A record of hydrocarbon input to San Francisco Bay as traced by biomarker profiles in surface sediment and sediment cores

San Francisco Bay is one of the world's largest urbanized estuarine systems . Its water and sediment receive organic input from a wide variety of sourc es; much of this organic material is anthropogenically derived. To document the spatial and historical record of the organic contaminant input, surfic ial sediment from 17 sites throughout San Francisco Bay and sediment cores from two locations-Richardson Bay and San Pablo Bay-were analyzed for bioma rker constituents. Biomarkers, that is, 'molecular fossils', primarily hopa nes, steranes, and n-alkanes, provide information on anthropogenic contamin ation, especially that related to petrogenic sources, as well as on recent input of biogenic material. The biomarker parameters from the surficial sed iment and the upper horizons of the cores show a dominance of anthropogenic input, whereas the biomarker profiles at the lower horizons of the cores i ndicate primarily biogenic input. In the Richardson Bay core the gradual up core transition from lower maturity background organics to a dominance of a nthropogenic contamination occurred about 70-100 years ago and corresponds to the industrial development of the San Francisco Bay area. In San Pablo B ay, the transition was very abrupt, reflecting the complex depositional his tory of the area. This sharp transition, perhaps indicating a depositional hiatus or erosional period, dated at pre-1952, is clearly visible. Below, t he hiatus the biomarker parameters are immature; above, they are mature and show an anthropogenic overlay. Higher concentrations of terrigenous n-alka nes in the upper horizons in this core are indicative of an increase in ter rigenous organic matter input in San Pablo Bay, possibly a result of water diversion projects and changes in the fresh water flow into the Bay from th e Delta. Alternatively, it could reflect a dilution of organic material in the lower core sections with hydraulic mining debris. (C) 1999 Elsevier Sci ence B.V. All rights reserved.