CS08 Biogeochemical Cycles
Date: Monday, June 10, 2002
Time: 3:00:00 PM
Location: Saanich
 
LaphamLL, University of North Carolina at Chapel Hill, Chapel Hill, USA, llapham@unc.edu
Martens, C, S, University of North Carolina at Chapel Hill, Chapel Hill, USA, cmartens@email.unc.edu
 
CONTROLS ON THE ISOTOPIC COMPOSITION OF DISSOLVED METHANE IN THE POREWATERS OF SEDIMENTS CONTAINING DISSEMINATED GAS HYDRATE OFF OF CASCADIA MARGIN
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Dissolved methane and sulfate were measured in porewaters taken from gravity cores in sediments surrounding two gas hydrate sites off of Cascadia Margin, British Columbia, Canada. Samples were collected from 1) a vent site in 1200-1300 meters water characterized by a seismic wipeout zone and 2) a site where massive amounts of hydrate were previously found in 840 meters water. Dissolved sulfate varied in all sites between constant seawater concentrations down-core and subsurface minimum between 0.5 to 6 meters below sea floor. Observed methane concentrations were as high as 1mM below the sulfate reduction zone; however, all samples experienced degassing during core retrieval and in situ concentrations were presumably higher. Porewater d13C-CH4 values varied from -80 to -110, indicating a biogenic origin. Depth distributions exhibited a systematic 40 shift controlled by a combination of production and anaerobic oxidation processes. The lightest isotopic values occurred at the base of the sulfate reduction zone and became heavier toward the sediment-water interface and with depth into the methane production zone.