About half of the surface oil floating on the Gulf of Mexico in the aftermath of the 2010 Deepwater Horizon spill was transformed into oxygenated hydrocarbons (OxHC) within days to weeks. These OxHC persist for years in oil/sand aggregates in nearshore and beach environments, and there is concern that these aggregates might represent a long-term source of toxic compounds. However, because this OxHC fraction is a continuum of transformation products that are not well chemically characterized, it is not included in current oil spill fate and effect models. This challenges an accurate environmental risk assessment of weathered oil. Here, we used molecular and bulk analytical techniques to constrain the chemical composition and environmental fate of weathered oil samples collected on the sea surface and beaches of the Gulf of Mexico. We found that approximately 50% of the weathering-related disappearance of saturated and aromatic compounds in these samples was compensated by an increase in OxHC. Furthermore, we identified and quantified a suite of oxygenated aliphatic compounds that are more water-soluble and less hydrophobic than its presumed precursors, but only represent <1% of the oil residues' mass. Lastly, dissolution experiments showed that compounds in the OxHC fraction can leach into the water; however, the mass loss of this process is small. Overall, this study shows that the OxHC fraction is prevalent and persistent in weathered oil/sand aggregates, which can act as a long-term source of dissolved oil-derived compounds.

The occurrence, trends and sources of soot black carbon (BC) in coastal sediments are poorly understood, particularly during the Anthropocene. Two sediment cores, covering the last ∼100 years from the US East Coast, off North Carolina and in the Florida Straits, were analyzed for organic carbon (OC), BC fluxes and BC sources. BC fluxes were 0.1 g cm−2 year−1 at both sites and accounted for 8%–22% of total OC. Carbon stable isotope values indicated OC to be of marine origin, while the BC was mostly terrestrially derived, C3‐plant material. Radiocarbon values revealed BC originating mostly from fossil fuels or pre‐aged carbon (fraction modern of 14%–31%) at North Carolina, while in the Florida Strait the BC was mostly derived from biomass burning (fraction modern of 70%–74%), in‐line with continental (NC) or marine (FS) air mass origins. Ratios of polycyclic aromatic hydrocarbons broadly supported different BC sources at the two sites.

Photoproducts can be formed rapidly in the initial phase of a marine oil spill. However, their toxicity is not well understood. In this study, oil was irradiated, chemically characterized, and tested for toxicity in three copepod species (Acartia tonsa, Temora longicornis, and Calanus finmarchicus). Irradiation led to a depletion of polycyclic aromatic hydrocarbons (PAHs) and n-alkanes in oil residues, along with an enrichment in aromatic and aliphatic oil photoproducts. Target lipid model-based calculations of PAH toxicity units predicted that PAH toxicities were lower in water-accommodated fractions (WAFs) of irradiated oil residues (“irradiated WAFs”) than in WAFs of dark-control samples (“dark WAFs”). In contrast, biomimetic extraction (BE) measurements showed increased bioaccumulation potential of dissolved constituents of irradiated WAFs compared to dark WAFs, mainly driven by photoproducts present in irradiated oil. In line with the BE results, copepod mortality increased in irradiated WAFs compared to dark WAFs. However, low copepod toxicities were observed for WAFs produced with photo-oxidized oil slicks collected during the Deepwater Horizon oil spill. The results of this study suggest that while oil photoproducts have the potential to be a significant source of copepod toxicity, dilution and dispersion of these higher solubility products appear to help mitigate their toxicity at sea.

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