HR: 15:10h
AN: B53C-07    [Abstracts]
TI: Methane concentrations and biogeochemical gradients within acoustic wipe-out zones at a Gulf of Mexico cold seep
AU: * Lapham, L
EM: lapham@ocean.fsu.edu
AF: University of North Carolina, Dept of Marine Sciences 340 Chapman Hall, CB#3300, Chapel Hill, NC 27599, United States
AU: Chanton, J
EM: jchanton@mailer.fsu.edu
AF: Florida State University, Dept of Oceanography 105 N. Woodward Ave. P.O. Box 3064320, Tallahassee, FL 32306,
AU: Martens, C
EM: cmartens@email.unc.edu
AF: University of North Carolina, Dept of Marine Sciences 340 Chapman Hall, CB#3300, Chapel Hill, NC 27599, United States
AU: Sleeper, K
EM: ksleeper@olemiss.edu
AF: University of Mississippi, Center for Marine Resources and Environmental Technology 220 Old Chemistry Bldg, University, MS 38677, United States
AU: Woolsey, J
EM: jrw@olemiss.edu
AF: University of Mississippi, Center for Marine Resources and Environmental Technology 220 Old Chemistry Bldg, University, MS 38677, United States
AB: The spatial distribution of methane concentrations and biogeochemical gradients were assessed within surficial sediments overlying acoustic wipe-out zones at a cold seep, Mississippi Canyon 118, Gulf of Mexico. We hypothesized that the wipe-outs were caused from saturated methane entrained within upward fluxing hydrocarbon-rich fluids and, since these fluids have been shown to stimulate sulfate reduction, anaerobic oxidation of methane, and methanogenesis, we also hypothesized that we would observe steep biogeochemical gradients, indicating high microbial activity, in sediments overlying these features. We tested these hypotheses by collecting thirty sediment gravity cores both within and outside the wipe-out zones and measuring resultant pore-fluids for dissolved methane and sulfate concentration gradients, methane and dissolved inorganic carbon isotope gradients, and organic matter chemical composition. Outside the wipe-outs, fifteen cores resulted in methane concentrations below 10 uM, limited down-core sulfate or methane concentration gradients, and down- core d13C-CH4 values averaging -52+/-2 ppt. While these background cores exhibited low microbial activity, the cores collected within the wipe-outs resulted in moderate to high activity. Moderate activity was exhibited in eight cores where methane concentrations reached 20 uM and had gentle sloping sulfate and methane concentration gradients. The d13C-CH4 values showed little change with depth and averaged -67+/-4 ppt while the d13C-DIC values decreased from -7ppt at the SWI to -32ppt at the bottom of the cores at a rate of 0.22ppt/cm. In stark contrast, high activity was seen in four cores collected within the wipe-outs. Methane concentrations reached above 4 mM, sulfate was depleted by ~50 cmbsf, and down-core profiles of d13C-CH4 and d13C-DIC were indicative of distinct depth zones of sulfate reduction coupled to anaerobic methane oxidation and methanogenesis. Bulk organic matter analysis suggested that the high activity cores were being supported by a source that is enriched in carbon (C:N=15) and depleted in d15N and d13C compared to other activity groups, possibly due to petroleum influx or chemosynthetic carbon. In the high activity cores, the isotopic values of the DIC were similar to the authigenic carbonates whereas in the moderate activity cores they were not, suggesting not only spatial but temporal variability in microbial processes. While the wipe-outs were correlated with saturated methane and were indicators of active microbial processes, the cause of the wipe-outs was not solely methane saturated pore-fluids. This novel data set was then compiled into a seep characterization model to determine the relative upward fluid flux.
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 3004 Gas and hydrate systems
SC: Biogeosciences [B]
MN: 2007 Fall Meeting