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Organic carbon contents and kerogen properties (hydrogen/carbon, nitrogen/carbon, carbon isotopic ratios) were determined for 29 core samples from Ocean Drilling Program Site 893. The samples, which span the last 160,000 years, are bioturbated and laminated silty-clay and clayey-silt hemipelagic/pelagic deposits accumulated under oxic to suboxic conditions. Organic carbon contents average 1.5 wt% with the highest values occurring in the laminated sediments. H/C and N/C atomic ratios vary between 0.8 and 1.2 and between 0.028 and 0.063, respectively. These variations are weakly correlated. Carbon isotopic compositions (δ ! C) for 20 of the kerogens range from -22%e to -25%o. Four kerogens have δ values varying between -25.5%o and -21 .\%o and the remaining five kerogens have δ values between -21.1%c and -29.5‰. The kerogens with low δ values tend to have low H/C and N/C ratios. Wide variations in both elemental and isotopic data indicate marked changes in the sources of the organic matter deposited in the Santa Barbara Basin. Kerogens with δ values in the range -22%e to -24%c reflect typical hemipelagic/pelagic debris predominantly composed of photosynthetically derived marine organic matter with elemental ratios varying in proportion to the amount of terrestrial organic matter and the degree of bioturbation. In contrast, the δ values lower than -21.0‰ are not typical of any sources of organic matter reported for the modern California shelf. Kerogen with low δ values could represent episodic inputs of C-depleted organic matter that was produced either by methanotrophs or by chemosynthetic bacteria (e.g., sulfide oxidizers) utilizing C-depleted inorganic carbon derived in part from the oxidation of CH4. Reports of both high concentrations of methane and abundant gas hydrates in marine sediments from offshore northern California suggest that periodic releasing of CH4 is possible, making episodic recycling of CH4 plausible.

Episodically Strong Recycling of Methane Recorded by 13C/12C, N/C, and H/C Ratios for Kerogens from the Last 160,000 Years in the Santa Barbara Basin at Hole 893A