Reassessment of the Hydrocarbons in Prince William Sound and the Gulf of Alaska: Identifying the Source Using Partial Least-Squares

There has been considerable exchange in the literature regarding the source of the background hydrocarbons in Prince William Sound and the Gulf of Alaska. Page and coworkers suggest oil-based sources while Short and coworkers suggest coal. The multivariate statistical methodology of partial least-squares (PLS) has been used to reassess the percentage contribution of coal, seep oil, shales, and rivers to the hydrocarbon loading in the Gulf of Alaska. Data have kindly been provided by Short (NOAA) and Page (Bowdoin College, for Exxon), and these have been analyzed using selected sites as sources in order to develop signatures. These signatures are based on 40 and 136 compounds, respectively, and include the PAH and terpane/sterane biomarkers in the case of the Exxon data. The principal components describing these sources are then fitted to the data for other sites around the Prince William Sound (PWS) and Gulf of Alaska (GoA) to determine the proportion of the variability described by each source. Using the Exxon data, a mixed source of coal, seep oil, eroding shales, and rivers (1 and 2) sources described approximately 13%, 18%, 24%, 26%, and 20%, respectively, of the variance in PWS and GoA data. The rivers 1 signature was very similar to that of coal, while rivers 2 was more similar to the eroding shales. New coal data (Short, unpublished work) also indicated considerable overlap with the Exxon seep oil. With the NOAA data, spatial plots of the explained variance indicate that the prespill background has a wide range of explained fits. There is considerable overlap in the signatures developed from the data, and Coomans' Plots identify those compounds which are the most diagnostic: for the Exxon seep oil signature, naphthalene and methyl- and dimethylnaphthalene are the best markers, whereas for the NOAA prespill background, Exxon coals and shales are best defined by the larger PAHs such as benzo[ghi]perylene. The evidence suggests mixed sources whose contributions vary significantly across the sampling area.