Resolving the origin of the petrogenic hydrocarbon background in Prince William Sound, Alaska

The dominant sources of the petrogenic hydrocarbon background in benthic se diments of Prince William Sound, AK (PWS), site of the 1989 Exxon Valdez oi l spill, are eroding Tertiary shales and residues of natural oil seepage. M ass balance considerations and statistical analyses of hydrocarbon fingerpr ints independently indicate that coal contributes generally less than 1% of the polycyclic aromatic hydrocarbons (PAH) and chemical biomarkers in this background. This is environmentally significant because of presumed differ ences in the bioavailability of PAH in coal, seep oil residues, and shales. Coal particles are present in PWS sediments, but their PAH and chemical bi omarker contributions are overwhelmed by those of seep oil residues and org anic particles from shales of low-to-high thermally maturity. In the late T ertiary or early Quaternary, the currently exposed and eroding shale format ions were heated into the oil-generation window and, consequently, are now relatively rich in extractable PAH and chemical biomarkers. The exposed and eroding coals in the area, in contrast, experienced long hot burial and ar e now thermally overmature with respect to oil generation. The concentratio ns of thermally sensitive PAH and biomarker compounds in PWS sediments are not consistent with a mature coal origin but are consistent with the low-to -high maturity shales and seep oils in the area.