Fallout plume of submerged oil from Deepwater Horizon

The sinking of the Deepwater Horizon in the Gulf of Mexico led to uncontrolled emission of oil to the ocean, with an official government estimate of ∼5.0 million barrels released. Among the pressing uncertainties surrounding this event is the fate of ∼2 million barrels of submerged oil thought to have been trapped in deep-ocean intrusion layers at depths of ∼1,000–1,300 m. Here we use chemical distributions of hydrocarbons in >3,000 sediment samples from 534 locations to describe a footprint of oil deposited on the deep-ocean floor. Using a recalcitrant biomarker of crude oil, 17α(H),21β(H)-hopane (hopane), we have identified a 3,200-km² region around the Macondo Well contaminated by ∼1.8 ± 1.0 × 10⁶ g of excess hopane. Based on spatial, chemical, oceanographic, and mass balance considerations, we calculate that this contamination represents 4–31% of the oil sequestered in the deep ocean. The pattern of contamination points to deep-ocean intrusion layers as the source and is most consistent with dual modes of deposition: a “bathtub ring” formed from an oil-rich layer of water impinging laterally upon the continental slope (at a depth of ∼900–1,300 m) and a higher-flux “fallout plume” where suspended oil particles sank to underlying sediment (at a depth of ∼1,300–1,700 m). We also suggest that a significant quantity of oil was deposited on the ocean floor outside this area but so far has evaded detection because of its heterogeneous spatial distribution.The sinking of the Deepwater Horizon in the Gulf of Mexico led to the discharge of ∼5.0 million barrels of petroleum from the Macondo Well. The discharge occurred at a water depth of ∼1,500 m and gave rise to intrusion layers (1) in the deep ocean that included both water-soluble hydrocarbons in the dissolved phase (2–6) and small particles of water-insoluble hydrocarbons (7–11). These intrusion layers were found primarily at a depth of 1,000–1,300 m and may have hosted the majority of the environmental discharge, including all the natural gas and ∼2 million barrels of liquid oil (12). Although the most abundant of the water-soluble hydrocarbons underwent rapid biodegradation during the spill (4, 6, 8, 9, 13–15), the fate and impacts of the insoluble hydrocarbons in the deep ocean have remained uncertain (16).The intrusion layers that hosted hydrocarbon contamination persisted for 6 mo or more and at distances >300 km from the well, but available evidence suggests that particles of submerged oil were particularly concentrated during the first 6 wk of discharge and within ∼15 km of the well (8, 9, 11). Thus, initial partitioning of hydrocarbon particles to the intrusion layers appears to have given way to transport or removal by undefined deep-ocean processes. Such processes might include sedimentation, buoyant rise toward the sea surface, incorporation into pelagic biota, biodegradation, or interventions at the wellhead. Mechanisms exist that support several of these options (9, 17–20), but uncertainty as to oil’s actual partitioning, the effect of chemical dispersant (21), and the impacts of a changing microbial community (6, 8, 9, 13–15, 17, 22–24) have precluded further understanding of the processes that acted on the oil.In this study we focus on testing the hypothesis that oil particles suspended in the deep intrusion layers were deposited on the sea floor over a broad area. To do so, we use publicly available data generated as part of the ongoing Natural Resource Damage Assessment (NRDA) process (Supporting Information) to assess the spatial distribution of petroleum hydrocarbons in the deep-ocean sediments of the Gulf of Mexico. We focus on the recalcitrant compound 17α(H),21β(H)-hopane (hereafter referred to as “hopane”) as a conserved tracer for crude oil deposition to sediments (25); we treat hopane as a degradation-resistant proxy for Macondo’s liquid-phase oil (26). Analysis of the spatial distribution of hopane allows us to define both a regional background level and a depositional footprint of oil from the Deepwater Horizon event. In combination with other lines of evidence, this analysis leads us to conclude that significant quantities of particulate oil sank from the intrusion layers to rest on the underlying sea floor.