Differential contribution of bacteria to sedimentary organic matter in oxic and anoxic environments, Santa Monica Basin, California

Two sediment box-cores in the Santa Monica Basin (SMB), one from the depocenter (900 m water depth) with anoxic bottom water and one from the periphery (840 m water depth) with oxic bottom water, were comparatively studied to assess the role of bottom water oxygen conditions to organic matter preservation and the differential contribution of bacteria to sedimentary organic matter accumulation and composition. Fatty acids from the sediment extracts were analyzed for their abundance and carbon isotopic compositions. Molecular and isotopic evidence indicates that most of the short-chain FAs from the SMB sediment extracts are sourced by bacteria as a result of intense microbial recycling and resyntheses of organic matter, especially in the aerobic environment at the periphery site. Long-chain fatty acids are probably derived from terrestrial sources and partially sourced by microbial processes. Very depleted δ13C values (−30 to −45‰) of bishomohopanoic acids and triglycerides suggest the presence of chemoautotrophic bacteria in the anoxic water column and in the sediments. There is a much greater contribution from anaerobic microbial biomass and chemoautotrophic bacteria in the depocenter sediments relative to the periphery sediments. In addition, organic matter exhibits bulk and molecular characteristics of enhanced preservation under the condition of anaerobic decomposition at the depocenter site, especially at the sediment/water interface and the first couple of centimeters. Together, enhanced preservation with the addition of microbial biomass could explain the higher organic accumulation rates and H/C ratios in the basin depocenter and the periphery sediments and the trend of decreasing δ13C values of bulk organic matter in the depocenter sediments. This study has implications for understanding the role of anoxia and associated bacterial processes in controlling the geochemical characteristics ancient organic carbon-rich sediments deposited under oxic and anoxic conditions.