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Partial photochemical oxidation was a dominant fate of Deepwater Horizon surface oil Collin P. Ward, Charles M. Sharpless, David L. Valentine, Deborah P. French-McCay, Christoph Aeppli, Helen K. White, Ryan Patrick Rodgers, Kelsey M. Gosselin, Robert K. Nelson, and Christopher M. Reddy Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05948 • Publication Date (Web): 24 Jan 2018 Downloaded from http://pubs.acs.org on January 26, 2018

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Environmental Science & Technology

Partial photochemical oxidation was a dominant fate of Deepwater Horizon surface oil Collin P. Ward1*, Charles M. Sharpless2, David L. Valentine3, Deborah P. French-McCay4, Christoph Aeppli5, Helen K. White6, Ryan P. Rodgers7, Kelsey M. Gosselin1, Robert K. Nelson1, and Christopher M. Reddy1 1

Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA 2 Department of Chemistry, University of Mary Washington, Fredericksburg, Virginia 22401, USA 3 Department of Earth Science and Marine Science Institute, University of California, Santa Barbara, California 93106, USA 4 RPS ASA, South Kingstown, Rhode Island 02879, USA 5 Bigelow Laboratory for Ocean Sciences, East Boothbay, Maine 04544, USA 6 Department of Chemistry, Haverford College, 370 Lancaster Avenue, Haverford, Pennsylvania 19041, USA 7 National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32310, USA *Correspondence to: Collin P. Ward Department of Marine Chemistry and Geochemistry Woods Hole Oceanographic Institution Woods Hole, MA 02543 Tel: 508-289-3452 Email: [email protected]

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Abstract:

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Following the Deepwater Horizon (DWH) blowout in 2010, oil floated on the Gulf of Mexico

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for over 100 days. In the aftermath of the blowout, substantial accumulation of partially oxidized

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surface oil was reported, but the pathways that formed these oxidized residues are poorly

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constrained. Here we provide five quantitative lines of evidence demonstrating that oxidation by

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sunlight largely accounts for the partially oxidized surface oil. First, residence time on the sunlit

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sea surface, where photochemical reactions occur, was the strongest predictor of partial

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oxidation. Second, two-thirds of the partial oxidation from 2010-2016 occurred in less than 10

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days on the sunlit sea surface, prior to coastal deposition. Third, multiple diagnostic

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biodegradation indices, including octadecane to phytane, suggest that partial oxidation of oil on

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the sunlit sea surface was largely driven by an abiotic process. Fourth, in the laboratory, the

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dominant photochemical oxidation pathway of DWH oil was partial oxidation to oxygenated

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residues rather than complete oxidation to CO2. Fifth, estimates of partial photo-oxidation

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calculated with photochemical rate modeling overlap with observed oxidation. We suggest that

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photo-oxidation of surface oil has fundamental implications for the response approach, damage

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assessment, and ecosystem restoration plan in the aftermath of an oil spill, and that oil fate

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models for the DWH spill should be modified to accurately reflect the role of sunlight.

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Introduction:

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Accounting for the fate of the five million barrels1 of oil and gas discharged into the Gulf of

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Mexico (GoM) during the Deepwater Horizon (DWH) spill of 2010 is of considerable interest to

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a wide range of public and private sector stakeholders. Fate models indicate that the low

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molecular weight, primarily soluble hydrocarbons (boiling points < ~n-C5) entrained and

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dissolved in deep waters2,3 were completely oxidized to CO2 (i.e., respiration) or fixed into

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microbial biomass on timescales of days to weeks.4,5 Higher molecular weight, insoluble

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hydrocarbons (boiling points > ~n-C5), rose to the sea surface and encountered one of many

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physical, chemical, and biological processes. One notable omission from these fate models was

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the finding that higher molecular weight, insoluble, non-volatile hydrocarbons (boiling points >

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~n-C16; referred to here as “surface oil”)3 were converted into persistent, partially oxidized

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compounds evidenced in residues deposited in coastal ecosystems and collected from GoM

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beaches (referred to here as “deposited oil”). These partially oxidized compounds were not

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present in the spilled oil and are quantitatively relevant, comprising upwards of 50% of the mass

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of surface oil residues.6–12

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Despite the occurrence of these partially oxidized oil residues, the kinetics and mechanisms of

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partial oxidation are not known. Previous studies hypothesized that partial oxidation during

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transit on the sea surface to GoM beaches was faster than oxidation following deposition on

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GoM beaches, potentially indicating a shift in oxidative pathways.6,10 However, the limited

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number of residues collected within 100 days of the spill hinders an accurate assessment on

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whether the majority of oxidation occurred during transit on the sea surface or after deposition

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on GoM beaches. The limited sampling coverage in the weeks and months following the DWH


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spill has also challenged our ability to identify the mechanisms that governed surface oil

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oxidation. Microbial and photochemical oxidation have been proposed as likely candidates for

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the observed oxidation;6 however, the relative importance of these proposed oxidative

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mechanisms is unknown.

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Based on incubations conducted in the absence of natural sunlight, GoM microbial communities

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possessed the metabolic potential to oxidize a substantial fraction of the surface oil.13–15 Despite

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this metabolic potential, blooms of microbial biomass in the slick were not observed.13

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Furthermore, hopane normalized concentrations of straight-chain alkanes (n-C22 to n-C29) in

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deposited oil residues collected on GoM beaches 12-18 months after the spill were upwards of

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80% of their concentration in fresh Macondo well oil.16 The stability of these n-alkanes in the

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field over year-long timescales stands in contrast to month-long half-lives based on laboratory

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microbial incubations.15 Potential explanations for this discrepancy between microbial oxidation

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of surface oil in the field versus laboratory include: (i) nutrient limitations,13 (ii) physical

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weathering processes that preferentially remove the lower molecular weight compounds

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expected to be labile to microbes,4,5,16,17 (iii) oxidative stress to the microbial community from

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photochemically generated reactive oxygen species, such as singlet oxygen, hydroxyl radical, or

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peroxides,18–20 or (iv) surface area limitations.15 Nonetheless, the discrepancy between field and

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laboratory observations suggests that microbial oxidation of DWH surface oil may have been

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less important than alternative processes, such as photochemical oxidation.

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A wide range of oils are susceptible to oxidation by sunlight.8,9,18–25 Light-absorption by oil

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sensitizes the formation of reactive oxygen species (e.g., singlet oxygen19 or radicals22) that can



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either partially oxidize oil to less volatile residues that remain in the water or completely oxidize

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oil to CO2.21 Despite the evidence that sunlight is capable of transforming oil into oxidized

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residues, including DWH oil,8,9,24 photo-oxidation is considered a longer-term, relatively

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inconsequential weathering process for the fate and mass balance of oil spilled into aquatic

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ecosystems.26–35 For example, the United States National Academies of Sciences states that

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“photo-oxidation is unimportant from a mass balance consideration.”28 There are likely two

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reasons to explain why photo-oxidation is often considered a negligible term in the

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environmental fate of oil spilled to surface waters. First, no study has ever measured an apparent

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quantum yield spectrum for the partial or complete photo-oxidation of crude oil, and thus

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environmentally relevant rates of partial or complete photo-oxidation of crude oil are unknown.

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Second, the vast majority of photochemical oxidation studies have focused on the oxidation of

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polycyclic aromatic hydrocarbons, a compound class that constitutes a minor fraction of crude

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oils ( 2.4. The stability of n-C18:phytane of DWH surface oil during transit on the sea

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surface was consistent with a previous chemical forensics study based on 66 DWH floating oil

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residues.51 Once deposited in the coastal zone in 2010, the minimum n-C18:phytane of the

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deposited oil residues value was 2.02, and >70% (34 of 46) had a n-C18:phytane > 2.4. After

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2014, neither n-C18 nor phytane were above the lower limit of detection (Fig. 3). The constancy

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of multiple indicators for biological degradation while the oil was floating on sea surface (Fig. 3

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and Tables S1 and S5),51 suggests that abiotic processes largely drove the rapid partial oxidation


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on the sea surface. In contrast, the indices suggest that the gradual oxidation that occurred after

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deposition in the coastal zone was largely driven by biotic processes (Fig. 3 and Table S3 and

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S5). Given the field-based evidence suggesting that the majority of partial oxidation occurred on

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the sunlit sea surface through abiotic processes (Figs. 1-3, S2, and S3, Tables S1 and S5), we

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experimentally manipulated DWH oil in the laboratory to determine if rates of photochemical

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oxidation were sufficient to account for the observed partial oxidation.

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(II)

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The rates of complete oxidation to CO2 and partial oxidation to residues that remain on the

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surface were quantified in laboratory experiments by concurrently measuring photochemical CO2

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production and O2 consumption by oil films on water (see Methods). Complete oxidation to CO2

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was directly quantified as dissolved inorganic carbon production. Partial oxidation was

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indirectly calculated by conservatively assuming that one mol of O2 is consumed per mol of CO2

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produced, and thus any excess O2 consumption represents the oxygen that was incorporated into

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the oil to yield partially oxidized residues. In order to estimate the spectral efficiency (i.e.,

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apparent quantum yield) and daily rates of complete and partial photo-oxidation, photochemical

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reactivity was quantified under two light regimes: (i) ultraviolet + visible, and (ii) visible light.

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The DWH oil consumed O2 under both light regimes, but did not produce CO2 under either

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regime (Fig. S5), thereby indicating that the consumed O2 was incorporated into the oil to yield

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partially oxidized residues. Apparent quantum yields for partial photo-oxidation decreased with

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increasing wavelength, from 0.60 ± 0.06 mmol O2 per mol photons at 280 nm to 0.30 ± 0.01

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mmol O2 per mol photons at 600 nm (Fig. S5, ± SD, N = 4).

Quantifying rates of partial photo-oxidation on the sea surface

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Daily rates of partial photo-oxidation during the DWH spill (mmol O2 m-2 d-1) were calculated

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using the apparent quantum yields (Fig. S5), solar irradiance on the sea surface during the spill

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(Fig. S6), and the extent of surface oiling.52 Calculations were performed for both thin (~0.1

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µm) and thick (~70 µm) oil films as representative of the range detected on the sea surface using

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synthetic aperture radar.52 Due to the slower rates of light absorption by the thin vs. thick oil

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film, daily depth integrated rates were substantially slower for the thin vs. thick oil film (Fig. 4).

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Despite the slower rates of partial photo-oxidation in the thin film, multiplying daily rates by the

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surface area of the different films demonstrates that total O2 uptake was comparable for each

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film type detected on the GoM (Fig. 4). There was no discernible relationship between total O2

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uptake and irradiance (thin: R2 = 0.16, thick: R2 = 0.17), as is expected for photochemical

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processes in aquatic ecosystems. Instead, total O2 uptake was strongly correlated to the extent of

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surface oiling (thin: R2 = 0.92, thick: R2 = 0.86), reflecting the rapid (daily) and substantial

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changes to the extent of surface oiling.52

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Integration of daily photochemical oxygen uptake across the 102 days that oil was detected on

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the sea surface results in 159 Mmol-O2 uptake by the thin film (95% CI, 96, 235), 201 Mmol-O2

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uptake by the thick film (95% CI, 53, 299), and a summed total of 360 Mmol-O2 uptake (95%

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CI, 149, 634). For comparison, multiplying estimates for insoluble, non-volatile hydrocarbons

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that comprised the surface oil (218 Gg-HC)1,3 with the increase in bulk oxygen content of the

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surface oil residues during transit (5.8 ± 1 %; Fig. S2) gives an estimated “observed” oxygen

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uptake of 340 Mmol-O2 (95% CI, 281, 399). The overlap between the photochemical model

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calculation and the observed values argues strongly for a photochemical basis of the oil oxidation

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on the GoM surface.


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Discussion:

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Five quantitative lines of evidence strongly suggest that oxidation by sunlight largely accounts

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for the partially oxidized surface oil produced during transit to the coastal zone. First, three

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measures of partial oxidation were correlated to the time that surface oil resided on the sunlit sea

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surface prior to collection (Fig. 1). Second, field-based evidence suggested that the majority of

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partial oxidation from 2010-2016 occurred in less than ten days on the sunlit sea surface before

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the oil residues were deposited in the coastal zone (Figs. 1, 2, and S2). Third, multiple

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diagnostic indicators for biological oxidation suggested that partial oxidation on the sea surface

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was driven by abiotic processes (Fig. 3 and S3, Table S5). Fourth, laboratory experiments

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indicated that DWH oil was more susceptible to partial oxidation rather than complete photo-

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oxidation (Fig. S5). Fifth, a photochemical model indicated that rates of partial photochemical

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oxidation were indeed fast enough to account for the observed oxidation; that is, estimates of

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partial photo-oxidation of DWH surface oil (i.e., Fig. 4; 360 Mmol-O2 (95% CI; 149, 634))

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overlap with observed values (i.e., 340 Mmol-O2; 95% CI, 281, 399)). Collectively, these results

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strongly suggest that partial photochemical oxidation governed the oxidation of DWH surface

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oil.

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The amount of surface oil partially oxidized by sunlight is comparable to the amount of oil and

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gas hydrocarbons respired by microbes in deep waters, a widely-discussed environmental fate of

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the spilled oil and gas.4,5 Photochemical O2 uptake by the surface oil from April 24th to August

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3rd, 2010, totaled 360 Mmol-O2 (Fig. 4). By conservatively assuming that 0.2 mol O was

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consumed per mol C oxidized (i.e., the O/C of the partially photo-oxidized products was 0.2, and



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surface oil residues were 80% C by mass6), 0.05 Tg-HC were oxidized by sunlight on the sea

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surface. Similarly, 0.14 Tg-HC were estimated to be respired by microbes in the deep plume by

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3-August-2010,5 the last day that a surface oil was detected,52 and the estimate totaled 0.18 Tg-

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HC by mid-September when the deep plume was no longer detectable.5 Accordingly, our

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conservative calculations indicate that the amount of hydrocarbons partially oxidized by sunlight

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on the sea surface was at least 25% of the amount respired by microbes in the deep plume. The

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key difference between photochemical and biological processing is the transformation products.

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Sunlight partially oxidizes surface oil into residues that remain in the water and contribute to

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water column and ecosystem impacts (e.g., smothering of or toxicity to near-surface water

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column biota), while microbial degradation transforms hydrocarbons into CO2 and microbial

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biomass, thereby potentially mitigating ecosystem impacts.

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The dominant photochemical oxidation pathway of DWH oil was partial oxidation rather than

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complete oxidation to CO2. Consistent with the finding that partial photo-oxidation was the

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principal photo-fate of DWH oil, exposure of many crude and refined oil sources to sunlight has

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been reported to yield oxidized compounds that were not present in the parent oil.8,9,19–25 Lack of

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complete photo-oxidation of DWH oil to CO2 (Fig. S5) was consistent with the report that only a

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trace fraction of Kuwait crude oil (99% of the residue is self-shaded

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from the photochemical production of reactive oxygen species that can be harmful to microbial

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communities and suppress activity (e.g., singlet oxygen, hydroxyl radical, or peroxides).18–20

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Third, compared to pelagic surface waters, coastal waters in the Gulf are nutrient replete. Further

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supporting this interpretation, the decrease in n-C18:phytane (Fig. 3) was accompanied by a shift


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in the microbial community composition towards a hydrocarbon-degrading consortium57,58

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capable of degrading fossil carbon present in the deposited oil residues.59,60 Thus, while it is

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possible that deposited oil residues were photo-oxidized, as has been suggested,61 it is

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improbable that photochemical oxidation was the dominant oxidation pathway.

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While we did not explicitly test microbial degradation in the laboratory, field-based evidence

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presented in this study (Fig. 3 and S3, Tables S1 and S5) and a previous study,51 suggest that

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models of DWH surface oil (boiling points > ~n-C16) should be modified to accurately reflect the

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timescales of microbial degradation (Fig. S7). The smaller, primarily soluble hydrocarbons

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entrained in the deep plume (boiling points < ~n-C5) were degraded rapidly by microbes over

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day- to week-long timescales.4,5,17 In contrast, analysis of hundreds of field residues suggests

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that the surface oil was largely resistant to microbial degradation during the week- to month-long

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transit time to the coastal zone (Fig. 3 and S3, Tables S1 and S5).51 However, upon deposition in

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the coastal zone, the surface oil residues were susceptible to microbial degradation over year-

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long timescales (Fig. 3 and S3, Tables S4 and S5).51

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Implications:

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Our findings on the photochemical transformation of DWH surface oil (> ~C16) are central to the

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three key components of evaluating an oil spill: response, damage assessment, and restoration.

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The efficacy of response tools, including sorption, burning, and the use of chemical agents (e.g.,

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dispersants, solidifiers, herders, or emulsion treating), is typically quantified under controlled

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laboratory conditions using crude or evaporated oil62 with a gap in knowledge as to how partial

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photo-oxidation of oil impacts the efficacy of these tools. For example, resolving the debate63,64



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over the efficacy of chemical dispersants to stimulate microbial degradation of surface oil likely

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requires refined experimental designs to consider the impacts of sunlight on surface oil chemical

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and physical properties, as well as microbial community activity. Presently, as part of a damage

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assessment, injuries are assessed by established relationships between exposure of target

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compounds and the associated effects on aquatic life based on bioassays.28,65–67 However,

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relationships between the chemical composition of partially oxidized residues and toxicity to

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aquatic life are poorly constrained, in large part because characterizing these relationships is

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analytically challenging for three key reasons. First, the diverse mixture of photo-products that

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comprise oxidized oil residues falls outside of the analytical window traditionally used to

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characterize oil (i.e., gas chromatography).7,68,69 Second, less than 15% of oxidized oil residue is

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soluble in solvents recommended for use in toxicity bioassays (Fig. S8).70,71 Third, the vast

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majority of toxicity bioassays are conducted in the absence of natural sunlight.28,67 Accordingly,

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the challenges associated with characterizing the chemistry and toxicity of photo-oxidized

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residues hinders our capacity to fully assess the injury to near-surface water column biota

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resulting from a spill, and thus to plan appropriate ecosystem restoration efforts.

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The current understanding is that photo-oxidation is a longer-term, relatively inconsequential

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weathering process for the fate and mass balance of oil spilled into aquatic ecosystems.26–35

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Using the DWH spill as a case study, we challenge the completeness and therefore accuracy of

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these fate models by demonstrating that the amount of oil partially oxidized by sunlight on the

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sea surface is comparable to the amount of hydrocarbons respired to CO2 by deep water

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microbes, a universally accepted environmental fate of DWH oil and gas.4,5,17 These results

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suggest that budget estimates and fate models for DWH surface oil should be revised to account


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for the extensive and rapid photochemical transformations that occurred during transit on the sea

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surface, yielding persistent partially oxidized residues that are still present in the coastal zone

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today. Furthermore, these results suggest that oil spill textbooks,26–29 as well as trajectory and

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response handbooks33–35 should be modified to reflect the substantial role of sunlight on the fate

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of oil released to aquatic ecosystems.

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Acknowledgments:

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We thank Robert Ricker and Greg Baker at NOAA for helping secure the surface and deposited

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oil residues from 2010, Robert Holmes (WHRC), Jonathon Sanderman (WHRC), Jennifer

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Bowen (Northeastern), Aleck Wang (WHOI), Ben Van Mooy (WHOI), and Rose Cory (UM) for

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access to analytical equipment, Joy Matthews (UC Davis) for advice on preparing the oil

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residues for elemental oxygen analysis, and John Farrington (WHOI), John Hayes (deceased),

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Rose Cory (UM), and George Kling (UM) for discussions about our findings. Thanks to Ian

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MacDonald (FSU) for providing the oil film surface area database. The irradiance data was

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provided by the United Stated Department of Agriculture UVB Monitoring Climatological and

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Research Network. This work was supported, in part, by National Science Foundation grants

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RAPID OCE-1043976, OCE-1333148, OCE-1333-26, and OCE-1333162, and the Gulf of

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Mexico Research Initiative grants -015, SA 16-30, and DEEP-C consortium. All data is publicly

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available on the Gulf of Mexico Research Initiative Information and Data Cooperative

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(GRIIDC).

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Supporting Information:

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Appendices describing the chemical characterization of surface and deposited oil residues; 23 ACS Paragon Plus Environment


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photochemical rate model calculations. Tables showing the chemical composition of the surface

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and deposited oil residues; fitting terms of the exponential growth model; indicators of biological

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oxidation. Figures showing relationship between sample distance and date vs. oxidation;

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comparison of the oxidative state of surface oil vs. deposited oil residues; comparison of

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biodegradation observed in the field vs. laboratory; description of experimental set-up; results

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from photochemical incubation; results from photochemical rate modeling; proposed revision to

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oil spill fate model; solubility of photo-oxidized oil in common solvents.

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Figure 1. Bulk oxygen content (black circles), carbonyl infrared stretching at 1712 cm-1 normalized to alkane stretching at 2925 cm-1 (blue squares), and oxidized hydrocarbon content determined by thin layer chromatography coupled with flame ionization detection (red triangles) vs. residence time of the surface oil prior to collection (days). The relationships were fit to a pseudo-first-order exponential production kinetic model in the form of y = A(1-e-kt). The fitting terms A (%) and k (d-1) were: 5.1 and 0.6 (bulk oxygen), 6.5 and 0.5 (carbonyl), and 54.0 and 1.2 (oxidized hydrocarbon). For all three models, the calculated chi-squared was lower than the critical chi-squared at α = 0.01 (Table S2).

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Figure 2. Distribution of oxidized hydrocarbon content (%) for deposited oil residues collected on Gulf of Mexico beaches from 2010 to 2016. Different letters indicate significant differences in oxidized hydrocarbon content across years (α = 0.05). Oxidized hydrocarbon content of residues collected from 2011-2014 was previously reported.6,10

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Figure 3. Octadecane (n-C18) to phytane ratios of the spilled oil (green square), surface oil residues (black triangle), and deposited oil residues (red circle) vs. time after the start of the spill. Residues where the abundance of octadecane and phytane was below the analytical detection limit are presented beneath the dashed line as “n/d.”

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Figure 4. (A) Daily water column photo-oxidation rates of the thin (~0.1µm) and thick (~70 µm) surface film from 24-April to 3-August-2010, the 102-day window of surface oiling on the Gulf of Mexico. (B) Daily photochemical O2 uptake for the thin and thick surface films determined by multiplying daily water column rates by the daily surface oil area (km2).52

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