derived hydrothermal fluids, and a range of precipitation temperatures from ambient conditions up to $\sim$180$^{\circ}$C at depth. Sulfur contents in bulk-rock serpentinites indicate a significant addition of sulfur during serpentinization with S-isotope compositions close to seawater values. $\delta$$^{34}$S ratios of extracted sulfides suggest that their formation is linked to sulfate reduction. Collectively, the isotopic compositions of the vents and basement rocks, measured fluid temperatures, and $^{14}$C ages of the vents, sediments and veins provide evidence that hydrothermal fluid flow and temperatures of $\sim$100$^{\circ}$-150$^{\circ}$C have likely been sustained in the basement for many tens of thousands of years.
B13A-0199 1340h
Sulfur Isotope Geochemistry of the Lost City Hydrothermal Vent Fluids
At the Lost City Hydrothermal Vent Field (Mid-Atlantic Ridge, $30\deg$N), reactions between seawater and ultramafic rocks produce high alkaline (pH 9 to 11) fluids that are venting at temperatures of 40 to $90\deg$C and result in the formation of up to 60m tall carbonate-brucite structures. The fluids are enriched in hydrogen, methane and other hydrocarbons, and support dense microbial communities. We present sulfur isotope data of dissolved sulfate and coexisting sulfide in the fluids venting at Lost City, which together with C-isotope data provide constraints on the links between chemical and biological processes associated with serpentinization. The sulfur isotope composition of sulfate increases from seawater values of +21$\permil$ (VCDT) in fluids with sulfate concentrations of 28 mM to values of up to +30$\permil$ in the low sulfate-, high pH end-member hydrothermal fluids. Sulfide concentrations range between 50 and 2780 micromolar. Sulfur isotope compositions of the sulfides lie in a narrow range of +34 to +37$\permil$ (VCDT) and show no clear correlation with concentrations. The isotopic compositions of dissolved inorganic carbon vary between -0.5$\permil$ (VPDB) in the high sulfate samples and -18$\permil$ in the low sulfate samples. This covariance indicates active sulfate reduction in the vent structures and/or in the shallow serpentinite subsurface. Sulfate reduction likely contributes to the variability of carbon isotope compositions observed in both the dissolved inorganic carbon and the carbonate minerals forming the structures. These data, together with C- and O-isotope data of the vent structures, provide evidence that methane oxidation coupled with sulfate reduction during mixing of the more pristine, hydrogen and methane-rich hydrothermal end-member fluids with seawater is an important process in hydrothermal carbonate precipitation at Lost City. Our results are consistent with previous microbiological and organic geochemical studies, which indicate a close association of methane-cycling archaea with sulfate-reducing bacteria in the vent structures.
http://www.lostcity.washington.edu
B13A-0200 1340h
The Use of Stable Hydrogen Isotopes as a Geothermometer in Hydrothermal Systems
Terrestrial geothermal work by Arnason in the 1970's demonstrated the utility of stable hydrogen isotopes as a geothermometer[1]. However, with the exception of two data points from 9°N in a study by Horibe and Craig[2], the value of this geothermometer in hydrothermal systems has never been rigorously assessed. Equilibrium fractionation factors for H2-H2O and H2-CH4 have previously been determined experimentally and theoretically over a range of temperatures and provide an expression relating alpha (fractionation) and temperature. We have measured the dD of H2(g), CH4(g) and H2O from a diverse selection of hydrothermal vent localities including Lost City, Middle Valley, Endeavour, Guaymas, Logatchev, Broken Spur, and SWIR. These samples were chosen to represent a wide range of fluid temperatures and a variety of environmental settings. We see a strong correlation between measured vent temperature and predicted vent temperature using both the hydrogen-water and the methane-hydrogen geothermometers over a temperature range of 25-400°C. In the case of the H2-H2O geothermometer, the predicted temperatures are slightly elevated with respect to the measured temperatures at the low temperature Lost City site, and are in good agreement at high temperature vent sites. The H2-CH4 geothermometer predicts temperatures that are 40-80°C elevated with respect to the measured temperature in both the low and high temperature sites. These measurements demonstrate that the hydrogen isotope geothermometer in the hydrogen-methane-water system is robust in hydrothermal systems and may be a useful tool in determining the temperature of the root zone. 1. Arnason, B., The Hydrogen-Water Isotope Thermometer Applied to Geothermal Areas In Iceland. Geothermics, 1977. 5: p. 75-80. 2. Horibe, Y. and H. Craig, D/ H fractionation in the system methane-hydrogen-water. Geochimica et Cosmochimica Acta, 1995. 59(24): p. 5209-5217.
B13A-0201 1340h
Trace Element Distributions and Size Fractionation in the Edmond Hydrothermal Plume, Central Indian Ridge
Because cycling of the entire ocean volume through hydrothermal plumes is rapid relative to thermohaline circulation (order 10$^{3}$ years), understanding the processes active within these plumes is crucial if we are to assess their impact on global geochemical cycles. Preliminary results from hydrothermal plume particle analyses at the Kairei and Edmond systems (Indian Ocean) have indicated that key processes, previously identified to be important in the Atlantic Ocean, also appear broadly applicable to the Indian Ocean. This was not immediately expected because parallel work has shown that the rate of dissolved iron (II) oxidation in hydrothermal plumes decreases systematically from the Atlantic to the Indian and Pacific Oceans. Here, we examine dissolved-particulate trace metal distributions in greater detail within one Indian Ocean plume (Edmond), together with the complementary vent-fluid data, to investigate these processes further. Upon oxidation, dissolved iron (II) initially forms colloidal iron (III) which then aggregates to form particulate iron (III) - the oxyhydroxide particles which apparently co-precipitate and adsorb dissolved metals from the surrounding seawater. What has remained unstudied, however, is the role that colloidal rather than aggregated particulate Fe may play in these systems. To investigate this we have combined studies of large-volume plume-particle samples (1.0$\mu$m filters) with a series of dissolved, colloidal and finer-grained particles collected using a CTD-rosette. Here, we will discuss the distributions of Fe and the relative fractionations of Mn, Cu (representative of the chalcophile elements) and P (representative of the oxyanions) within and between different hydrothermal "pools": dissolved, colloidal, fine particles and coarse particles as determined from filtration through 0.1, 0.4 and 1.0$\mu$m filters.
B13A-0202 1340h
Physical Constraints On The Formation And Storage Of Brines In Mid-Ocean Ridge Hydrothermal Systems
Mid-ocean ridge hydrothermal systems are known to vent fluids with salinities substantially different from seawater. This is attributed to phase separation and the segregation of the resulting vapor and brine phases. Time series of vent temperature and salinity (chlorinity) show that some black-smoker vent fields have vented fluids with salinities well below seawater for over a decade. This raises important questions concerning chloride mass conservation and the fate of brines in these systems. One widely accepted model is that high-density brines formed during super-critical phase separation sink efficiently to the base of hydrothermal systems, leading to the development of a two-layer system in which a re-circulating brine layer underlies a single-pass seawater cell. However, there is no conclusive evidence for such a two-layer configuration or for the assumption that a brine layer will convect. In this study we first present theoretical arguments to constrain the dynamics of such a deep brine layer. From an analysis of brine properties in the two-phase area, we conclude that, if brines are stored in a layer at the base of high-temperature mid-ocean ridge hydrothermal systems they are unlikely to convect because phase separation will lead to a stable stratification. One consequence of this result is that the brine layer beneath black systems has to be thin ($<$ 10m) to match the high heat fluxes. However, estimates of the rate at which brines are accumulating in the crust below the Main Field on the Endeavour segment of the Juan de Fuca Ridge and below vents near $9\deg$50'N on the East Pacific Rise suggest that the brine layer is likely at least 100 meter thick. To resolve this apparent paradox we propose an alternative model which we support with both conceptual arguments and inferences from single-phase numerical models. It is generally believed that the pressure gradients in mid-ocean ridge hydrothermal systems are close to cold hydrostatic. At the high temperatures and pressures characteristic of the deeper parts of these systems brines with salinities as high as 20-30 wt% NaCl have densities around 800-900 kg/m$^{3}$ and will be buoyant in a cold-hydrostatic system. We argue that interfacial tensions between fluid and solid phases will likely favor the segregation of vapor into the main fractures and brine into the smaller fissures and backwaters. This allows the vapor to flow efficiently through the system and transport large heat fluxes while most of the porosity in the lower part of the system fills with brines that will rise only slowly because of their higher density and viscosity and the low permeability of brine filled fissures. Our numerical models suggest that brines that rise will reach a level of neutral buoyancy as they cool and enter high permeability regions in which the pressure gradients decrease.
B13A-0203 1340h
Internal Structure of Fault Zones in Mid-Oceanic-Ridges: implications for
Normal fault in oceanic crust are important elements of all Mid-Oceanic Ridges. Although their surface expression is known, almost nothing is known about the internal structure of these fault zones. The brittle character of the mafic oceanic crust promotes the formation of massively dilatant systems and makes these fault zones different from normal faults in weaker rocks, with important consequences on fault zone structure and transport properties. We present a scaled analogue model combined with field studies of the Koa'e Fault system (Big Island/Hawai'i) to provide insight in the faulting of oceanic crust. The Koa'e is a normal fault system with sub-vertical fault scarps up to 20 m. The formation of large gaping fissures on the footwall is connected with underground openings up to several meters width. Fault scarps are accompanied by a large number of mode-I cracks along existing weakness planes like cooling-joints and bedding features. In our scaled analogue model we used a fine-grained cohesive powder with a tensile strength of 33 Pa. The curved yield locus of the material presents a good mechanical analogue to basalt with a scaling ratio of approximately 1:5.000-40.000. Time-lapse imagery and Particle ImagingVelocimetry was used to analyze the displacement field and fracturing process at high resolution. Boundary conditions include the presence of a buried fault, to simulate the volcanic growth faults in areas of active volcanism and tectonics. Results show structures very similar to those observed in the field: mode-I preceding mode-II movement, the formation of vertical fissures and fault scarps, fragmentation due to mechanical stratigraphy, the presence of large open fissures and filling of fault gaps with wall fragments falling downwards. The analogue experiments and the field observations imply that massively dilatant structures are common features in Mid-Oceanic-Ridges, to a depth of several hundred meters. The highly open internal structure and the resulting transport properties have a large effect on the network of hydraulic systems. The meter-scaled cavities may provide habitats for the chemosynthetic life forms associated with such black-smoker systems.
B13A-0204 1340h
How Lithospheric Stress Constrains Dike Intrusions in Mid-Ocean Ridges
It's believed that more than 90% of the spreading in mid-ocean ridges is accommodated by dike intrusions. Recent advances in ocean bottom observations makes feasible the direct measurement of seafloor displacement during dike intrusions, and will provide a tool to probe into the magmatic and faulting processes in mid-ocean ridges. Here we use numerical modeling to explore the effect of lithospheric stress on dike intrusions. We firstly calculate lithospheric stress prior to dike intrusion in mid-ocean ridges using visco-elastoplastic rheology and show that the distribution and magnitude of lithospheric stress are different for ridges with different spreading rate and thermal structure. Then we assume the stress state in the dike changes into lithostatic during dike intrusion, and numerically model the corresponding seafloor displacement and the variation of dike width along the depth. Also we show that lithospheric stress may determine the maximum depth of dike opening, and the bottom of the dike may be well below the magma chamber or even may penetrate down into the asthenosphere on the axis.
B13A-0205 1340h
Lithospheric Accretion and the Nature of Anomalously Thick Oceanic Moho Transition Zone
The oceanic Moho transition zone (MTZ) separates layered gabbros of the crust derived by magma crystallization from the uppermost residual peridotites, generally harzburgites, representing mantle rocks. Mapping of the Oman and the Bay of Islands ophiolite complexes, both of which are inferred to be composed of obducted oceanic lithosphere formed at fast spreading ridges, has shown that the MTZ is mostly composed of sills and lenses of gabbro intruded into dunite. Thickness of the MTZ can vary from a few meters to over two kilometres. Within the thick MTZ, individual gabbro sills and lenses can reach thickness of a few hundred meters. Thermal modelling, tomography, compliance and PmS converted wave studies support the geologic evidence and suggest presence of gabbroic melt accumulations within the MTZ, in the vicinity of fast and intermediate spreading centres. However, seismic reflection imaging, which has been instrumental for determining the structure of the oceanic crust and for defining the geometry of axial magma chambers, has not yet been successful at imaging the gabbro sills and gabbro-melt lenses imbedded into dunite, casting some doubt on their existence within the present day oceanic lithosphere. Here we show images of a series of groups of subcrustal reflection events that resulted from our analysis of some 1500 km of multichannel seismic data collected in 2002 across the Juan de Fuca ridge flanks as part of the EW0207 cruise. Because the Moho discontinuity is well imaged along most of the survey track and the inferred crustal thickness is remarkably uniform, the location of these events as being within the MTZ is well constrained. We provide evidence that the imaged events are true subcrustal reflections and discuss why imaging the structure of thick MTZs is challenging when both dunite - gabbro and dunite - gabbro-melt interfaces are strong reflectors of acoustic energy. We also discuss mechanisms for the emplacement of gabbro sills within the dunites of the thick MTZ, and their subsequent thinning and/or recycling deeper into the mantle. For the thick MTZs located tens of km away from the ridge axis, we discuss the processes that led to their preservation.
B13A-0206 1340h
Diking, Magma Lenses, and Location of Hydrothermal Sites at Mid-Ocean Ridges
Magma chambers beneath fast spreading mid-oceanic ridges appear in the form of thin lenses in the cross- axis sections. Furthermore, the recent data indicate that such a magma lens also lies beneath the intermediate spreading rate Endeavor segment on the Juan de Fuca Ridge. This shape indicates that the pressurization of the magma chamber should result in the stress concentration near the tips of the lens while the rest of the host rock would be in the state of compression. It is likely, therefore, that if an episode of magma replenishment in the magma lens results in diking, the dikes will initiate near the lens tips. The further propagation of dikes can be described by the principles of fracture mechanics. Our calculations suggest that the dikes propagate almost vertically towards the seafloor from the lens tips. Because diking is likely to generate a region of high permeability near its margin in addition to heat, hydrothermal activity may be localized by diking events. This suggests that hydrothermal vent fields may be located above the tips of the magma lenses away from the ridge axis (i.e., in the cross-axis profile of the lens). Alternatively, the diking may result from solidification of the magma lens. Because the density of magma is lower then that for the fresh rock, crystallization leads to the pressure decrease in the lens. Consequently, the stress distribution in the host rock changes and becomes tensile in the middle part of the magma lens and compressive in the tip regions. In this scenario, the dikes are likely to initiate from the central areas and the hydrothermal sites would be more expected to occur above the central part of the magma lens. Comparing observations of the location of the hydrothermal sites with respect to the magma lens location may be useful for constraining the mechanisms of magma lens evolution. For example, the Salty Dawg hydrothermal site on the Endeavor segment appears to be located near the western tip of the seismically imaged magma lens. Our model suggests that the venting activity at this site may be localized by diking triggered by the lens pressurization. The fact that hydrothermal venting at the EPR occurs above the center of the magma lens suggets that the rapid hydrothermal cooling and magma crystallization in the lens may be important in controlling the permeability distribution at that site. As has recently been argued in the literature, the pressurization of the magma lens may be occuring by the dyking originating from the underlying, deep magma chambers (also with a lens shape). Our computations suggest this hypothesis. Furthermore, not only may these dykes pressurize overlying shallow magma lenses, but these can then also lead to the subsequent dyke propagation from the tips of these shallow lenses to the seafloor.
B13A-0207 1340h
A Model for Hydrothermal Convection in the Near-Axis Region
Although high-temperature ridge axis convection and low-temperature circulation have received extensive study, relatively little is known concerning the transition zone between these environments. Recent seismic studies of the East Pacific Rise at 9$\deg$ 30'N indicates the presence of a large partial melt region, extending approximately 3 to 4 km on either side of the ridge axis. The partially molten body is capped by a thin melt lens, approximately 2 km wide at a depth of approximately 1.5 km, and the sides of the melt body are nearly vertical to a depth of 6 km, but gradually deepen over the next 6-7 km to a depth of roughly 8 km. This large region of partial melt is essentially defined by the 1150$\deg$C isotherm. Except very near the near-vertical sides of the partial melt zone, where isotherms are high compressed, the temperatures appear to increase nearly linear with depth. We consider two styles of hydrothermal convection in the oceanic crust in an attempt to explain these observations. In the first model, hydrothermal circulation is constrained the rate of heat conduction from below; and we assume the 1150$\deg$C isotherm simply deepens conductively as hydrothermal circulation extracts heat from the crust. We further assume that convection is constrained to lie above the 500$\deg$C isotherm, the crust being effectively impermeable at higher temperatures. Conductive cooling would place the 1150$\deg$C isotherm at a depth of 6 km in approximately 3.6 x 10$^{5}$ yr, but at a 6 cm/yr half-spreading rate, this depth would occur $\sim$ 20 km from the ridge axis. This is much farther than the seismic data indicate. In the second model, we assume that the steep- sided magma chamber drives a narrow vertical convective boundary layer. The application of boundary layer theory suggests that a boundary layer $\sim$ 100 wide would form if the permeability in the frozen gabbros were $\sim$ 10$^{-14}$ m$^{2}$. The heat transfer in the boundary layer appears to be roughly the same as the rate of heat input to the ridge axis to maintain seafloor spreading. Further from the axis, the linear isotherm distribution suggests weak cellular convection, perhaps restricted to the upper 2 km (pillows and dikes) of the crust.
B13A-0208 1340h
Numerical Models of Layered Convection in a Heterogeneous Medium and the Relationship Between Focused and Diffuse Flow in Ridge Crest Hydrothermal Systems
One of the outstanding issues in submarine hydrothermal research concerns the connection between focused high-temperature venting and nearby, low temperature diffuse flow. Some systems such as the Galapagos do not exhibit high-temperature venting at all despite geochemical evidence of a high-temperature end member fluid. We explore this issue be constructing a number of 2-D simulations using the two-phase finite difference code GTH, set to operate in the single-phase mode. We considered a rectangular system one kilometer deep and two kilometers wide with a heterogeneous permeability distribution to simulate a deep single-pass circulation system. To drive the single-pass system we constructed a 750 m-wide recharge zone with permeability k = 10$^{-13}$ m$^{2}$ and 50 m-wide cross-flow and discharge zones with k = 10$^{-12}$ m$^{2}$. In the interior of the cell, we assumed k = 10$^{-15}$ m$^{2}$. The side boundaries were impermeable and insulated; the top boundary was open and held at fixed temperature. The lower boundary was also impermeable and either constant temperature or constant heat flux, with values appropriate for black smoker flows, conditions were imposed. We then explored the effect of a surface layer of high permeability pillow basalts with thicknesses ranging between 100 m and 400 m and 10$^{-14}$ m$^{2}$ $<$ k$_{p}$ $<$ 10$^{-10}$ m$^{2}$. At low values of k$_{p}$, convection in the pillows was negligible and the pillow had little affect on the high-temperature system. At high values of k$_{p}$, convection in the pillows significantly damped the high-temperature output. At intermediate values cold diffuse flow occurred at varying distances from the main high temperature vent. The results of these simulations thus show that fluid convection in the pillows can significantly impact deep-seated circulation. Depending upon the permeability and thickness of the pillow either low-temperature Galapagos-type venting or combined focused-diffuse flow venting may evolve.
B13A-0209 1340h
Numerical Simulations of 1-D Two-Phase Flow with Non-Zero Mass Fluxes: Application to Phase Separation at 9$\deg$N (EPR) and the Main Endeavour Vent Field (JDF)
Numerical simulations of vertical one-dimensional two-phase flow with non-zero mass fluxes can help to enumerate and describe the different parameter space regimes at which phase separation can occur in submarine hydrothermal systems. Our simulations suggest that there are at least two such quasi-steady state regimes, corresponding to low and high mass fluxes imposed at the bottom of the system. For relatively low mass fluxes (10$^{-5}$ - 10$^{-4}$ kg/m$^{2}$-s), a low salinity vapor-rich region forms near the top of the system, and high-salinity brine flows downward, raising the salinity in the lower half of the system. For high mass fluxes (&\sim$ 10$^{-3}$ kg/m$^{2}$-s), both vapor and brine flow upward, with the vapor moving more quickly than the brine. In this case, phase separation results in higher salinities near the top of the system, corresponding to a mixture of brine and vapor, and salinities approaching that of normal seawater lower in the system. Bulk surface salinities in the high-flux regime are qualitatively consistent with vent fluid salinities at hydrothermal systems such as 9$\deg$N on the East Pacific Rise and some of the vents of the Main Endeavour Field on the Juan de Fuca Ridge. Our future work will further test this preliminary analysis.
B13A-0210 1340h
Numerical Modelling of MOR Hydrothermal Systems: The Need to Use Compressible Fluids, Realistic EOS and High-resolution Meshes
We use a simplified model of convection in a porous medium to investigate the transport of mass and energy in sub-seafloor hydrothermal systems with high permeabilities. Previous studies have addressed the issue either taking the Boussinesq approximation and/or using too coarse meshes. Here we argue that both steps result in erroneous results and typically lead to artificial and non-realistic steady state solutions. The highly non-linear thermodynamic properties of water cause plumes to form around temperatures of 400$^\circ$C where energy transport is maximised. This cannot be modelled accurately using a hypothetical Boussinesq fluid for which the variations in fluid properties are linearized. Further we show that high-resolution meshes are essential in representing the full dynamics of the system. Coarse meshes tend to suppress density instabilities and result in non-realistic steady-state solutions. When using high-resolution meshes the convection pattern will remain non-steady. Based on our recent simulations of convection of H$_2$O-NaCl liquid+vapor fluid mixtures around granitic intrusions, we expect that convection patterns for seawater compositions will be even more unstable.
B13A-0211 1340h
A new InterRidge Working Group : Biogeochemical Interactions at Deep-sea Vents
A new Working Group on `Biogeochemical Interactions at deep-sea vents' has been created at the initiative of the InterRidge programme. This interdisciplinary group comprises experts in chemistry, geochemistry, biogeochemistry, and microbial ecology addressing questions of biogeochemical interactions in different MOR and BAB environments. The past decade has raised major issues concerning the interactions between biotic and abiotic compartments of deep-sea hydrothermal environments and the role they play in the microbial turnover of C, S, N, Fe, fluxes from the geosphere to hydrosphere, the formation of biominerals, the functioning of vent ecosystems and life in extreme environments, the deep-biosphere, and the origin of life. Recent multidisciplinary studies have provided some new insights to these issues. Results of some of these studies will be presented here. They point out the variability and complexity of geobiological systems at vents in space and time and highlight the need for interactions across the fields of chemistry, geochemistry, biogeochemistry, and microbial ecology of hydrothermal environments. Limitation for advances in these fields include the availability of seafloor observation/experimentation time, and of underwater instrumentation allowing quantitative, in situ measurements of chemical and biological fluxes, as well as physical and chemical sensing and sampling along small scale gradients and repeated observation of study sites. The aim of this new Working Group is to strengthen the scientific exchange among chemists, geochemists, biogeochemists and microbial ecologists to favor collaboration in field studies including intercomparison of methods and planning of integrated experiments. The Biogeochemical Interactions working group will also foster development of underwater instrumentation for in situ biogeochemical measurements and microscale sampling, and promote exchange and collaboration with students and scientists of neighboring disciplines, particularly with vent biologists, ecologists and geologists .
B13A-0212 1340h
Abiotic Synthesis of Methane Under Alkaline Hydrothermal Conditions: the Effect of pH in Heterogeneous Catalysis
Abiotic formation of methane in hydrothermal reaction zones at mid-ocean ridges likely occurs by Fischer-Tropsch catalytic processes involving reaction of CO2-bearing fluids with mineral surfaces. The elevated concentrations of dissolved methane and low molecular weight hydrocarbons observed in high temperature vent fluids issuing from ultramafic-hosted hydrothermal systems, in particular, suggest that Fe and Cr-bearing mineral phases attribute as catalysts, enhancing abiotic production of alkanes. The chemi-adsorption of dissolved CO2 on the catalytic mineral surface, however, might be influenced by a pH dependent surface electron charge developed within the mineral-fluid interface. Thus, a series of experiments was conducted to evaluate the role of pH on rates of carbon reduction in fluids coexisting with Fe-oxides at 390 degree C and 400 bars. At two distinct pH conditions, acidic (pH = 5) and alkaline (pH = 8.8), the abiotic production of isotopically labelled CH4(aq) was monitored during FeO reaction with aqueous NaCl-NaHCO3-H2-bearing fluid (0.56 mol/kg NaCl, 0.03 mol/kg NaH13CO3). Despite the lower H2(aq) concentrations (120 mmol/kg) in the high pH system, concentrations of abiogenic methane attained values of 195 umol/kg and 120 umol/kg respectively, suggesting enhanced catalytic properties of mineral under moderately high pH. X-ray photoelectron spectroscopy (XPS), performed on unreacted and final solid products, reveal the significantly greater abundances of alkyl (C-C-) groups on the surface of FeO oxidized at elevated pH, in comparison with mineral reacted at low pH conditions. Thus, enhanced adsorption of dissolved CO2 and the resulting Fischer-Tropsch formation of alkyl groups likely contributes to methane production observed at alkaline conditions. Introducing the effect of pH in the Fischer-Tropsch mechanism of alkane formation has important implications for the recently discovered Lost City ultramafic-hosted hydrothermal system, where elevated pH and high CH4 concentrations in the moderate temperature vent fluids are observed.
B13A-0213 1340h
Abiotic Organic Chemistry in Hydrothermal Systems.
Abiotic organic chemistry in hydrothermal systems is of interest to biologists, geochemists and oceanographers. This chemistry consists of thermal alteration of organic matter and minor prebiotic synthesis of organic compounds. Thermal alteration has been extensively documented to yield petroleum and heavy bitumen products from contemporary organic detritus. Carbon dioxide, carbon monoxide, ammonia and sulfur species have been used as precursors in prebiotic synthesis experiments to organic compounds. These inorganic species are common components of hot spring gases and marine hydrothermal systems. It is of interest to further test their reactivities in reductive aqueous thermolysis. We have synthesized organic compounds (lipids) in aqueous solutions of oxalic acid, and with carbon disulfide or ammonium bicarbonate at temperatures from $175-400\deg$C. The synthetic lipids from oxalic acid solutions consisted of n-alkanols, n-alkanoic acids, n-alkyl formates, n-alkanones, n-alkenes and n-alkanes, typically to C$_{30}$ with no carbon number preferences. The products from CS$_{2}$ in acidic aqueous solutions yielded cyclic thioalkanes, alkyl polysulfides, and thioesters with other numerous minor compounds. The synthesis products from oxalic acid and ammonium bicarbonate solutions were homologous series of n-alkyl amides, n-alkyl amines, n-alkanes and n-alkanoic acids, also to C$_{3}$0 with no carbon number predominance. Condensation (dehydration) reactions also occur under elevated temperatures in aqueous medium as tested by model reactions to form amide, ester and nitrile bonds. It is concluded that the abiotic formation of aliphatic lipids, condensation products (amides, esters, nitriles, and CS$_{2}$ derivatives (alkyl polysulfides, cyclic polysulfides) is possible under hydrothermal conditions and warrants further studies.
B13A-0214 1340h
Distribution of bacteria and associated minerals in the gill chamber of the vent shrimp Rimicaris exoculata and related biogeochemical processes
The shrimp Rimicaris exoculata dominates the megafauna of some mid-Atlantic Ridge hydrothermal vent fields. This species harbors a rich bacterial epibiosis inside its gill chamber. At the Rainbow vent field, the epibionts are associated with iron oxide deposits. Investigation of both bacteria and minerals by scanning electron microscopy (SEM) and X-ray microanalysis (EDX) shows the occurrence of three distinct compartments in the gill chamber: (1) the lower pre-branchial chamber, housing bacteria, but devoid of minerals, (2) the "true" branchial chamber that contains the gills and remains free of both bacteria and minerals, and (3) the upper pre-branchial chamber housing the main ectosymbiotic bacterial community and associated iron oxides. According to our chemical and temperature data, abiotic iron oxidation appears to be kinetically inhibited in the environment of the shrimps and this would explain the lack of iron oxide deposits in the first two areas. We propose that, in the third area, iron oxidation is microbially promoted. The discrepancy between the spatial distribution of bacteria and minerals suggests that different bacterial metabolisms are involved in the two compartments. A possible explanation lies in the modification of physico-chemical conditions downstream of the gills, that would reduce the oxygen content and favor the development of bacterial iron-oxidizers in this Fe II-rich environment. A potential role of such iron-oxidizing symbionts in the shrimp diet is suggested. This would be unusual for hydrothermal ecosystems, where most previously described symbioses rely on sulphide or methane as an energy source.
B13A-0215 1340h
Investigating Microbial Habitats in Hydrothermal Chimneys using Ti-Thermocouple Arrays: Microbial Diversity
In order to examine the changes that occur in the microbial community composition as a deep-sea hydrothermal vent chimney develops, we deployed Ti-thermocouple arrays over high temperature vents at two active sites of the Guaymas Basin Southern Trough. Chimney material that precipitated around the arrays was recovered after 4 and 72 days. Chimney material that precipitated prior to deployment of the arrays was also recovered at one of the sites (Busted Shroom). Culture-independent analysis based on the small subunit rRNA sequence (cloning and DGGE) was used to determine the microbial diversity associated with subsamples of each chimney. The original Busted Shroom chimney (BSO) was dominated by members of the {\it Crenarchaeota} Marine Group I, a group of cosmopolitan marine Archaea, $\epsilon$-{\it Proteobacteria}, and $\gamma$-{\it Proteobacteria}, two divisions of Bacteria that are common to deep-sea vents. The 4 days old Busted Shroom chimney (BSD1) was dominated by members of the {\it Methanocaldococcaceae}, hyperthermophilic methanogens, and the 72 days old chimney (BSD2) by members of the {\it Methanosarcinaceae}, mesophilic and thermophilic methanogens. At the second site, Toadstool, the 72 days old chimney material that had precipitated around the array (TS) revealed the dominance of sequences from uncultured marine Archaea, the DHVE group I and II, and from the $\epsilon$-{\it Proteobacteria}. Additionally, sequences belonging to the {\it Methanocaldococcaceae} and {\it Desulfurococcaceae} were recovered next to thermocouples that were at temperatures of 109$\deg$C (at Busted Shroom) and 116$\deg$C (at Toadstool), respectively. These temperatures are higher than the upper limit for growth of cultured representatives from each family.
B13A-0216 1340h
Microbial Colonization of Mineral Substrates on Loihi Seamount
Seafloor weathering involves the biologically influenced alteration of basaltic rocks during reaction with seawater. The dynamics of this process have implications for global climate by removing carbon dioxide from seawater and ocean chemistry via elemental exchange during weathering. Additionally, seafloor weathering communities may represent a significant fraction of the subsurface biosphere. To understand the ecology and geochemistry of seafloor weathering, it is important to understand the population dynamics and activities of the microbiological populations colonizing rock surfaces on the seafloor during weathering. Various naturally occurring substrates common to the seafloor (including basalt and pyrite) were incubated in situ for one year on the seafloor at different distances from hydrothermal vents on the Loihi seamount, the newest basaltic shield volcano in the Hawaiian arc. Before placement on the seafloor, the mineral samples were sterilized, well-characterized and mounted on slides. The slides were collected in sample baskets which were then delivered to the seafloor via submersible in December 2002 and retrieved in December 2003. We are using DAPI staining and fluorescent in-situ hybridizations (FISH) to characterize the microbial populations which colonizing these substrates. FISH is performed with domain-specific oligonucleotide probes (for bacteria [eub338] and archaea [arch915]) to determine the relative proportions of the two domains in the colonizing communities, as well as to assess total cell numbers, as functions of both substrate and conditions in situ. Our observations indicate that both mineralogy and local chemical/physical conditions significantly influence colonization and weathering of the substrates reacted.
B13A-0217 1340h
Tracing Alteration Textures in Young to Ancient Pillow Lavas: A Petrographic Signature of Early Life on Earth
Alteration of basaltic glass in pillow lava rims and hyaloclastites in the upper oceanic crust may involve a combination of abiotic and biotic processes depending on thermal conditions. Abiotic alteration yields smooth fronts of banded dark yellow to brown material of approximately equal thickness on both sides of fractures. Textures we ascribe to a biogenic origin occur as granular and tubular types, and differ markedly from the characteristic abiotic features. The granular type comprises individual coalesced spherical bodies 0.2-0.6 microns in diameter, and appears as solid bands, semicircles or irregular patches protruding into the fresh glass. The tubular type consists of thin, sometimes branching tubes, with widths of 1-4 microns, and minimum lengths commonly between 10-100 microns, that define a highly irregular alteration front. The granular and tubular structures have the right size, shape and distribution to have resulted from microbial corrosion of the glass. Further alteration causes them to be filled by authigenic minerals. The microbial alteration textures always occur in places where seawater could penetrate into the glass, such as along fractures, around vesicle walls and near varioles. Their biogenicity is substantiated by carbon isotope data, element distribution of C, N, P and S, as well as the presence of DNA in young samples. The granular texture is the most common of the two types and occurs at any depth down to 500m into the lava pile; both types are, however, most abundant in the 100-300m depth range. Even though these microstructures are easily destroyed during deformation and metamorphic mineral growth, an ultimate goal of our research is to trace biotextures in pillow lava as far back in time as possible. For that purpose we have investigated undeformed and low grade metamorphosed pillow lavas and interpillow hyaloclastite from the $\sim$3.5 Ga Barberton Greenstone Belt in South Africa. Petrographically the original glass rim of the pillow lavas and the shards consist of predominantly fine-grained chlorite hosting fracture-rooted titanite-filled tubular structures. These structures are of similar width and length as those found in pillow lava rims of modern oceanic crust, produced by microbial corrosion. The tubular structures are commonly partly overgrown by chlorite and are thus clearly pre-metamorphic. We suggest these tubular structures represent traces of microbial activity formed during etching of the original glassy material as microbes colonised the surface along fractures soon after eruption on the sea floor $\sim$3.5 billion years ago.