Carbon cycling within the upper methanogenic zone of continental rise sediments: An example from the methane-rich sediments overlying the Blake Ridge gas hydrate deposits

Data from piston cores collected from Carolina Rise and Blake Ridge, and from many DSDP/ODP sites indicate that extreme C-13-depletion of methane and Sigma CO2 occurs within the uppermost methanogenic zone of continental rise sediments. We infer that C-13-depleted methane is generated near the top of the methanogenic zone when carbon of C-13-depleted Sigma CO2, produced by microbially-mediated anaerobic methane oxidation, is recycled back to methane through CO2 reduction. Interstitial water and gas samples were collected in 27 piston cores, 16 of which penetrated through the sulfate reduction zone into methane-bearing sediments of the Carolina Rise and Blake Ridge. Isotopic measurements (delta(13)C(CH4)), delta(13)C(CO2), Methane samples from two distinct isotopic pools. (1) Methane from a seafloor seep site shows a mean delta(13)C(CH4) value of -69 +/- 2 parts per thousand, mirroring values found at greater than or equal to 160 mbsf from a nearby DSDP site. (2) Twenty, areally-separated sites (sample depth, 10 to 25 mbsf) have delta(13)C(CH4) values ranging from -85 to -103 parts per thousand, and delta(13)C(CO2) as negative as -48 parts per thousand. The very low delta(13)C values from the methane and CO2 pools highlight the importance of carbon cycling within continental rise sediments at and near the sulfate-methane boundary. (C) 1997 Elsevier Science B.V.