Award: OCE-1333026

Summary: Photochemistry plays a more significant role in the early stage fate of marine oil spills than was previously known, and we estimate that the amount of Deep Water Horizon oil whose fate was controlled by photochemistry was only slightly smaller than the amount believed to have been degraded by microbes in the deep sea. Along with new, fundamental knowledge about oil spill science, our research also provided four undergraduate students with professional laboratory and field training on oil sampling and advanced analysis. Every few years, manmade spills focus attention on environmental issues surrounding oil extraction and draw public concern about the fate of spilled oil and how quickly it can dissipate so that impacted ecosystems can recover. Scientists have frequently used past spills as uncontrolled experiments to study the processes by which oil "weathers," a general term encompassing various transformations of oil by environmental and biological forces. Through such work, in part, it has been found that a substantial portion of the tar-like material that persists years after a spill appears to be produced by photochemical reactions initiated by sunlight. These cause the oil to incorporate oxygen into its structure, becoming partially oxidized, not to carbon monoxide or dioxide, but instead to a water-insoluble polymer that physically resembles asphalt-like tar. Recent research refers to these partially oxidized hydrocarbons as OxyHC. We examined the formation rates of OxyHC in marine oil slicks. The focus on marine systems was driven by several factors. For one, much of the world?s petroleum reserves, including most U.S. reserves, are located off-shore, and spills during marine extraction produce residual OxyHC that can severely impact coastal ecosystems. Improved understanding of OxyHC chemistry could enhance spill response efficiency; for example, responders don?t currently factor in OxyHC formation when deciding the timing and amounts of dispersant application, yet OxyHC is not susceptible to control by dispersants. We also focused on marine systems because a substantial amount of oil, equal to or, in some estimates, exceeding that of accidental releases, enters the ocean via natural seeps (albeit at substantially slower rates than accidental releases); the environmental fate of natural seep oil is not well-constrained, and as such represents a gap in current understanding of this portion of the carbon cycle. Lab studies reproducibly formed OxyHC by irradiating crude oil slicks on water with simulated sunlight (Figure 1). These reactions occurred very quickly, requiring only a few days to convert 20% to 30% of the oil to OxyHC. The lab experiments involved fairly thick oil films, more characteristic of spill conditions than the thin films encountered near natural seeps. Extrapolating lab conditions to what might be expected from thin films suggested that just one day of sunlight exposure during summer in the Gulf of Mexico could produce comparable chemical changes. During field research, we followed a natural seep slick in the Gulf over the course of a day, and we observed more rapid production of OxyHC than the lab studies predicted, suggesting that partially weathered seep oil may be more susceptible to partial oxidation than unweathered well-head oil (Figure 2). Follow up lab studies measured the exact efficiency for conversion of solar energy to oxygen uptake. This quantity is a key parameter for modeling the photochemical formation rate of OxyHC in the environment, and as a case study, we chose to retrospectively model OxyHC formation during the Deep Water Horizon (DWH) disaster of 2010. By combining chemical analyses and fluid dynamics modeling, we showed that the best predictor of OxyHC formation in the spilled oil was the time that the oil spent floating on the ocean surface (Figure 3); due to currents and wind, this is not well-correlated to distance from the well. Figure 3 also shows that photochemical formation of OxyHC is extremely rapid and can be expected in the first few days of a spill subjected to sunlight. Next, we used lab measurements of the photochemical efficiency along with measurements of the sunlight intensity in the Gulf during the DWH spill to predict the daily rate of oxygen consumption by oil as a function of wavelength. Figure 4 shows the calculated results for May 31, 2010 and demonstrates that comparable amounts of the chemistry are driven by ultraviolet and visible light. A comprehensive model of oil oxidation over the course of the spill gave results for oxygen uptake that agreed well with estimates from field samples. Furthermore, the model suggests that photochemical formation of OxyHC was the fate for a significant fraction of the spilled oil: we estimate that the amount of oil partially oxidized to OxyHC on the Gulf surface was at least 25% as big as the amount of hydrocarbons degraded by microbes in the deep sea oil plume. Last Modified: 12/30/2017 Submitted by: Charles M Sharpless