Oxygen Isotopes (δ18O) Trace Photochemical Hydrocarbon Oxidation at the Sea Surface

Although photochemical oxidation is an environmental process that drives organic carbon (OC) cycling, its quantitative detection remains analytically challenging. Here, we use samples from the Deepwater Horizon oil spill to test the hypothesis that the stable oxygen isotope composition of oil (delta O-18(Oil)) is a sensitive marker for photochemical oxidation. In less than one-week, delta O-18(Oil) increased from -0.6 to 7.2, a shift representing similar to 25% of the delta O-18(OC) dynamic range observed in nature. By accounting for different oxygen sources (H2O or O-2) and kinetic isotopic fractionation of photochemically incorporated O-2, which was -9 parts per thousand for a wide range of OC sources, a mass balance was established for the surface oil's elemental oxygen content and delta O-18. This delta O-18-based approach provides novel insights into the sources and pathways of hydrocarbon photo-oxidation, thereby improving our understanding of the fate and transport of petroleum hydrocarbons in sunlit waters, and our capacity to respond effectively to future spills. Plain Language Summary Sunlight alters the physical and chemical properties of petroleum hydrocarbons released into aquatic ecosystems. These alterations impact the performance of chemical tools used in response to oil spills, and thus there is substantial interest in improving approaches to track these alterations and our understanding of the driving mechanisms. In this study, we developed a stable oxygen isotope-based approach to track photochemical oxidation of oil and validated it using samples from the 2010 Deepwater Horizon oil spill. Our isotope-based approach revealed two key insights into the photochemical oxidation mechanism. First, the major source of oxygen added to the oil by sunlight appears to be molecular oxygen rather than water. Second, molecular oxygen kinetic isotope fractionation during photochemical oxidation was remarkably similar for a wide range of organic carbon types, such as crude oil and dissolved organic carbon, raising the possibility of a common step in the oxidation mechanism. This new stable oxygen isotope-based approach will improve our capacity to respond effectively to future oil spills, and can be applied to track the cycling of a broad range of organic carbon sources.