B23F-01 INVITED
Volatile Behavior at the 9° 50'N Hydrothermal Field through a Full Volcanic Cycle
The Ridge2000 Integrated Study Site (ISS) on the East Pacific Rise (EPR), 9-10° north latitude, was impacted by several volcanic eruptions in the months preceding April 2006. In June 2006 during the RESET06 cruise we used the DSV Alvin to collect gas-tight samples from the hydrothermal system at this site. During the preceding decade, "M" and "Q" Vents in the northern part (9°50.7'N) of the area had been the most gassy vents in the 9N system, continuously venting over 100 mmol/kg CO2. In June 2006, "M Vent" was inactive and the "Q Vent" CO2 concentration was decreased by almost half compared to recent years. The CO2 concentration at "P Vent" (9°50.3'N) increased by about a factor of one-third following the new eruptive activity. Methane concentrations were not remarkably different than prior to the eruption but H2 concentrations increased by over an order of magnitude at all the vents sampled, with the exception of "Q Vent" which remained unchanged. In Dec 2006, "Q Vent" was found to be inactive but the CO2 concentrations in the "P Vent" area remained above 100 mmol/kg while H2 concentrations had fallen dramatically from those seen the previous June. In Dec 2007, both "M" and "Q" Vents were still inactive but, interestingly, Biovent (north of "M" and "Q") was still venting CO2 concentrations in excess of 100 mmol/kg. Vents in the Bio 9 and "P" area were also still venting CO2 above 100 mmol/kg but H2 concentrations had again dropped significantly from the previous year. Based on CO2 concentrations, it appears that one effect of the most recent eruption has been to shift the major residual magma lens south from the "M/Q" area to the Bio 9/"P" region.
B23F-02
Constraints on 226Ra-210Pb Decoupling in Fast-Spreading Mid-Ocean Ridge Magma Systems from Hydrothermal Vent Fluid Compositions, 9°46'-9°52'N East Pacific Rise
Though mid-ocean ridge magma systems are responsible for producing two-thirds of Earth's surface, the time scales associated with magma migration into and through these systems remain enigmatic. A recent study by Rubin et al.(1) made progress on this front using U-series techniques on mid-ocean ridge basalt (MORB) erupted in 1991 from the EPR 9°50'N. Their results suggest a 210Pb-226Ra- 230Th disequilibrium that, in a system closed since melting, would have taken only sixty-six years to form, significantly faster than most geophysical models have proposed. Hydrothermal vent fluid chemistry provides an ideal means of evaluating the relative significance of one important crustal process, radon degassing, which could potentially impact these U-series signatures. High levels of CO2 and 3He/heat in vent fluids collected in November 2003 and December 2004 from the 9°46'-9°52'N vent field area may be associated with shallow level magmatic degassing (2). If degassing occurs at shallow levels, Rn may partition into the convecting seawater, producing an in growth of 210Pb that is not supported by 226Ra decay. Excess 210Pb from vent fluids associated with the high CO2 would therefore be indicative of 222Rn fractionation from a shallow degassing magma source. Since the Pb content of vent fluids varies as a function of chlorinity, temperature and other parameters, the 210Pb data must be normalized to the concentrations of stable Pb in the fluids(3). During the last decade, regular sampling at the Ridge2000 Integrated Study Site (ISS) has provided a spatially diverse range of samples to characterize the evolution of vent fluid chemistry throughout the eruption cycle (1991-2006). This unique dataset includes samples collected before and after the 2005 eruption and represents the full range of CO2 at this site. Vent sites located at the maximum of the CO2 degassing signal (9°50.3'-50.8'N) range in 210Pb/Pb ratios of 1.15 to 5.68 dpm/μg. Those south of the degassing maximum (9°46.3'-47.2'N) are used to establish a non-degassing 210Pb/Pb range which ranges from 0.22 to 0.54 dpm/μg. Preliminary results show a positive trend between 210Pb/Pb ratios with end member CO2 concentrations, thereby implying that Rn degassing does occur at shallow levels and must be taken into consideration in melt ascent models based on U-series. (1) Rubin et al., Nature, 2005. (2) Lilley et al., Nature, 2003. (3) Von Damm, Annu. Rev. Earth Planet Sci., 1990.
B23F-03
Characteristics of the crustal magma body in the 2005-06 eruption area at 9°50'N on the East Pacific Rise from a 3D multi-channel seismic investigation
In July-August 2008 the first 3D academic multi-streamer seismic survey was carried out aboard R/V Langseth, focusing on the 9°50'N Integrated Study Site (ISS) at the East Pacific Rise. Preliminary results from 2D processing of along-axis and across-axis grid lines provide insight into the architecture of the magmatic system at the ISS and its relationship with hydrothermal activity and volcanic/dyking events associated with the 2005-06 eruption. Segmentation at length scales of about 5-10km of the axial magmatic system of the 9°50'N area can be mapped based on variations in two-way travel time, reflection strength and along-axis dip of the axial magma chamber (AMC) reflector on the stack sections, as well as the presence of edge diffractions. The main hydrothermal vents around 9°50'N are located over the region of shallowest AMC, which can be divided into two sub-regions, between 9°45.2'N - 9°48.7'N and 9°48.7'N - 9°51.9'N, respectively, separated by a discontinuity that has been interpreted as a potential hydrothermal downflow zone based on microseismicity studies. The northern lens event is rather flat-lying, while the southern one is more complex, dips south and shows pronounced edge diffractions. Both of these shallow lenses display distinctly weaker amplitudes than AMC reflections located immediately to the north and south. Lava flows from the most recent eruption overlie these two 'dim' lenses, and also extend over part of the north-dipping, brighter AMC to the north. Though detailed quantification is required, such along-axis variations in reflection strength are suggestive of a lower present-day melt percentage between 9°45.2'N - 9°51.9'N, an observation consistent with melt being drained by the 1991 and 2005-06 eruptions.
B23F-04
Integrated Temporal and Spatial Studies of Biological Community Structure, Fluid Geochemistry, and Geological Perturbations on the East Pacific Rise
Over the past 19 years, temporal and spatial changes in vent faunal community structure, fluid chemistry, and geological features have been tracked on the East Pacific Rise. Specifically, co-located studies of temporal and spatial changes within the Integrated Study Site region of the EPR have included the: 1) photographic documentation of more than 30 diffuse-flow habitats; 2) characterization of in situ fluid chemistry, temperature, and pH in these habitats; 3) characterization of basalts influenced by venting to enumerate microbial, meiofaunal, and metazoan colonists; 4) characterization of low-temperature hydrothermal fluids for microbial and chemical analyses; and 5) characterization of colonization substrates along with time-lapse camera systems and autonomous chemical and temperature sensors; and 6) characterization of seismic activity spatially targeted at these experimental locations. The goal of this work has been to address first order questions and the existence of correlative patterns among microbial communities, metazoan colonization, and fluid chemical composition- an essential prerequisite for further detailed studies of biological interactions and community dynamics at hydrothermal vents. These co-located activities, which now span a volcanic eruption, have yielded individual and interdependent datasets, which are currently at different stages of analyses and synthesis toward the examination of the interactions of subseafloor perturbations, fluid chemistry and microbes on faunal colonization. Ongoing analytical activities for integration and modeling include: 1) characterizing the composition of microbial colonizers; 2) assigning physiological and functional attributes of microbial colonizers; 3) comparing microbial community composition and physiology with patterns of metazoan colonization; 4) correlating relationships between microbial and metazoan colonization to relationships between fluid chemistry, pH, temperature; 5) relating results of strategic sampling to the observed changes in hydrothermal fluid flux and biological community structure (e.g., succession) at pre- and post-eruptive hydrothermal vents. Our integrative goals include providing input and continuity to long-term, time-series data sets to yield models and insights into temporal and spatial scales over which ecological processes occur on fast-spreading mid-ocean ridges.
B23F-05
Time-Series Chemical and Temperature Habitat Characterization of Diffuse-Flow Hydrothermal Sites at 9°50'N East Pacific Rise Using an Autonomous Analyzer
Sulfide, oxygen and temperature were monitored during 6 deployments of an in-situ electrochemical analyzer between December 2003 and January 2007 in diffuse-flow sites at the 9°50'N East Pacific Rise hydrothermal vent field for up to two weeks at a time. Sensors were placed near the mussel Bathymodiolus thermophilus, and the tubeworms Riftia pachyptila and Tevnia jerichonana to monitor the physical and chemical conditions in which these organisms reside. Our results show that these organisms live in a highly dynamic environment that is changing on time-scales of seconds to days. Variance analysis reveal that a diffuse-flow site needs to be observed for at least one tidal cycle to monitor the full range of conditions an organism living there experiences. The sulfide to temperature relationship at a given site varied over short time scales (hours to days); however, the variance remained centered on the mean. Thus, temperature can be used as a proxy for chemistry at diffuse-flow sites, as long as the site is well characterized. Data were also used in conjunction with previous lab based studies of metabolite uptake to calculate the rates of sulfide and oxygen uptake near mussels and tubeworms. The results suggest Bathymodiolus favored high O2, low H2S uptake environments, Tevnia favored low O2, high H2S uptake environments, and Riftia are found in between.
B23F-06
Two-Dimensional Models of Poroelastically-Controlled Earthquake Triggering at the East Pacific Rise
Recent analyses of microearthquakes at the East Pacific Rise (EPR) Integrated Study Site suggest that
earthquake triggering along this section of the mid-ocean ridge is partially controlled by poroelastic pressure
perturbations generated by earth tides and ocean tidal loading. Microearthquakes within the hydrothermal
system are preferentially triggered at different phases relative to the tidal potential, and the spatial pattern of
triggering indicates that waves of fluid pressure travel through the hydrothermal system on tidal timescales.
In order to explore this hypothesis we have developed a series of two-dimensional numerical models of young
upper oceanic crust that incorporate the effects of poroelasticity, ocean tidal loading and earth tides. We
use these models to determine the range of parameter distributions that can generate the inferred pressure
perturbations. Simple spatial distributions of reasonable permeability values can produce pore pressure
changes that are consistent with the observed seismicity. We are now working to determine if local variations
in the elastic or transport properties of the fluid or rock matrix can lead to similar pressure perturbations. We
are also working to establish effective Coulomb failure stress estimates for a range of fault orientations and
distributions within the model domain to further constrain the effects of tidally produced pressure changes on
earthquake triggering in these environments. When paired with models of poroelastic pressure perturbation,
high-density microearthquake datasets may provide significant insights into the hydrogeological structure of
these systems.
http://www.fluidcontinuity.org/
B23F-07 INVITED
Integrating Radiometric, Geophysical and Thermal Signals of Volcanic Unrest and Eruption in 2005-06 at 9 deg 50'N EPR
In 2005-06, volcanic eruptions at the East Pacific Rise near 9° 50'N produced extensive lava flows along 18 km of the ridge crest, impacting the local hydrothermal system and seafloor vent ecology. The timing and duration of the volcanic episode are constrained by ocean bottom seismograph data (from instruments deployed near the ridge axis between 9° 50'N and 51'N), regional, eastern equatorial Pacific hydro-acoustic data, a time series of in situ hydrothermal fluid temperature data, and high resolution 210Po-210Pb radiometric ages of newly erupted lavas. Each of these methods provide unambiguous signals of volcanic unrest in the same localized area of seafloor and over the same time frame, yet differ in detail as to when and where activity was most intense. We summarize these observations, and discuss how the important differences in spatial-temporal signals from these methods can be integrated to better understand the timing and dynamics of this volcanic episode. Lava ages indicate that the eruption initiated in late June/early July 2005 over most of the eruptive fissure system and culminated in Jan. 2006. ~80% of the lava volume is inferred to have erupted by Sept. 2005, based on geologic mapping and ages of samples collected from the lava flow field. By Oct. 2005 eruptive activity focused to one or two small fissure segments near 9° 50.4'N. Fluid temperatures measured at the M vent chimney (9° 50.6'N) were stable at ~360°C from Apr. 2004 to Aug. 2005, followed by several sharp ~50°C drops and rapid recoveries to ~360°C over the next few months, a subsequent nearly instantaneous fall to below detection limits (150°C) on 11 Jan. 2006, a sudden climb above ~360°C nearly two weeks later, and an increase to ~380°C on 1 Mar., after which recording ceased. Ocean bottom seismographs show a steady increase in seismicity over 2+ years leading to a period of intense tremor and seismic activity culminating on 22 Jan. 2006, followed by a rapid decline in seismicity rate. The seismicity peak has been interpreted as an eruption date, although high levels of seismic activity in the months preceding it may be associated with smaller, or more distant, volcanic events. The regional EPR hydrophone array detected 250 earthquakes at this site from 1 to 20 Jan. 2006, 250 more on 22 Jan., and essentially none in the 1 year prior. The detected hydroacoustic events cluster near the northern and southern extremes of the young lava flow field (at 9° 45'N and 9° 55'N) alternating from the north to the south and back north over a 1 hr. period.