Petrocarbon evolution: Ramped pyrolysis/oxidation and isotopic studies of contaminated oil sediments from the Deepwater Horizon oil spill in the Gulf of Mexico

RESEARCH ARTICLE Petrocarbon evolution: Ramped pyrolysis/ oxidation and isotopic studies of contaminated oil sediments from the Deepwater Horizon oil spill in the Gulf of Mexico a1111111111 a1111111111 a1111111111 a1111111111 a1111111111 OPEN ACCESS Citation: Rogers KL, Bosman SH, Lardie-Gaylord M, McNichol A, Rosenheim BE, Montoya JP, et al. (2019) Petrocarbon evolution: Ramped pyrolysis/ oxidation and isotopic studies of contaminated oil sediments from the Deepwater Horizon oil spill in the Gulf of Mexico. PLoS ONE 14(2): e0212433. https://doi.org/10.1371/journal.pone.0212433 Editor: Lee W. Cooper, University of Maryland Center for Environmental Science, UNITED STATES Received: July 26, 2018 Accepted: February 2, 2019 Published: February 28, 2019 Copyright: © 2019 Rogers et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: Data are publicly available through the Gulf of Mexico Research Initiative Information & Data Cooperative (GRIIDC) at https://data.gulfresearchinitiative.org, 10.7266/ N7KH0KWJ and 10.7266/N7Q52N7D. Funding: This research was made possible by grants from The Gulf of Mexico Research Initiative through its consortiums: Ecosystem Impacts of Oil & Gas Inputs to the Gulf (ECOGIG), The Center for the Integrated Modeling and Analysis of the Gulf Kelsey L. Rogers ID1¤*, Samantha H. Bosman1, Mary Lardie-Gaylord2, Ann McNichol2, Brad E. Rosenheim3, Joseph P. Montoya4, Jeffrey P. Chanton1 1 Department of Earth, Ocean and Atmospheric Science, Florida State University, Tallahassee, Florida, United States of America, 2 NOSAMS, Woods Hole Oceanographic Institute, Woods Hole, Massachusetts, United States of America, 3 College of Marine Science, University of South Florida, St. Petersburg, Florida, United States of America, 4 School of Biological Sciences, Georgia Institute of Technology, Atlanta, Florida, United States of America ¤ Current address: Department of Geosciences and Natural Resources Management, University of Copenhagen, Copenhagen, Denmark * Abstract Hydrocarbons released during the Deepwater Horizon (DWH) oil spill weathered due to exposure to oxygen, light, and microbes. During weathering, the hydrocarbons’ reactivity and lability was altered, but it remained identifiable as “petrocarbon” due to its retention of the distinctive isotope signatures (14C and 13C) of petroleum. Relative to the initial estimates of the quantity of oil-residue deposited in Gulf sediments based on 2010–2011 data, the overall coverage and quantity of the fossil carbon on the seafloor has been attenuated. To analyze recovery of oil contaminated deep-sea sediments in the northern Gulf of Mexico we tracked the carbon isotopic composition (13C and 14C, radiocarbon) of bulk sedimentary organic carbon through time at 4 sites. Using ramped pyrolysis/oxidation, we determined the thermochemical stability of sediment organic matter at 5 sites, two of these in time series. There were clear differences between crude oil (which decomposed at a lower temperature during ramped oxidation), natural hydrocarbon seep sediment (decomposing at a higher temperature; Δ14C = -912‰) and our control site (decomposing at a moderate temperature; Δ14C = -189‰), in both the stability (ability to withstand ramped temperatures in oxic conditions) and carbon isotope signatures. We observed recovery toward our control site bulk Δ14C composition at sites further from the wellhead in ~4 years, whereas sites in closer proximity had longer recovery times. The thermographs also indicated temporal changes in the composition of contaminated sediment, with shifts towards higher temperature CO2 evolution over time at a site near the wellhead, and loss of higher temperature CO2 peaks at a more distant site. PLOS ONE | https://doi.org/10.1371/journal.pone.0212433 February 28, 2019 1 / 21 Ramped pyrolysis/oxidation of oil contaminated sediment Ecosystem (C-Image), and Deep Sea to Coast Connectivity in the Eastern Gulf of Mexico (Deep-C) and the Resuspension, Redistribution and Deposition of DWH Recalcitrant Material (ReDirect) project. This is ECOGIG Contribution # 521. Funding was also provided by the National Ocean Sciences Accelerator Mass Spectrometry Facility (NOSAMS) Graduate Student Internship Program (NSF OCE-1239667). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing interests: The authors have declared that no competing interests exist. Introduction The results of a number of field studies indicate unambiguously that oil residues from the Deepwater Horizon (DWH) oil spill were deposited on the seafloor [1–7]. Of the total oil released, an estimated 0.5–14.4% was deposited on the seafloor [1,3]. Passow and Ziervogel [8] argued that these estimates were low because they failed to consider the formation of marine oil snow over the total spread of the surface oil slicks, which could have resulted in a greater extent of seafloor deposition. The bulk of the sedimented oil-residue was limited to the surface sediment as defined by radiocarbon [3], hopane [1,4], and other radioisotopes [2]. The severity of impacts on benthic communities depends on the nature of the petroleumderived material which was deposited on the seafloor. It has been suggested that biodegradation and dissolution of oil in the water column prior to deposition on the seafloor moderated these impacts [5,7,9]. We used ramped pyrolysis oxidation (RPO) to assess the biodegradation state of the material present on the seafloor due to the blowout. With RPO we examined 5 sites in all, 3 contaminated sites, two in time series and one as a function of depth, a control uncontaminated site and a natural seep site. Several studies have analyzed the recovery of contaminated sediments and have shown a reduction in the overall extent of contamination and have estimated degradation rates. Stout et al. [4] and Adhikari et al. [10] showed reduced coverage of elevated levels of hopane and polycyclic aromatic hydrocarbons (PAHs) in the years following the blowout. Studies by Stout and Payne [5] and Bagby et al. [9] analyzed biodegradation rates of multiple hydrocarbons in the sediment, showing that biodegradation continued on the seafloor after the deposition of the sedimented oil-residues. In contrast to focusing on specific petroleum compounds, studies by Pendergraft et al. [11] and Pendergraft and Rosenheim [12] employed ramped pyrolysis/oxidation paired with carbon isotope analysis on bulk coastal sediments. We applied their approach to the deep-sea floor. RPO is an approach to determine the thermochemical stability of organic matter [13]. When paired with δ13C and Δ14C isotopic analysis, the source of the carbon can be inferred as a function of thermal (...truncated)