Sediment-Microbe Interactions in Permeable Sediments at Hydrocarbon Seeps in the Santa Barbara Channel, California

Treude, Tina¹, Wiebke Ziebis¹ (1) University of Southern California, Los Angeles, CA

Sediments of marine cold-seep areas exhibit high rates of hydrocarbon discharge and are unique dynamic systems with specific microbial communities connected to methane and/or oil degradation processes. Microbial activity in such systems is in general tightly coupled to advective transport mechanisms of fluids and hydrocarbons as well as to geochemical gradients in the sediment. Seeps in coastal shallow-water areas are, in contrast to deep-sea mud, characterized by sandy permeable sediments, thus enabling enhanced substrate exchange due to accelerated pore water transport processes. We investigated sandy sediments off Coal Oil Point (Santa Barbara Channel, California), one of the world's largest hydrocarbon-seep area, to study the effect of pore water transport processes on microbial hydrocarbon turnover rates, biogeochemical gradients as well as microbial community structure. Our results demonstrate that microbial activity, such as anaerobic oxidation of methane and sulfate reduction, is accelerated in comparison to deep-sea seep environments in areas where reduced fluids, rich in hydrocarbons, seep through permeable surface sediments. Biogeochemical parameters of vertical and horizontal sulfide, oxygen, methane, and sulfate concentration reveal gradients and small-scale distribution patterns that differ from cold-seep systems of the deeper oceans. We suggest that these gradients and the resulting microbial activity are a result of pore water transport processes, where the supply of substrates is not limited to diffusion. Our preliminary data indicate that fast substrate supply and removal of inhibitory end products is an important factor enabling efficient microbial consumption of hydrocarbons in marine sediments.