Biogeochemical Processes at Mississippi Canyon 118 Amanda Tazaz, Laura Lapham, Jeff Chanton Abstract A combined biogeochemical and geophysical approach was used to understand the surface expression of hydrocarbon seepage at a hydrate-containing Gulf of Mexico cold seep. One such seep, Mississippi Canyon (MC) 118, located offshore southern Louisiana in 850 meters of water, is the site of a long-term seafloor observatory. Geophysical scanning, including side-scan and CHIRP, show areas of high acoustic reflection that indicate areas of upward fluid flow. We sought to characterize the spatial distribution of biogeochemical processes within the context of these reflections at MC 118. We hypothesized that areas of high acoustic reflectivity contain saturated concentrations of methane and have higher microbial activity in comparison to outside areas. This hypothesis was tested by collecting 17 surficial cores from sediment overlaying and outside of areas of high acoustic reflectivity. The porewaters were measured for down core gradients of sulfate and methane concentrations as well as methane stable isotopes. Preliminary results indicate that regions of high microbial activity were confined to areas within high acoustic reflectivity zones with a few notable exceptions. Conclusion In contrast to previous results, regions of high microbial activity were evident outside of high acoustic reflectivity Thermogenic hydrate was found near core 14 and there is also evidence for microbial methane production. With the addition of this new data, we are beginning to have a comprehensive birds eye view of the microbial processes at MC118 Core Profiles PFA #2 PFA #1 Low Relative microbial activity Moderate High Core 14 Core 11 Objective To characterize the spatial distribution of biogeochemical processes within the context of the geophysics. Core 12 Biogeochemical Processes Sulfate reduction SO42- + 2 CH2O  H2S + 2 HCO3- Anaerobic Methane Oxidation SO42- + CH4  HS - + HCO3- + H2O Methane production CO2  CH4 + 2 H2O CH3COOH  CH4 + CO2 Future Research Long term monitoring of MC118 via deployment of PFA Extensive coring expedition to focus on SE crater and area around core 14 Core 10 Acknowledgments We would like to thank Rachel Wilson and Jimmy Neslon for assistance in the field; Carol Lutken, Brian Noakes, Matt Lowe, and Andy Gossett for all their help with coring activities; and the captain and crew of the R/V Pelican for expert support of shipboard operations. We would also like to extend a special thanks to the Gulf of Mexico Hydrate Research Consortium (GOMHRC). Pore-fluid chemistry 13C-CH4 (‰) Depth (cm) SO4 (mM) CH4 (uM) Sulfate reduction Anaerobic Methane Oxidation Methane production Core 1 (black) and 14 (orange)