Study Assessed Aging Oil Spill Material on the Seafloor and Found Recovery

October 09, 2019

Scientists analyzed radiocarbon isotopes, which identify the source of carbons in compounds such as oil and methane, and applied those “fingerprints” to quantify recovery of deep-seafloor sediment contaminated by Deepwater Horizon. Comparisons of sediments collected from 2010-2017 showed a gradual return to baseline conditions. Recovery in areas further away from the Macondo wellhead (~90km) took about four years and areas nearer the wellhead (~17km) took about five-six years. There was a shift from lower to higher thermochemically-stable hydrocarbon compounds over time, indicating that petroleum-based material was being degraded. That shift, which is related to recovery speed, depended on contamination level, the distance from the wellhead, and the distance that oil traveled in sub-surface plumes before sinking.

The authors published their results in PLOS ONE: Petrocarbon evolution: Ramped pyrolysis/oxidation and isotopic studies of contaminated oil sediments from the Deepwater Horizon oil spill in the Gulf of Mexico.

Nearly half of the hydrocarbons released from the Macondo wellhead rose to the ocean’s surface and formed a thick oil slick. Large oil droplets (>100 μm) sank relatively rapidly from the surface slick through the water column while smaller droplets (marine oil snow aggregates that sank to the seafloor from fall 2010 through early 2011. The flux of oil-contaminated material to the seafloor reduced the size and diversity of the benthic community, which affected bioturbation processes.   

This study’s team assessed sediment at three oil-impacted sites, a natural seep site, and an uncontaminated site to better understand the current conditions of contaminated sediment. Their measurements showed increasing isotopic enrichment, indicating recovery of bulk radiocarbon and stable carbon isotopes in sediments.

Study author Kelsey Rogers explained their methods, “We used ramped pyrolysis/oxidation paired with carbon isotope analyses (¹³C and ¹⁴C) to study how contaminated sediment recovered following the Deepwater Horizon blowout. During ramped oxidation, the sample is slowly heated up to ~900-1000C, releasing CO₂ as the carbon compounds are oxidized. The temperature at which the CO₂ comes off the sample tells us how labile the compounds are (easy/difficult it can be broken down), with low temperatures meaning it is easier, high temperatures it is more difficult. Combing this idea of the compounds’ lability with its ¹³C and ¹⁴C isotope signatures, we can determine what fraction is from petrocarbon verses modern sources.” Rogers acknowledged the National Ocean Sciences Accelerator Mass Spectrometry facility’s graduate student interns as having greatly assisted with the analyses.

The authors also noted differences between carbon sources in oiled samples (having a surface veneer of fossil carbon overlying more ¹⁴C enriched or “younger” carbon) and natural seep site samples (relatively uniform ¹⁴C depleted fossil carbon signature through all depths). This study also provided insights into how far plume-associated oil travelled (up to 190 km southwest of the wellhead) and the origin of sedimentary organic matter in the Gulf of Mexico (see Elementa Science of the Anthropocene by Chanton et al).

Rogers put their results into context, “This is just one measurement in the whole of the Gulf of Mexico environment and is part of the bigger picture of petrocarbon degradation. Several factors influence the degradation of petrocarbon, which is much slower once it is deposited on the sediment than it is in the water column. Hopefully a spill like the Deepwater Horizon blowout will never happen again but given the amount of oil operations in the Gulf of Mexico, the more we know about how the sediment and ecosystem of the recovers, the better prepared we will be for any future events.”

Data are publicly available through the Gulf of Mexico Research Initiative Information & Data Cooperative (GRIIDC) at DOI:10.7266/N7Q52N7D and DOI:10.7266/N7KH0KWJ. 

The study’s authors are Kelsey L. Rogers, Samantha H. Bosman, Mary Lardie-Gaylord, Ann McNichol, Brad E. Rosenheim, Joseph P. Montoya, and Jeffrey P. Chanton.

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This article was written by Nilde Maggie Dannreuther and originally appeared online here. Contact maggied@ngi.msstate.edu with questions or comments.

This research was made possible in part by a grant from the Gulf of Mexico Research Initiative (GoMRI) to the Ecosystem Impacts of Oil & Gas Inputs to the Gulf 2 (ECOGIG 2), the Center for the Integrated Modeling and Analysis of the Gulf Ecosystem-II (C-Image-II), the Center for the Integrated Modeling and Analysis of the Gulf Ecosystem-III  (C-IMAGE-III), the Deep Sea to Coast Connectivity in the Eastern Gulf of Mexico (Deep-C) consortium, and to the University of Southern Mississippi for the project Resuspension, Redistribution, and Deposition of Deep Water Horizon (DwH) Recalcitrant Hydrocarbons to offshore Depocenters.

Funding was also provided by the National Ocean Sciences Accelerator Mass Spectrometry Facility (NOSAMS) Graduate Student Internship Program (NSF OCE-1239667).

The Gulf of Mexico Research Initiative (GoMRI) is a 10-year independent research program established to study the effect, and the potential associated impact, of hydrocarbon releases on the environment and public health, as well as to develop improved spill mitigation, oil detection, characterization and remediation technologies. An independent and academic 20-member Research Board makes the funding and research direction decisions to ensure the intellectual quality, effectiveness and academic independence of the GoMRI research. All research data, findings and publications will be made publicly available. The program was established through a $500 million financial commitment from BP. For more information, visit http://gulfresearchinitiative.org/.