Weathering and the Fallout Plume of Heavy Oil from Strong Petroleum Seeps Near Coal Oil Point, CA

Abstract

The Coal Oil Point (COP) seeps offshore Goleta, CA, are estimated to release 20-25 tons of oil daily, providing an ideal natural laboratory to investigate the fate of oil in the coastal ocean. To address the long-term fate of COP oil, we collected 15 sediment samples down current from the seeps and quantified petroleum content and individual biomarkers using traditional and comprehensive two-dimensional gas chromatography. Similarities in the distributions of hopane biomarkers link the oil in the sediments to fresh seep oil (n=5) and underlying reservoirs (n=3), although sediment oil is heavily weathered. The spatial distribution of oil forms a plume along the continental shelf that we suggest represents a chronic fallout pattern for heavy oil from the persistent surface slicks; average surface currents appear to modulate the distribution of the fallout over a period of 0.4-5 days. The extent of hydrocarbon loss is consistent for all sediments, indicating a common limit to oil weathering with contributions from evaporation, biodegradation, and dissolution. Considering the amount of oil and quantity of sediment impacted, we estimate a sediment oil burden of 0.3 x 10(12) to 3 x 10(12) g in the study area, equivalent to 8-80 spills of the Exxon Valdez accident of 1989.