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). 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. 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. 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 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). 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. Using a recalcitrant biomarker of crude oil, 17α(H),21β(H)-hopane (hopane), we have identified a 3,200-km 2 region around the Macondo Well contaminated by ∼1.8 ± 1.0 × 10 6 g of excess hopane. 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. 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. 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.

Fallout by Lisa Henry