Several lines of evidence indicate that partial oxidation by sunlight was a critical environmental fate of Deepwater Horizon (DWH) surface oil, transforming over half of the floating oil mass within days of surfacing. Using a combination of field- and laboratory-based approaches, here we (i) corroborate previous reports that sunlight drove the rapid and extensive oxidation of DWH oil on the sea surface, (ii) resolve the stable oxygen isotopic (δ18O) mass balance of DWH surface oil, and (iii) propose that δ18O is a novel, sensitive tracer of partial photo-oxidation of hydrocarbons in sunlit surface waters. In less than one week, δ18O of DWH surface oil increased by nearly 8‰, an enrichment comparable to 25% of the known dynamic range of δ18O of organic matter on Earth. By exposing oil to sunlight in the presence and absence of molecular oxygen in the laboratory, we demonstrate that nearly 90% of photochemical O-incorporation is sourced from atmospheric O2 (δ18O=24‰), which is isotopically enriched relative to seawater (δ18O=0‰). Mass-dependent isotopic fractionation of δ18O-O2 due to photochemical oxidation of crude oil was experimentally determined to be -9‰. The magnitude of isotopic fractionation of δ18O-O2 was similar for two oil types with widely different physical and chemical properties, suggesting that the rate-limiting step of partial photochemical oxidation is independent of oil type. To close the isotopic mass balance of DWH surface oil, roughly 20% of total O-incorporation would have had to exchange with water following production by sunlight. This hypothesized isotopic exchange is consistent with several studies reporting surface oil photo-products are comprised of exchangeable functionalities (e.g., ketones, carboxyl, aldehydes), shedding additional light on the chemical composition of the transformation products. Collectively, δ18O is a novel, sensitive tracer of the partial photo-oxidation of petroleum hydrocarbons, with potential applications for tracking the fate and transport of additional reduced hydrocarbon sources in sunlit surface waters, such as lipids, black carbon, and pollutants.

Stable oxygen isotopes as a novel, sensitive tracer of hydrocarbon oxidation on the sunlit sea surface

2019 Spring National Meeting

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.