T31A-0472 INVITED
Overview of the Ridge 2000 Integrated Studies Sites
The Ridge 2000 program is in its fourth year and fieldwork at each of the Integrated Studies Sites (ISS) is in full swing. Multidisciplinary monitoring continues at the EPR ISS with seismic, temperature, and current data being continuously recorded. Long-term fluid sampling programs aimed at furthering our understanding of temporal variations in the chemistry of high-temperature hydrothermal vents are continuing. In situ fluid chemistry monitors have been deployed for weeks, and longer deployments are planned as the technology matures. Nested within these monitoring studies are experiments addressing larval dispersal and changes in microbial and macrobiological communities. In early 2006, geodetic monitoring will begin, with an array of pressure gauges as well as a detailed compliance study. By early 2007, a 3-D multichannel seismic survey will have provided unprecedented details of the crustal structure at $9°50\'{ }N. Together these studies provide a strong framework for an interdisciplinary understanding of the links between the forces that produce a mid-ocean ridge spreading center and their manifestation on the seafloor. Fieldwork on the Endeavour segment of the Juan de Fuca ridge in 2005 also included a balance of monitoring, experimental, and sampling programs across a wide range of disciplines. Four interdisciplinary field programs were conducted to maintain and expand ongoing Ridge 2000 and proto-NEPTUNE experiments. These research programs continued development and testing in situ chemical and microbial sensors, conducted co-registered sampling of fluids, fauna, and chimney material, and recovered moorings that measured heat and chemical fluxes at the segment scale. High-resolution mapping was also completed at this site, which has been chosen for one of the two initial NEPTUNE Canada nodes to prepare the way for the collaborative, cabled observatory projects. The mapping cruise included 5 secondary school teachers as part of the REVEL outreach and education program. Live transmission of high-definition video from the seafloor to land stations provided an exciting preview of the potential of high-bandwidth communication with the seafloor. The first round of fieldwork at the East Lau Spreading Center ISS was completed in 2005. Building upon the two R2K-funded cruises in 2004, three cruises in 2005 sampled 7 hydrothermal vent sites. Four of these sites were discovered by the collaborative efforts of R2K scientists, working together across cruises, and one site by Japanese colleagues collaborating with R2K scientists in 2004. Another of the sites was discovered during the first R2K cruise of 2005. The SM2000 mounted on {\it Jason II} in 2005 was used to create fine-scale bathymetric maps of six of the sites and high-resolution imagery was collected for photomosaics of selected areas of hydrothermal activity within the sites. These maps and imagery guided even finer scale surveys, equipment deployments and sampling of basalt, hydrothermal deposits, vent fluids, microbial mats, and benthic organisms. Some of the fauna collected are still alive and under study in pressure vessels in R2K-supported laboratories. Results from these cruises have improved our understanding of this back-arc spreading center, "from mantle to microbe," and are invaluable for selection of the focus area, or bull\'{ }s eye, for the next generation of integrated, interdisciplinary studies in this region.
T31A-0473
Opposing Trends in Crustal Thickness and Spreading Rate Along the Back-arc Eastern Lau Spreading Center: Controls on Ridge Morphology, Faulting, and Hydrothermal Activity
At mid-ocean ridges (MORs), crustal production and spreading rate are tightly coupled primary variables that are thought to control major ridge features such as morphology, faulting, and hydrothermal activity. At back-arc ridges (BARs), in contrast, crustal thickness is observed to vary systematically with the position of the spreading center relative to the arc volcanic front, independent of spreading rate. This is thought to be due to hydration and depletion effects in the subduction mantle wedge which affect the magmatic productivity of the spreading center and is unlike at MORs where unusual crustal thickness variations are often attributed to mantle temperature anomalies. In the Lau back-arc basin the ~400 km long Eastern Lau Spreading Center (ELSC) approaches the arc volcanic front from north to south as the basin narrows. Correspondingly, ridge spreading rates decrease by more than half (97-39 mm/yr) as near-axis crustal thickness nearly doubles (~5.5-9 km). The opposing variations between spreading rate and crustal thickness at the ELSC thus allows observations, generally unavailable at MORs, of their decoupled effects on ridge morphology, volcanism, faulting, and hydrothermal activity. We investigate these effects as part of the first phase of RIDGE2000 Integrated Studies in the Lau back-arc basin. We used deep-towed side-scan sonar instruments (DSL120A and IMI30) to map the near-axis region within broader-coverage ship multibeam bathymetry and side-scan imagery swaths. An array of miniature autonomous plume recorders (MAPRs) attached to the deep-towed sonar's tow cables concurrently measured sea water optical backscatter in a vertical swath. Subsequent hydrocasts were made at identified plume sites to obtain water and particulate samples for chemical analysis. The data show that variations in ridge morphology are opposite to spreading rate trends at MORs, forming a peaked volcanic high at slow rates where the crust is thick and a deeper flat axis at fast spreading rates where the crust is thinner. Faults along the ELSC become larger and more widely spaced northward, also opposite to trends with spreading rate at MORs. Hydrothermal activity, as measured by plume incidence, increases with spreading rate, as at MORs, but reaches levels higher than the global MOR trend. The observations indicate that crustal thickness has a greater control on ridge morphology and faulting than spreading rate, even at fast rates where the thermal lithosphere should be thin. However, spreading rate, perhaps in combination with increased fault permeability, has a greater control on hydrothermal activity than crustal thickness and the magmatic robustness of the ridge.
T31A-0474
Axial Magma Chamber Properties of the Lau Back-arc Basin Spreading Centers
From multichannel seismic data we have observed a nearly continuous axial magma chamber along much of the Central Lau Spreading Center, southern Eastern Lau Spreading Center, and southern Valu Fa Ridge. However, little is known about the seismic properties of the melt generating the reflectors. By employing stacking and waveform inversion methods from a similar study of the East Pacific Rise [{\it Singh et al.,} 1998], we distinguish seismic signals from the standard P-wave reflection as well as from a converted P- to S-wave reflection, referred to as P$_{melt}$S. Based on the inability of molten magma to withstand shear stresses, this later signal provides insight into where the axial magma chamber is nearly pure melt versus a more mushy mixture of melt and crystals. In the Lau Basin, the P$_{melt}$S signal has appeared at several locations along the Central Lau Spreading Center and southern Valu Fa Ridge. Correlating these sites of pure melt with the locations of hydrothermal vents and recent magmatic eruptions offer additional clues into the processes driving oceanic spreading, particularly in a back-arc environment.
T31A-0475
Spreading evolution in the southern Lau Basin based on geophysical surveys
Lau Basin and Havre Trough are back-arc basins related to Pacific-Australian plate convergence. Seafloor spreading occurs in the Lau Basin and Havre Trough is in a rifting stage. At present, the spreading propagator has a tectonic tip at the southern end of the Valu Fa Ridge(VFR)($22°40S). This tip has advanced at the rate of 120mm/yr. South of the tip, well defined linear feature of VFR loses it_fs morphological identity and the topography changes into a disordered rough area. New bathymetry data revealed that this area can be divided into three zones across the basin. The westernmost zone has flat topography indicating sedimented basement. The central part lies in the extended area of the topographic high in the northern Havre Trough($23°30S~$24°30S). The easternmost zone is deeper compared to the west and has grabens divided by short narrow ridges. A deep graben of depth 3400m~3500m has been observed. This graben is located about 10km east from the extended area of VFR. The trend of this graben can be extended southward to another deep graben in the northern Havre Trough. Side-scan data obtained by SeaBeam2112 show intensity difference between the zones. Strong intensity comes from the eastern zone indicating active rifting. Within this active zone, strongest intensity comes from two segmented areas:1)VFR and it_fs southern extension, 2)along the graben 10km offset from VFR trend. This indicates segmented rifting occurring in the active zone. The rifting may connect the propagating spreading at VFR with the deep graben in the Havre Trough. Gravity anomaly and sea surface magnetic anomaly data are also presented. High Bouguer anomaly is observed around the eastern graben. This may be due to thinner crust. Variations in magnetic intensity will be presented to show whether basaltic intrusions can be suggested.
T31A-0476
High-Resolution Micro-Bathymetry Mapping in the Lau Basin: Examples From the Tui Malila and Mariner Vent Sites
High-resolution SM2000 (200 kHz) multibeam sonar data were collected at six vent areas on the Lau Basin spreading center in April 2005. Data were acquired during near-bottom surveys conducted with the ROV Jason II at altitudes ranging from 5 to 20 m. High altitude (20 m) bathymetric surveys were complemented by near-bottom visual surveys, which provided ground-truth observations of the seafloor. Combined with Doppler and Long Baseline (LBL) Navigation, these bathymetry data provide sub-meter resolution of seafloor features, and reveal individual vent structures, faults and fissures. We present bathymetry data from two sites located 22 km apart, which are geologically and biologically distinct and exhibit contrasts in venting styles and biota. The Mariner vent field contains massive vent structures, many of which are taller than 25 m, with active venting from their bases and sides. Fluids exit as vigorous, high-temperature (< $363°C) black smoker fluids through chalcopyrite-lined conduits, and as less focused flow from porous beehive structures. Inactive structures are friable and are composed of iron- and copper-oxides. There was little evidence of faulting or fracture at the vent field, but we note the presence of collapsed volcanic dome structures. The vent fauna at Mariner is very limited; only {\it Bythograeid} and {\it Galatheid} crabs, and one {\it Brisingid Seastar}, were found. Tui Malila, by contrast, is characterized by shorter and wider branched vent structures with coalesced spires, the tops of which were actively venting. There is extensive faulting and fracture at this site, as well as a number of large flanges and areas of diffuse flow. At Tui Malila fluids exit tall structures through chalcopyrite- and zinc-lined conduits (at temperatures < $312°C), from beneath flanges, and directly from andesite. Hydrothermal breccias are also present. Tui Malila hosts a more typical vent community, with greater abundances of both {\it Bythograeid} and {\it Galatheid} crabs, mostly within 4 m of the vent field. {\it Ifremeria Nautilei} and {\it Bathymodiolus Brevior} were prolific within 2 m of the vent field, and sponges and anemones abundant from 4 to 32 m away.
T31A-0477
Characterization of Six Vent Fields Within the Lau Basin
Six active vent fields on the Valu Fa Ridge and Eastern Lau Spreading Center were successfully characterized on R/V Melville cruise TUIM05MV. The Kilo Moana (20deg3.2'S), Tow Cam (20deg19'S), and ABE (20deg45.6-46'S) vent fields were first noted during two R/V Kilo Moana cruises (in April (F. Martinez, Chief Scientist) and Sept (C. Langmuir, Chief Scientist) 2004). A fourth vent field, Mariner at 22deg10.8'S, was located Sept 2004 during the Shinkai 6500 program (K. Takai, Chief Scientist) using data from previous cruises. The fifth vent field examined was the Vai Lili vent field at 22deg12.95'S (Fouquet et al., 1991, Nature 349). On the TUIM05MV cruise, a sixth large active vent field was found at 21deg59.4'S using Jason2 and data from multiple CTD tow-yos, following up on plume observations provided by the Martinez et al. cruise. Tasks conducted to characterize each vent field included 1) detailed SM2000 bathymetric surveys; 2) down-looking and/or forward-looking camera surveys to create photomosaics of megafauna distributions; 3) recovery of biological, rock (basalt and sulfide), and fluid samples, and microbiological sampling of sulfide and diffuse fluid samples; 4) MOCNESS tows to recover larval plankton from plumes; 5) CTD casts to collect plume particles. At the Kilo Moana, Tow Cam, ABE and Tui Malila vent fields, hydrothermal activity occurs in proximity to major faults. At Kilo Moana, active venting occurs in three areas from ~5m tall, branched structures as focused, high temperature flow (to 333C) through chalcopyrite (Cp)- and/or wurtzite (Zn)- lined conduits and as diffuse flow through beehive-type structures. At Tow Cam high temperature (to 330C) active venting occurs in two areas at the base of a western fault from Cp- and/or Zn-lined conduits; diffuse flow exits basalt east of the black smoker areas. Within the ABE vent field there are three large areas of active venting spaced 150 to 300 m apart along NNE trending faults and benches; fluid exits from Cp- and/or Zn - lined conduits (at temperatures to 309C), diffuse spires, and beneath flanges; low to moderate temperature diffuse flow is also observed. At Tui Malila fluids exit tall structures through Cp- and Zn-lined conduits (at temperatures to 312C), and from beneath flanges, and directly from andesite; hydrothermal breccias are also present. Within the Mariner and Vai Lili vent fields, hydrothermal activity is not related to faulting, but may be concurrent with magmatic processes. At Mariner, venting occurs from the base and sides of 10 to 25 m tall narrow structures. Fluids exit as vigorous, high temperature (to 363C) black smoker fluids through Cp-lined conduits and less focused flow from porous beehive structures. Inactive structures are friable, composed of Fe- and Cu-oxides. At Vai Lili, the highest temperature fluids observed (121C) exit from beneath marcasite-lined flanges, and 70C clear fluids exit from an Fe-oxide chimney. The geochemistry and mineralogy of deposits from these six vent fields, and their relation to vent chemistry and biology, will be presented.
T31A-0478
Aqueous Volatiles in Lau Basin Hydrothermal Fluids
The Lau Basin is a back-arc spreading center characterized by widespread hydrothermal activity. High and low temperature vent fluids were collected from six vent fields along the Eastern Lau Spreading Center (ELSC) and the Valu Fa Ridge (VFR) using isobaric gas-tight samplers during R/V Melville cruise TUIM05MV. Fluids were analyzed for the abundances of H2, H2S, CH$_{4}$, CO2, and CO to assess chemical environments inhabited by biological vent communities and constrain fluid-rock reactions and magmatic processes in subsurface environments. Maximum measured temperatures for focused venting in these areas varied from 309 to $363°C. Water depths decreased from ~2700m for sites sampled at the northern end of the ELSC to ~1725m for the southern most site sampled on the VFR. Endmember concentrations of dissolved H2 at the Kilo Moana, TowCam, and ABE vent fields on the ELSC varied from 0.054 to 0.498 mmol/l and showed a systematic interfield decrease from north to south along the ridge crest. A similar spatial trend was observed for endmember H2S concentrations that varied from 2.6 to 6.6 mmol/l. In contrast to H2 and H2S, aqueous CH$_{4}$ abundances that varied from 0.028 to 0.057 mmol/l increased from north to south. In general, fluids from the Tui Malila, Mariner, and Vai Lili vent fields on the VFR showed greater compositional variability than fluids venting along the ELSC and an absence of systematic along strike chemical trends. Endmember H2, H2S, and CH$_{4}$ abundances at VFR ranged from 0.0029 to 0.178 mmol/l, 0.010 to 9.6 mmol/l, and 0.0029 to 0.043 mmol/l, respectively. Endmember concentrations of dissolved CO at ELSC and VFR varied from 0.01 to 0.1 umol/l and showed systematic variations with dissolved H2 and CO2 abundances. Assessment of the CO, CO2, and H2 concentrations within a thermodynamic framework suggests that these species have attained equilibrium states at measured vent temperatures and pressures. The higher degree of compositional variability observed in vents fluids from VFR likely reflects the occurrence of three distinct styles of hydrothermal activity along this portion of the ridge-crest. While the Tui Malila field appears most similar to vent fields along the ELSC with fault-related hydrothermal activity, higher temperatures and abundances of magmatic CO2 at Mariner point to a significant role for ongoing magmatic processes in regulating vent fluid chemistry. In contrast, absence of focused flow, maximum temperatures of $121°C, and low volatile abundances at Vai Lili suggests that this location may be in the waning stages of hydrothermal activity.
T31A-0479
Vent Fluid Chemistry From Six Hydrothermal Fields Along the Eastern Lau Spreading Center From 20°03'S to 22°13'S.
As part of a series of R2K and NSF funded explorations of the Eastern Lau Spreading Center, we participated in a cruise between April and May of 2005 aboard the R/V Melville. Eighty-one fluid samples were collected from six hydrothermal fields that span from 20°03'S to 22°13'S (from north to south: Kilo Moana, TowCam, Abe, Tui Malia, Mariner, and Vai Lili) using the ROV Jason 2. Fluids samples were collected using both isobaric gas-tight and major samplers from focused and diffuse vents to determine vent fluid composition and establish the hydrogeologic and geochemical processes occurring at depth. We report here on dissolved ion concentrations from vent fluids. The maximum vent temperature and magnesium concentration from individual orifices ranged from 11 to 363 ° C and from 51.7 to 0.65mmol/kg, respectively. The Mariner site had the lowest pH of 2.5 and the greatest H2S concentration of approximately 9mmol/kg. All samples had alkalinities less than seawater. Chlorinities were both depleted and enriched relative to seawater (around 538 mmol/kg), ranging from 530 to 650mmol/kg. Fe endmember concentration is about 12mmol/kg at Mariner and 3mmol/kg at Kilo Moana, but less than 0.3mmol/kg for the other sites. Fe/Mn molar ratios range from 4.7 at Kilo Moana to 0.1 at Vai Lili. Systematic along strike variations were observed in the endmember abundance of some aqueous species. For example, values for Si and Li are greater to the north whereas concentrations of K are greater to the south. Compilations of endmember compositions for individual orifices and fields are being compiled for all major, minor and certain trace ions in seawater. Trends will be incorporated with geological and biological data to elucidate hydrothermal processes along this geologically diverse transect. We are particularly interested in assessing the response of fluid chemistry to variations in the composition and structure of the crust resulting from the increase influence of subduction related processes to the south.
T31A-0480
Understanding the Composition and Distribution of Megafauna at Diffuse Flow Sites in the East Lau Basin Back-Arc Spreading Center
Ridge 2000 research at the Lau Basin Integrated Studies Site spans a range of scales, from hundreds of kilometers to fractions of a centimeter. Here we report on an interdisciplinary study of the relations between megafaunal distribution and diffuse flow chemistry, conducted in an area of about 30 m2. This work was enabled by, and interpretation will be nested within, the results from the three cruises that preceded our field effort. In June 2005, the remotely operated vehicle (ROV) {\it Jason II} collected a series of geo-referenced, high-resolution images using the Insite Scorpio digital still camera at the ABE vent site ($176°11\'{ }29\'{ }\'{ }W, $20°45\'{ }47\'{ }\'{ }S) in the East Lau Basin back-arc spreading center. Twenty pictures were mosaicked together at sea, using a MatLab program (courtesy of Tim Shank, Dana Yoerger, Hanu Singh, and Vicki Ferrini of Woods Hole Oceanographic Institution) to create one seamless image of the biological community covering an area of approximately 30 m2. The mosaic was used to guide electrochemical and temperature scans during a subsequent dive to provide data on the concentrations of reactive sulfur species and oxygen, temperature, and pH at 60 different locations within the community. To allow quantification of the biomass within the imaged community and obtain data on smaller and/or cryptic fauna, five area-specific quantitative-samples of representative animal aggregations were collected from within the same area and analyzed. Separate layers for each dataset (imagery, chemistry, temperature, community composition, and biomass) are being created with GIS to visualize and analyze trends in each individual layer, such as the co-occurrence of different megafaunal species and changes in chemical speciation. Overlays and statistical analyses of these layers in GIS are used to elucidate correlations between datasets that will contribute to our ultimate goal of understanding the distribution patterns of the megafauna in this spatially and temporally complex hydrothermal vent environment.
T31A-0481
Bacterial and Archaeal Diversity From the Eastern Lau Spreading Center
Due to the diversity of venting styles, geological settings and variations in fluid geochemistry, the Valu Fa Ridge and Eastern Lau Spreading Center (ELSC) provide a unique opportunity to explore the effects geological and geochemical variables on patterns of microbial phylogenetic and metabolic diversity. High temperature sulfides, diffuse flow fluids and microbial mats were collected from six active vent fields on the Valu Fa Ridge and Eastern Lau Spreading Center during the R/V Melville cruise TUIM05MV. All samples were subsampled for molecular and microbial culturing purposes. The archaeal and bacterial 16S rRNA genes were amplified by PCR from a selection of samples. Additionally, the presence of Aquificales and an unidentified lineage, the DHVE archaeal group, was explored using PCR primers specific for these groups. A selection of DNAs were also screened for functional genes that are diagnostic for certain pathways, viz, aclB (reductive TCA cycle), mcrA (methanogenesis), nirS and nirK (nitrite reduction), amoA (ammonia oxidation). Culturing of thermophiles, both acidophiles and neutrophiles, was initiated. Over 20 hydrogen oxidizing (hydrogen and oxygen) or nitrate reducing (hydrogen and nitrate) chemolithoautotrophs were isolated as colonies and grow at 70 degrees C. All are related to {\it Persephonella hydrogenophila}, with the exception of 2 cultures that perhaps represent new species of {\it Hydrogenivirga} and {\it Aquifex}. Preliminary analysis of patterns of Aquificales diversity using both culturing and molecular approaches suggest that the distributions of this group alone are very different from that observed at other hydrothermal sites such as along the East Pacific Rise or Central Indian Ridge. As yet, the most commonly isolated Aquificales, {\it P. marina}, has not been detected in enrichment cultures from ELSC, and the diversity of Aquificales-related sequences is much greater than detected from sites along the EPR. It is therefore also likely, that patterns of archaeal and bacterial diversity will reflect the geochemical differences within ELSC and between EPR and ELSC.
T31A-0482
Geochemical and Morphological Characteristics of Metalliferous Deposits at the Endeavour Segment, Juan de Fuca Ridge
Metalliferous sediments of the Endeavour Segment, Juan de Fuca Ridge originating from fallout of particles from a hydrothermal plume and wasting of sulfide chimneys and their oxidation products, as well as deposits of primary Fe-Si-rich oxyhydroxides, were collected during Keck supported expeditions in 2003 and 2004. The analysis of sediment deposited within 35 m of an actively venting high-temperature chimney at the Main Endeavour Field and containing hydrothermal material derived from a plume fallout indicates that Fe, Cu and Zn settled as sulfides, constituting about 3%\ of the sediment, and as amorphous phases. The amorphous material is represented by Fe-Si-S phases and black aggregates of variable Fe-Mn-Si-S-P-Ca-Mg-Cu-Zn composition. The association of the amorphous material with biogenic debris and bacteria suggests that microorganisms have enhanced the scavenging of the metals and their settlement near the vents. Fine-grained sulfides, barite and other mineral particles may have been deposited near the vents as zooplankton fecal pellets. Barite and Fe-Mn oxyhydroxides are the hydrothermal phases in the sediments along 1.8 km of the axial valley. The sediments also contain Fe-rich montmorillonite of authigenic origin, illite, chlorite, quartz, feldspar and calcite. The study of a mound of old oxidized sulfides at the Main Endeavour Field shows that patchy accumulations of Fe oxyhydroxides are associated with increased concentrations of Si. It is suggested that the oxidation of sulfides was accompanied by precipitation of primary Fe-Si oxyhydroxides from diffuse low-temperature sources. At the Mothra Field, low-temperature solutions have deposited amorphous Fe-Si-Mn oxyhydroxides containing minor Ca, P, Mg, S, Zn and Cu as well as amorphous material of Fe-Mn-Si-S-P-Zn-Pb composition that are found as an external crust on the wall of an extinct chimney. In the oxyhydroxide deposits, the amorphous material is associated with a variety of mineralized microorganisms, which served as a substrate for its chemical precipitation or in some cases may have mediated the mineralization process. A diversity of unique filamentous, rod-shaped and coccoid microbes was observed in the oxyhydroxides at Mothra.
T31A-0483
Endeavour basalt geology and petrology
We report major and trace element and isotope data from 250 basalt samples recently collected by submersible from the axial valley and flanks of the Endeavour segment of the Juan de Fuca Ridge. Off-axis volcanism is abundant on both flanks which are mirror images of one another geologically. Axial valley walls up to 1 km off axis appear to be steps of in tact but variably fractured sheet, lobate, and hackly lava flows similar to the youngest lavas seen in collapse features in the axis. Coverage by pillow terrane increases with distance off axis and coverage becomes complete after 1 km. The similarity of the two flanks suggests that the currently asymmetric axial magma chamber (van Ark et al., 2004) may be shorter-lived than the off-axis volcanism. MgO contents range from 6.0-8.5% and generally are lower on the flanks consistent with consistently cooler chamber edges there. La/Yb ratios vary 3-fold within 100 m in the axial valley, with normalized La/Sm = 0.8-2.5 in contrast to constant Sr and Nd isotopes. However, Th/U and 230Th/232Th ratios vary only slightly in the axial valley, which may enable dating of off-axis samples. H2O/Ce is less than 170, typical of values throughout much of the Pacific. Variations in depth and degree of melting, and in source composition, are implied. At times, these heterogeneities escaped homogenization in axial magma chambers. Cl concentrations and Cl/K ratios are surprisingly low considering the active hydrothermal systems in close proximity and the potential for brine incorporation into the magma chamber.
T31A-0484
A particle sedimentation model of buoyant jets: observations of hydrothermal plumes
Existing transport and sediment deposition models rarely use measured (rather than predicted) flow velocity and particle concentration data. We are developing a sediment deposition model using observed velocities and concentrations in order to model the deposition and transport of hydrothermal plume particles and biologic material. Our prior work developing acoustic imaging techniques to measure the static and dynamic properties of seafloor hydrothermal plumes provides the necessary input to the preliminary model. The general approach is to assume exponential settling of particles from the margins of the plume. Exponential settling states that the rate of decrease in the mass concentration C of sediment suspended in a layer of thickness h is given by dm/dt = -vCA/h, where v is the terminal velocity, C is the mass concentration of particles of given diameter, A is the basal area of the volume through which the settling occurs, and h is the thickness of the layer. Assuming the total thickness h is the height of the plume, the vertically integrated sedimented mass becomes: $\int$$_{z}$dm/dz = $\int$$_{z}$(vCA/w)/$\int$$_{z}$(1/w), where $\int$$_{z}$(1/w) is the rise time over the region of interest and w is the local vertical velocity in a volume element. In this study, we calculate the terminal velocity (based on the estimated Reynolds number) and net force on the particles of a given size everywhere within the plume and its surroundings based on local velocity in gridded volume elements. In each volume element where the net force is negative (positive is upwards), the loss of sediment from the volume element is calculated. The loss of sediment is then summed vertically to determine the sediment mass deposited on the seafloor. The results for five particle sizes were then combined using three different assumed size distributions. This methodology is applied both to acoustic observations and to the output of an integral (time averaged) model of plume dynamics. Preliminary tests, using the predicted velocity and concentration from a integral model, produced the expected sedimentation maps, where larger grain sizes fall out near the vent, mean grain size decreases away from the vent and transport into the upper plume layers is greatest for the smallest particles. We also applied the sedimentation model to acoustic observations of hydrothermal plumes at Grotto Vent, on the Endeavour Segment of the Juan de Fuca Ridge. Acoustic backscatter data was used to infer particle concentration (as a function of assumed particle size); Doppler shifts in the same acoustic backscatter data were used to infer vertical velocity. The sedimentation model results showed decreasing mean particle size away from the vent. The mass sedimentation patterns are sensitive to the total mass available, which, as inferred from the volume backscattering strength, is also a function of particle size. Bending of the plume in local currents shifts the locus of sedimentation away from the vent, especially for smaller particle sizes. Future efforts will refine the basic settling model and incorporate predictions of the transport of biological material.
T31A-0485
Microearthquakes beneath the Hydrothermal Vent Fields on the Endeavour Segment of the Juan de Fuca Ridge: Results from the Keck Seismic/Hydrothermal Observatory
The W.M. Keck Foundation is supporting the operation of a small seismic network in the vicinity of the hydrothermal vent fields on the central portion of the Endeavour Segment of the Juan de Fuca Ridge. This is part of a program to conduct prototype seafloor observatory experiments to monitor the relationships between episodic deformation, fluid venting and microbial productivity at oceanic plate boundaries. The Endeavour seismic network was installed in the summer of 2003 and comprises seven GEOSense three-component short-period corehole seismometers and one buried Guralp CMG-1T broadband seismometer. A preliminary analysis of the first year of data was undertaken as part of an undergraduate research apprenticeship class taught at the University of Washington's Friday Harbor Laboratories and additional analysis has since been completed by two of the apprentices and by two IRIS undergraduate interns. Over 12,000 earthquakes were located along the ridge-axis of the Endeavour, of which ~3,000 occur within or near the network and appear to be associated with the hydrothermal systems. The levels of seismicity are strongly correlated with the intensity of venting with particularly high rates of seismicity beneath the Main and High Rise Fields and substantially lower rates to the north beneath the relatively inactive Salty Dawg and Sasquatch fields. We have used both HYPOINVERSE and a grid search algorithm to investigate the distribution of focal depths assuming a variety of one-dimensional velocity models. The preliminary results show that the majority of earthquakes occur within a narrow depth range and may represent an intense zone of seismicity within a reaction overlying the axial magma chamber at ~2.5 km depth. However, the mean focal depth is strongly dependent on the relative weights assigned to the S arrivals. We infer from the inspection of residuals that no combination of the P- and S-wave velocity models we have so far investigated are fully consistent with the arrival times. We have also determined focal mechanisms from first motions for well-recorded events within the array as well as their dependence on the uncertainty in earthquake location and depth. The preliminary results for 134 better-constrained focal mechanisms show a wide range of mechanisms and strikes.
T31A-0486
The Acoustic Signature of High-Temperature Deep Sea Hydrothermal Vents
Motivated by a desire to find new measurements that might be sensitive to flow rate variations within mid-ocean ridge hydrothermal systems, we have conducted field studies to collect passive acoustic measurements at black smoker hydrothermal vents using two versions of a simple dual-hydrophone recording device capable of collecting continuous acoustic data for about one week at sampling rates of 1000--2000 Hz. We deployed the first-generation instrument on the Sully sulfide structure in the Main Endeavour Field of the Juan de Fuca Ridge during September of 2004. We were able to collect approximately 48 hours of data before the instrument was partially destroyed by venting fluid. We are in the process of obtaining additional measurements in the same vent field with a second-generation instrument. For the 2004 deployment, the venting fluid produced an acoustic signal that was far above the background level at all measured frequencies. The acoustic spectrum contains a broadband signal that is weighted toward the low frequencies and extends to the Nyquist frequency at 500 Hz. The spectrum also contains several sharp peaks below 150 Hz. The signal is variable in time, with the broadband and peak amplitudes fluctuating by ~20 dB, and the frequencies of the sharp spectral peaks fluctuating by ~1--3 Hz. The complex nature of the acoustic signal suggests that more than one sound production mechanism is operating within the vent. The sharp peaks suggest the presence of a resonant mechanism such as pipe resonance excited by turbulent flow. The high level of the broadband signal is not predicted by theoretical investigations of low Mach number jet acoustics. It is likely that another broadband sound source is present, which could be related to phase separation or to the mixing of different density fluids. More observations will be required to fully understand the basic mechanisms of sound production within black smoker chimneys.
T31A-0487
Evolution of the Mothra Hydrothermal Field, Endeavour Segment of the Juan de Fuca Ridge
The Mothra Hydrothermal Field (MHF) is a 600 m long, high-temperature hydrothermal field. It is located 2.7 km south of the Main Endeavour Field at the southern end of the central Endeavour Segment. Mothra is the most areally extensive field along the Endeavour Segment, composed of six active sulfide clusters that are 40-200 m apart. Each cluster contains rare black smokers (venting up to $319°C), numerous diffusely venting chimneys, and abundant extinct chimneys and sulfide talus. From north to south, these clusters include Cauldron, Twin Peaks, Faulty Towers, Crab Basin, Cuchalainn, and Stonehenge. As part of the Endeavour Integrated Study Site (ISS), the MHF is a site of intensive interdisciplinary studies focused on linkages among geology, geochemistry, fluid chemistry, seismology, and microbiology. Axial valley geology at MHF is structurally complex, consisting of lightly fissured flows that abut the walls and surround a core of extensively fissured, collapsed terrain. Fissure abundance and distribution indicates that tectonism has been the dominant process controlling growth of the axial graben. Past magmatic activity is shown by the 200 m long chain of collapse basins between Crab Basin and Stonehenge, which may have held at least ~7500 m3 of lava. Assuming a flow thickness of 0.5 m, this amount of lava could cover over half the valley floor during a single volcanic event. At a local scale, MHF clusters vary in size, activity, and underlying geology. They range in size from 400-1600 m2 and consist of isolated chimneys and/or coalesced cockscomb arrays atop ramps of sulfide talus. In the northern part of the field, Cauldron, Twin Peaks, Faulty Towers, and Crab Basin are located near the western valley wall, bounded by basalt talus and a combination of collapsed sheet flows, intermixed lobate and sulfide, disrupted terrain, and isolated pillow ridges. The southern clusters, Cuchalainn and Stonehenge, are associated with collapse basins in the central valley and bounded by extensive lobate flows and disrupted terrain. At all clusters, active chimneys stand within meters of extinct chimneys, suggesting that flow in the shallow subsurface is both complex and transient. 1-2 m high mounds of sulfide talus and broken chimneys indicate that focused flow has been concentrated at the clusters for long periods, while extinct sulfide deposits between clusters and in collapse basins demonstrate that flow conduits have been rerouted and/or clogged by mineral precipitation. Two subsurface processes are responsible for hydrothermal venting at the clusters: tapping of magmatic heat near the lava drainbacks and tectonic movement along the steeply dipping, inward-facing normal faults at the western wall boundary. The interplay between these processes and fluid flow is synthesized in an evolutionary model of hydrothermal development at Mothra.
T31A-0488
In-situ Chemistry of Hydrothermal Fluids from Black Smokers in Main Endeavour Field, Juan de Fuca Ridge
After an off-axis earthquake swarm in 1999, dramatic changes were observed in vent fluids of Main Endeavour Field, Juan de Fuca Ridge. Three month latter, we also recorded this sudden variation using a high temperature in-situ chemical sensor. The results at that time indicated some of the vent temperatures as high as $374°C. This change was also characterized by relatively high in-situ pH, high dissolved H2, and H2S concentrations in the fluids that were in excess of 5, 0.7 mmol/kg and 20 mmol/kg respectively. In order to further track time dependent changes over the past 6 years, we revisited Main Endeavour Field during the recent AT 11-31 cruise in Aug.~Sept. 2005. The high temperature chemical sensor was again used on selected dives with DSV Alvin to conduct in-situ measurements of pH, dissolved H2 and H2S concentrations along with temperatures. The data were obtained in a real time mode of 3 seconds per-reading from a series of measurements at high temperature conditions in the depth of 2200 m. Conventional gas-tight samples were also collected for verification and further study. In this study, Puffer, Sully and Bastille black smoker vent sites were specifically investigated owing to the high fluid temperatures that characterize these vents in comparison with other vents in the area. The measured temperatures for these vents were $362°C, $358°C, and $361°C respectively, which were generally about 20~$30°C higher than the others currently in the area, but approximately $10°C lower than the highest temperatures measured in the aftermath of the 1999 seismic-magmatic event. Although the drops in vent temperatures were not substantial, the measured in-situ chemistry showed large departures from previous reported data. The in-situ pH values in these vents ranged from 4.43 to 4.89, in comparison with values above 5 in 1999. This difference may be linked directly to the decrease in temperature. The measured in-situ dissolved H2 and H2S concentrations were 0.12~0.18 mmol/kg and 7.96~8.90 mmol/kg, respectively. These concentrations clearly demonstrate substantially lower values than reported in the past, which cannot be simply the result of the observed decrease in temperature of the MEF vent fluids. These data reveal a strong departure from the pyrite-pyrrhotite-magnetite equilibrium state as observed in 1999 to more oxidizing conditions, consistent with the assemblage of anhydrite-anorthite-clinozoisite. Compared to dissolved H2, the decrease in H2S concentration is less obvious, but still significant in comparison with previous data. From these in-situ measurements, we suggest that the MEF vent systems have not yet fully returned to a pre-seismic condition. The comparatively high temperature and high dissolved H2S may be the cause for the current dense faunal communities observed most recently at the vent sites.
T31A-0489
Heat Flux From the Endeavour Segment of the Juan de Fuca Ridge
The very essence of a hydrothermal system is transfer of heat by a convecting fluid, yet the flux of heat remains a poorly known quantity. Past studies of heat flux consisted primarily of point measurements of temperature and fluid flow at individual vent sites and inventories of the neutrally buoyant plume above the field. In 2000 the Flow Mow project used the Autonomous Benthic Explorer (ABE) to determine heat flux from Main Endeavour Field (MEF) on the Juan de Fuca Ridge by intersecting the stems of rising buoyant plumes. ABE carries instruments to measure conductivity, temperature and depth, and a MAVS current meter to determine the vertical velocity of the fluid, after correcting for vehicle motion. Complementary work on horizontal fluxes suggests that the vertical flux measured by ABE includes both the primary high buoyancy focused "smoker" sources and also entrained diffuse flow. In 2004, ABE was again used to determine heat flux not only from MEF, but also from the other four fields in the Endeavour Segment RIDGE 2000 Integrated Study Site. In this four year interval the flux of heat from MEF has declined by approximately a factor of two. The High Rise vent field has the greatest heat flux, followed by MEF, then Mothra, Salty Dawg and Sasquatch (of order 500, 300, 100, 50 MW respectively; heat flux at Sasquatch was below detection).
T31A-0490
Repeat Hydrography at the Endeavour Integrated Study Site, 2004 - 2005
Significant differences exist between hydrographic transects made in 2004 and 2005 at the Endeavour Segment Integrated Study Site on the Juan de Fuca Ridge. Sections that describe the conditions above the segment utilize twenty-one nearly uniformly spaced hydrographic stations from south of Mothra to north of the Sasquatch hydrothermal vent fields. Criteria used in choosing station locations included depth, ~500 m spacing from other stations, and being centrally located in the valley. The resulting sections allow for rapid evaluation of the characteristics of the neutrally buoyant plume over each of the vent fields. Preliminary results indicate heat content over the northern vent fields, Salty Dawg and Sasquatch, significantly increased between the summers of 2004 and 2005. In 2004, the plumes over these vent fields were barely discernable while in 2005 prominent plumes existed with potential temperature anomalies over $0.1°C. Vent data being obtained by other RIDGE 2000 and UW Keck investigators will help constrain the underlying causes of these changes. Isopycnals in the 2005 sections are also elevated along the entire length of the transect by approximately 50 m or more. The potential temperature anomaly section from 2005 is indicative of a thicker (about 75 m) neutrally buoyant plume and substantially more heat at the north end of the valley. In 2004, the shallowest plume depth was 1900 m contrasted with 1830 m in 2005.
T31A-0491
Detailed Studies of the Axial Summit Trough of the East Pacific Rise 9-10N
The axial summit trough is a nearly continuous feature along the East Pacific Rise from 9-10 deg. N. The AST marks the central spreading axis of the ridge, is the site of the bulk of the volcanic eruptions, and hosts the majority of hydrothermal vent sites. In the past, the AST has been interpreted to be a purely tectonic or purely volcanic feature. Detailed analysis of sidescan imagery and high-resolution microbathymetry are used to identify the morphology and structural variability of the axial summit trough (AST) along the EPR crest in the study area. We use these data to address questions related to the causes for variability in width and character of the AST and how those parameters relate to the morphology and volcanic history of adjacent seafloor, relationships between lava flows and faults and fissures developed within ~4 km of the AST, and the frequency and size of volcanic eruptions. With constraints from microbathymetry data and knowledge of the detailed volcanic contacts along the EPR 9-10N AST derived from Alvin and towed camera data, we explore models of AST formation in an effort to better understand relationships between dike intrusion and how extensional strain is accommodated across the EPR axis. Assuming purely tectonic origins for the AST, 3D boundary element modeling suggests that dike widths and the levels to which dikes rise beneath the EPR axis are variable along-axis. For well-mapped portions of the AST we are able to infer the number of dikes needed to produce its current geometry, and as a result, the timescale of formation. Where the ridge is most volcanically active, we find that modeled dike widths and depths are unreasonable, and suggest that volcanic overprinting has modified the character of the AST. We conclude that both volcanic and tectonic processes are responsible for the formation of the AST and that the balance between these parameters controls the physical characteristics of the AST observed on the seafloor today.
T31A-0492
The Relationship of Changes in the 9 50'N EPR Hydrothermal System to Observed Seismicity: 2002-2004
There have been profound changes in the chemistry and temperature of hydrothermal vent fluids sampled in the $9°50'N area of the East Pacific Rise, one of the RIDGE2000 Integrated Study Sites. While hydrothermal fluids have been frequently sampled from this site since 1991, the deployment in September 2003 of an array to detect seismicity provides the opportunity to more quantitatively tie temperature, chemistry, and seismicity together than the previous, and very brief, seismometer deployments here in 1991 and 1995. Seismic swarms were noted in the $9°50'N area beginning on 11/19/2003 [Tolstoy et al. this session], five days after we sampled these fluids. Fluid samples were again collected in March 2004. We also deployed recording temperature probes in all of the high temperature vents at this site in early November 2003. The only vent fluids to show large chemical changes in the November 2003-March 2004 period were from M vent, where the iron concentrations dropped by one-third over this time period. The drop in iron is likely related to the (smaller) increase in the concentration of hydrogen sulfide in these same vent fluids. Fe concentrations are likely controlled by the solubility of Fe-S minerals such as pyrite (FeS2), which is dependent on the activity of sulfide squared. Hence relatively small changes in hydrogen sulfide can result in much larger changes in iron concentrations. While no significant temperature changes were noted in the vent fluids from the point measurements in November 2003 vs. March 2004, records from the recording temperature probes show an ~$7°C temperature rise in the Bio9" fluids in late November 2003. The M vent fluids show an ~$4°C temperature decrease lasting less than a day in March 2004, which occurs within 24 hours of another earthquake swarm. In most cases the changes in the fluid chemistry from November 2003-March 2004 are significantly smaller than those observed between February 2002 and November 2003. Correlations between variations (or lack thereof) in the fluid compositions and temperatures will be discussed relative to the seismic record.
T31A-0493
Emplacement of Oceanic Crust Can Continue for Several Hundred Thousand Years
We report on the seismic imaging of magma-filled sills located ~20 km off axis near the East Pacific Rise (EPR) between the Siqueiros and Clipperton fracture zones. The seismic data were collected during the UNDERSHOOT experiment, which was designed to test competing magma supply models for the EPR by mapping the pattern of magma delivery from the mantle to the crust along the entire length of a transform-bounded ridge segment. Previously reported delay-time tomographic images reveal a mantle low-velocity zone (MLVZ) beneath the EPR that defines both on- and off-axis centers of magma accumulation spaced at along-axis intervals of 15 to 25 km. The tomographic results can be explained by a 1-3% change in melt fraction; however, melt concentrations could be greater if distributed anisotropically. To find evidence of off-axis magma bodies, we analyze seismic waveform data recorded on 12 three-component ocean-bottom seismometers (OBSs) located 10 and 20 km off axis. We search data for evidence of P-wave shadow zones and anomalous P-to-S conversions consistent with magma-filled sills. In a region where the MLVZ is located 10-15 km east of the rise, we find compelling evidence for anomalously low crustal and mantle velocities. Data from an OBS located 20 km east of the rise reveal an amplified secondary phase on the radial channel consistent with a solid-melt interface. Additionally, a crustal-level P-wave shadow zone is detected near the off-axis MLVZ. We find no evidence for off-axis low-velocity anomalies in regions where the MLVZ is located beneath the rise. To better map the location and physical properties of the anomalous features, we are using a combination of travel time and full waveform modeling. Our preliminary interpretation is that in regions where the MLVZ is located off axis, magmatic processes modify both the crust-mantle transition zone and the lower crust. An unusual aspect of this discovery is that there are no seamounts nor any other bathymetric features in the region that would indicate the presence of accumulated melt below the seafloor. We conclude that while the majority of oceanic crust is emplaced at the rise axis, the process of crustal formation can continue for at least a few hundred thousand years, particularly when mantle upwelling is not centered beneath the rise.
T31A-0494
Upper Crustal Dynamics of the 9N OSC at the East Pacific Rise: Linking Surficial and Melt Sill Structures.
The Overlapping Spreading Center (OSC) at $9°03'N on the EPR is a non-transform discontinuity of the ridge axis which has been migrating southward with an average rate of 42 km/Ma since 2 Ma. The present OSC has been widely studied since the 80's, however, the magma supply and the mechanisms of migration of the OSC are still debated. The 1997 ARAD seismic survey allowed the mapping of the magma lenses in 3D beneath the OSC. Multibeam (Hydrosweep) bathymetry data was also acquired during the survey. Here, we present results from the joint analysis of seismic reflection data and geomorphologic data, with a particular emphasis on the relationship between magma chamber and seafloor volcanic and tectonic features. We provide a new structural map of the OSC using bathymetry obtained from multibeam and seismic reflection data, along with existing surface and deep towed side-scan sonar data. Seismic reflection data has higher resolution inline (25 m) than multibeam bathymetry (50m) that allows a better identification of the surface volcanic and tectonic structures. Our results show a misalignment between the melt lens and surface features orientation at the propagating tip, which is interpreted as a decoupling between the ductile magma chamber and the brittle upper crust. To explain the anomalous width of the melt lens and its offset from the bathymetric ridge, we propose a new model of magma supply where the locus of the neovolcanic zone is set by lithospheric constraints, and not by the deep supply. The structural characterization of the propagating and dying ridge tips gives new insights into the mechanisms of propagation and recession of the ridge segments. At the propagating tip, accretion occurs beyond the melt lens in the form of elongated and curved volcanic ridges. On the dying segment, the higher abundance of large scarps suggests reduced magmatism. The coexistence of structures of different orientations shows the different stress-field states experienced by the dying limb throughout the OSC migration.
T31A-0495
Increasing seismic activity at 9deg50'N on the East Pacific Rise RIDGE 2000 Integrated Studies Site from October 2003 through April 2004
Monitoring of micro-seismicity within the bull's-eye region of the R2K ISS at 9deg49'N - 9deg51'N on the East Pacific Rise has been ongoing since October 2003. Results from the first deployment (October 2003 - April 2004) will be presented with hypocenters determined using relative-relocation techniques. Analysis shows that there is a gradual and ongoing increase in the rate of activity over the 7 months of the deployment. Mean event rates increase from 31 events/day for the first quarter of the deployment period, to 55, 105, and 131 events per day for the 2nd, 3rd and 4th quarters, respectively. This gradual increase in activity suggests long-term changes in the magma body or changes in the hydrothermal cracking front. Preliminary analysis and event counts for the 2004-2005 deployment will be presented to assess whether or not the build up in activity seen in 2003-2004 continued. Numerous brief swarms are observed throughout the deployment and their locations will be studied relative to temporal changes in the vent temperature monitoring as well as variations in the fluid chemistry (see Von Damm et al., same session). Early analysis suggests two dominant areas of recurrent activity, between M-vent and Bio-9 and between Bio-9 and Tube-worm pillar. The exceptionally well-characterized and monitored seafloor at this site allows for unprecedented correlation of observed seismic activity with local biology, geology, geochemical and hydrothermal monitoring. As results from different monitoring activities continue to come in, a detailed understanding of the linkages should emerge.
T31A-0496
Discriminating Lava Flows From the EPR ISS (9 25'-9 55' N) Using Lava Morphology, Petrography, and Geochemistry
The neovolcanic zone of the East Pacific Rise (EPR) from $9°25' to $9°55' N is dominantly comprised of overlapping lava lobes, 0.25-1 km in length and 1-2 km in width, that emanate from the axial summit trough (AST) and extend up to a few km down the ridge flanks. This volcanic terrain is interspersed with sinuous to linear channel systems that are oriented perpendicular to the AST and serve to distribute lava off-axis, as well as elongate ridge-parallel pillow mounds that may have erupted from off-axis fissures at the edge of the neovolcanic zone. In order to better understand the distribution and geochemical heterogeneity of lavas that comprise the neovolcanic zone of the EPR and to re-evaluate models of upper oceanic crust construction at fast-spreading mid-ocean ridges, we have evaluated the degree of geochemical heterogeneity, petrographic diversity, and morphological variation within carefully mapped and sampled lava flow units. In addition to steep pillow flow fronts and sheet and lobate flow tops that comprise the flow units and form the overlapping terrain near the AST, ALVIN also collected samples from distributary lava channels and pillow mounds erupted off-axis. Lavas from a series of flow-fronts on either side of AST at $9°50' N and two channels at ~$9°29' N are mainly aphyric to sparsely plagioclase phyric (< 2%) basalts with normal incompatible element depletions (N-MORB). In contrast the more evolved (lower MgO) lavas from off-axis mounds exhibit greater depletion of the most incompatible elements and contain as much as 10% microphenocrysts suggesting origins distinct from the flows emanating from the axis. Overlapping flow units extending from the AST each exhibit a distinct trace element signature. Pillow basalts comprising the flow fronts of these eruptive units are consistently more evolved (0.2-0.77 lower wt.% MgO) than their associated lobate and sheet flows in the body of the flow and have slightly higher concentrations of incompatible elements. The quantities of microphenocrysts in pillow lavas at the flow front are ~1.0-2.4 vol.% greater than those in the body of the flow. Fractional crystallization models at low pressures (0.025 kbar) predict 4-14 vol.% crystallization of parental magmas are required to produce the observed changes in chemistry within individual flow units; at odds with the measured crystallinity variation. Furthermore, some mixing between primitive and evolved melts is required to explain all of the variations in major element and trace element characteristics of the flow units. The results of our study suggest that the majority of chemical imprinting of lavas precedes eruption onto the seafloor and that little variation occurs once magmas have been erupted.