Comment on ``PAH refractory index as a source discriminant of hydrocarbon input from crude oil and coal in Prince William Sound, Alaska'' by

The recent article by Hostettler et al. (1999) describes a source-dependent refractory PAH index, T/C [C26(R)+C27(S)-triaromatic steranes/1-methylchrysene], and uses that index to argue that coal is the dominant source of the natural petrogenic hydrocarbon background in the marine sediments of Prince William Sound (PWS), Alaska. However, our more complete sampling of potential sources from the region shows that, based on T/C only, coal is only one of several possibilities. Measurements of vitrinite/kerogen re ̄ectance, hydrocarbon mass balance calculations, and consideration of other molecular indicators, including the polycyclic aromatic hydrocarbons (PAH) and saturate biomarkers, clearly indicate that coal cannot be a dominant source. Inadequate sampling and failure to consider other chemical evidence resulted in the incorrect interpretation reported by Hostettler et al. (1999). In addition, Hostettler et al. (1999) do not consider the thermal dependency of the T/C index. Consequently, its application to sources having the wide range of maturities observed for the region is problematic. The sources of the natural petrogenic hydrocarbon background in PWS o€shore sediments occur in eastern Alaska, in the coastal region of the Gulf of Alaska (GOA) (e.g. Page et al., 1993, 1995, 1996; Hostettler et al., 1999; Short et al., 1999) (Fig. 1). Hydrocarbons from these sources and ®ne-grained sediments from glacial streams are incorporated into the Alaskan Coastal Current (ACC), transported westward into the Sound and deposited (Page et al., 1993, 1995; Bence et al., 1996). Analyses of Pb-dated deep cores from PWS indicate that this in ̄ux of hydrocarbons has continued for more than 160 years. The speci®c sources of these hydrocarbons are the subject of debate. Page et al. (1996) argue that the sources are a mixture of eroding petroleum source rocks (Tertiary shales) and oil residues from natural seeps. Short et al. (1999) and Hostettler et al. (1999) argue that the source is coal. This Comment summarizes arguments against the coal source and cautions against uncritical application of the T/C ratio.