Microbial biogeochemistry and heterotrophy in sediments of a marine hydrocarbon seep

Vertical profiles of sediment and pore-water constituents and rates of microbially mediated geochemical processes were determined in surficial sediments (0-7-cm depth) of three stations in and around Isla Vista hydrocarbon seep off Santa Barbara, California. Measurements were made of pore-water alkalinity (total and carbonate), pH, Eh, dissolved oxygen, sulfate, and sulfide, total sedimentary organic carbon (TOC) and nitrogen, ATP, sulfate reduction and dark bicarbonate uptake and incorporation, and oxygen flux across the sediment-water interface in benthic chambers. In general, alkalinity, pH, sulfide, TOC, ATP, and all rate processes were greatest in sediments of that station (A) with active seepage and decreased with increasing distance from the seep. Sulfate depletion and extremely low Eh values occurred in station A sediments. At a station (B) with lower seepage rates than station A, sedimentary and pore-water constituents and rate processes were intermediate those measured at station A and a station (C) without seepage. Pore-water and sedimentary constituents as well as oxygen flux were strongly correlated with total extractable sedimentary hydrocarbons (TEH) measured at each station and for each corresponding depth interval. All parameters exhibited seasonal differences that may have been temperature-dependent. The greater concentrations of TEH, sulfide, and alkalinity with increasing sedimentary depth indicate that seep sediments are a source of these constituents to the water column and a sink for 02 and S042-. The strong heterotrophic, and possibly chemoautotrophic, character of seep sediments resembles that of other organically enriched systems. The diagenesis of petroleum hydrocarbons is a function of the biogeochemical patterns and microbial heterotrophic activities in surficial sediment