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

Abstract

The 2010 Deepwater Horizon disaster introduced an unprecedented discharge of oil into the deep Gulf of Mexico. Considerable uncertainty has persisted regarding the oil’s fate and effects in the deep ocean. In this work we assess the compound-specific rates of biodegradation for 125 aliphatic, aromatic, and biomarker petroleum hydrocarbons that settled to the deep ocean floor following release from the damaged Macondo Well. Based on a dataset comprising measurements of up to 168 distinct hydrocarbon analytes in 2,980 sediment samples collected within 4 y of the spill, we develop a Macondo oil “fingerprint” and conservatively identify a subset of 312 surficial samples consistent with contamination by Macondo oil. Three trends emerge from analysis of the biodegradation rates of 125 individual hydrocarbons in these samples. First, molecular structure served to modulate biodegradation in a predictable fashion, with the simplest structures subject to fastest loss, indicating that biodegradation in the deep ocean progresses similarly to other environments. Second, for many alkanes and polycyclic aromatic hydrocarbons biodegradation occurred in two distinct phases, consistent with rapid loss while oil particles remained suspended followed by slow loss after deposition to the seafloor. Third, the extent of biodegradation for any given sample was influenced by the hydrocarbon content, leading to substantially greater hydrocarbon persistence among the more highly contaminated samples. In addition, under some conditions we find strong evidence for extensive degradation of numerous petroleum biomarkers, notably including the native internal standard 17α(H),21β(H)-hopane, commonly used to calculate the extent of oil weathering.

Petroleum dynamics in the sea and influence of subsea dispersant injection during Deepwater Horizon

“…In contrast, our results reveal that only 0.6-2.0% of the insoluble material released from the wellhead was channeled into the deep-sea intrusion, after the riser pipe was pared at the wellhead. Assuming that seafloor residues originated primarily from the deep-sea intrusion (24,26), this discrepancy is consistent with the likely possibility (discussed above) that the mass flow of liquid petroleum droplets into the deep-sea intrusion was higher before the riser pipe was pared on June 3. Alternatively, the seafloor residues may originate from other sources that supplemented contributions from the intrusion.…”

“…Although deposition processes were not included in our simulations, our results confirm the unambiguous presence of ≤130-μm liquid microdroplets in the deep-sea intrusion, which would have contributed to the "bathtub ring" previously observed in sediments where the intrusion impinged on the continental slope at 900-to 1,300-m depth (26). Our simulations further predict rapid dissolution of semisoluble compounds (up to C 2 -naphthalenes) from suspended ≤130-μm microdroplets in the intrusion (SI Appendix, Table S5 and SI Text, S-1.10), which would explain the finding (24,27) that petroleum residues deposited on the seafloor were severely depleted of two-ring and three-ring polycyclic aromatic hydrocarbons (PAHs).…”

“…In addition, previous reports have estimated that 1.9-14.8% (24,26) or 0.5-9.1% (28) of the released insoluble oil mass was deposited on the seafloor largely within 40 km of the wellhead, based on the observed concentrations of recalcitrant compounds such as hopane and n-C 38 in seafloor sediments (24,26). In contrast, our results reveal that only 0.6-2.0% of the insoluble material released from the wellhead was channeled into the deep-sea intrusion, after the riser pipe was pared at the wellhead.…”

“…These discrepancies suggest that paring the riser pipe decreased the flow of liquid petroleum compounds into the deep-sea intrusion. The outcomes from this technical intervention are relevant for interpreting bioavailability (10), ecological impact (20,21), and biodegradation (5,(22)(23)(24)(25) of petroleum compounds, all of which depend upon the droplet sizes, liquid/dissolved distribution, and altered chemical composition of petroleum residues in the deep sea.…”

“…Seafloor. Recent studies suggest that the deep-sea intrusion may have contributed to the extensive deposition of weathered petroleum residues observed on the seafloor within a 3,200-km 2 area near the wellhead (24,26,27). Although deposition processes were not included in our simulations, our results confirm the unambiguous presence of ≤130-μm liquid microdroplets in the deep-sea intrusion, which would have contributed to the "bathtub ring" previously observed in sediments where the intrusion impinged on the continental slope at 900-to 1,300-m depth (26).…”