Using Natural Abundance Radiocarbon To Trace the Flux of Petrocarbon to the Seafloor Following the Deepwater Horizon Oil Spill

Abstract

In 2010, the Deepwater Horizon accident released 4.6–6.0 × 10(11) grams or 4.1 to 4.6 million barrels of fossil petroleum derived carbon (petrocarbon) as oil into the Gulf of Mexico. Natural abundance radiocarbon measurements on surface sediment organic matter in a 2.4 × 10(10) m(2) deep-water region surrounding the spill site indicate the deposition of a fossil-carbon containing layer that included 1.6 to 2.6 × 10(10) grams of oil-derived carbon. This quantity represents between 0.5 to 9.1% of the released petrocarbon, with a best estimate of 3.0–4.9%. These values may be lower limit estimates of the fraction of the oil that was deposited on the seafloor because they focus on a limited mostly deep-water area of the Gulf, include a conservative estimate of thickness of the depositional layer, and use an average background or prespill radiocarbon value for sedimentary organic carbon that produces a conservative value. A similar approach using hopane tracer estimated that 4–31% of 2 million barrels of oil that stayed in the deep sea settled on the bottom. Converting that to a percentage of the total oil that entered into the environment (to which we normalized our estimate) converts this range to 1.8 to 14.4%. Although extrapolated over a larger area, our independent estimate produced similar values.

Persistence and biodegradation of oil at the ocean floor following Deepwater Horizon

“…Our approach is independently supported by the good spatial agreement found between the deposition footprint as defined by the MDI and the footprint as previously defined by hopane (28) and natural abundance radiocarbon (27) anomalies. The MDI offers an advantage over these methods in its ability to distinguish between seeped and spilled oil in individual samples: Among sediment samples that do not meet our MDI threshold, a distinct and coherent fingerprint emerges at greater distances from the Macondo Well and lower depths in the sediment column, likely representing weathered oil that originated in natural seeps.…”

“…We tested the validity of these relationships for the seafloor by comparing the residual fraction for each hydrocarbon remaining in the sediment 4 y after the spill began. Among the compounds examined, carbon skeletons range from nine to 37 atoms (aliphatics, 9-37; aromatics, 9-22; biomarkers, [23][24][25][26][27][28][29][30][31][32][33][34][35] and vary in complexity from the straight-chain aliphatic n-C9 to the pentacyclic, multiply substituted biomarker pentakishomohopane. This analysis provides an unparalleled window into the disposition of oil following the DWH event, in that the extent of biodegradation is quantified simultaneously for 125 petroleum hydrocarbons across wide-ranging contamination levels.…”

“…Some suspended oil was eventually deposited to the seafloor, likely via oilmineral aggregates or microbial flocs (8,19,20), with intense contamination within ∼5 km of the well (21)(22)(23)(24)(25)(26). Surficial sediments near the well were found to carry >1,000-fold-elevated concentrations of dioctyl sodium sulfosuccinate (4), an active ingredient of the chemical dispersant applied at the wellhead, and to exhibit a radiocarbon deficit consistent with oil deposition (27). We recently identified a 3,200-km 2 deposition footprint stretching southwest from the wellhead (28).…”

Scales of seafloor sediment resuspension in the northern Gulf of Mexico

“…The sedimentation of large amounts of oil via marine snow and its accumulation on the deep seafloor (>1,200 m) during and after the Deepwater Horizon (DwH) oil spill (Passow et al, 2012;Valentine et al, 2014;Brooks et al, 2015;Chanton et al, 2015;Daly et al, 2016;Joye, 2016;Joye et al, 2016;Passow, 2016) raised questions regarding the distribution and re-distribution processes of freshly sedimented material (marine snow) on the seafloor. Once on the seafloor, marine snow contributes to unconsolidated fluffy sediment layers (Gardner, 1978;Gardner et al, 1984;1985;Walsh et al, 1988;Pilskaln et al, 1998;Newell et al, 2005) that are subject to resuspension and the production of benthic nepheloid layers (BNLs).…”

Impact of Photooxidation and Biodegradation on the Fate of Oil Spilled During the Deepwater Horizon Incident: Advanced Stages of Weathering

“…20,21,25,54 The weathered sand patties may remain on the seafloor or be transported to beaches where they can settle in the swash zone. 15,18,55,56 where they are exposed to direct sunlight, continual wave action and the ambient microbial communities. In addition, the sand patties themselves likely represent a suitable habitat for the proliferation of microorganisms (see Chapter 1).…”