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 (delta¹³C) for 20 of the kerogens range from –22‰ to –25‰. Four kerogens have delta values varying between –25.5‰ and –27.1‰ and the remaining five kerogens have delta values between –27.7‰ and –29.5‰. The kerogens with low delta 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 delta values in the range –22‰ to –24‰ 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 delta values lower than –27.0‰ are not typical of any sources of organic matter reported for the modern California shelf. Kerogen with low delta values could represent episodic inputs of ¹³C-depleted organic matter that was produced either by methanotrophs or by chemosynthetic bacteria (e.g., sulfide oxidizers) utilizing ¹³C-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.

Date of initial receipt: 14 September 1994
Date of acceptance: 20 March 1995

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