Persistent Alaska North Slope crude oil: a quarter century of weathering

Persistent Alaska North Slope crude oil (ANSCO) in Prince William Sound (PWS) and Gulf of Alaska (GOA) beaches has weathered over the past quarter century. Weathering is discernable in alkane, polynuclear aromatic hydrocarbons (PAHs), and biomarkers. PAHs accumulated in passive samplers deployed near oil patches 22 to 23 years after the Exxon Valdez oil spill. This oil remains definitively identifiable with biomarkers, despite weathering (which is generally slight for biomarkers). A novel pattern matching identification model for biomarkers indicated the presence of ANSCO from 1989 to 2012 at most GOA and PWS sample sites (7 of 9) and distinguished this source from several other potential sources. The presence of ANSCO was confirmed with Nordtest forensics, demonstrating the veracity of the new method. The principal advantage of the new method is that it provides sample-specific identification, whereas the Nordtest approach is based on multi-sample statistics. Biomarkers were conserved relative to other constituents, thus concentrations (per g oil) in initial beach samples were greater than those in fresh oil because they were lost more slowly than more labile oil constituents such as straight-chain alkanes and aromatic hydrocarbons. However, biomarker concentrations consistently declined thereafter (1989 to 2014), though loss varied substantially among and within sites. Isoprenoid loss was substantially greater than tricyclic triterpane, hopane, and sterane loss. Loss rates of the largest steranes tended to be least.