Carbon isotopic composition and dissolved CO2 concentration in pore waters of ODP Leg 174A sites

Microbially mediated redox diagenetic processes in marine sediments are reflected in the amount and carbon isotopic composition of dissolved CO2 and CH4 (Claypool and Kaplan, 1974). Oxidation of organic matter gives rise to dissolved CO2 with about the same 13C/12C ratio as the starting organic matter. Subsequent reduction of CO2 to form CH4 involves a large (~70) kinetic isotopic effect, resulting in significant 13C depletion in the CH4, and 13C enrichment in the residual CO2. Ocean Drilling Program Leg 174A (offshore New Jersey) presented an opportunity to study these processes in shelf and upper slope sediments. Holes 1071A-1071D, 1071F, and 1072A were drilled on the shelf in water depths of 88.0-98.1 m. Hole 1073A was drilled on the slope in 639.4 m of water. Pore-water samples were collected for analysis at all three sites, whereas gas samples could only be obtained from Hole 1073A on the slope.