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.

海洋沉积物中微生物介导的氧化还原成岩作用，可通过溶解态二氧化碳（CO₂）与甲烷（CH₄）的含量及碳同位素组成得以体现（Claypool与Kaplan，1974）。有机质的氧化作用会产生溶解态二氧化碳，其¹³C/¹²C比值与初始有机质大致相当。后续二氧化碳还原生成甲烷的过程伴随显著（约70）的动力学同位素效应，导致甲烷中发生明显的¹³C贫化，而残留二氧化碳则出现¹³C富集。大洋钻探计划（Ocean Drilling Program）174A航次（新泽西州外海）为研究陆架与上斜坡沉积物中的上述过程提供了契机。研究团队在陆架区钻取了1071A-1071D、1071F及1072A号钻孔，作业水深介于88.0~98.1米之间；1073A号钻孔则设于斜坡区，作业水深为639.4米。所有三个站位均采集了孔隙水样品用于分析，而气体样品仅能从斜坡区的1073A号钻孔获取。