Hydrocarbons in Deep-Sea Sediments following the 2010 Deepwater Horizon Blowout in the Northeast Gulf of Mexico

May Hydrocarbons in Deep-Sea Sediments following the 2010 Deepwater Horizon Blowout in the Northeast Gulf of Mexico Isabel C. Romero 0 1 2 3 Patrick T. Schwing 0 1 2 3 Gregg R. Brooks 0 1 2 3 Rebekka A. Larson 0 1 2 3 David W. Hastings 0 1 2 3 Greg Ellis 0 1 2 3 Ethan A. Goddard 0 1 2 3 David J. Hollander 0 1 2 3 0 Funding: The authors acknowledge the British Petroleum/Florida Institute of Oceanography (BP/ FIO)-Gulf Oil Spill Prevention, Response, and Recovery Grants Program for funding several of the initial research cruises and laboratory analysis during 2010 and 2011. This research was made possible by funding from BP/The Gulf of Mexico Research Initiative (GOMRI), specifically the Center for 1 Data Availability Statement: Data are available from The Gulf of Mexico Research Initiative Information and Data Cooperative (GRIIDC) , UDI: R1. x135.119:0004 2 Academic Editor: Wei-Chun Chin, University of California , Merced, UNITED STATES 3 1 University of South Florida, College of Marine Science , St. Petersburg, Florida, 33701 , United States of America, 2 Eckerd College , St. Petersburg, Florida, 33711 , United States of America The Deepwater Horizon (DWH) spill released 4.9 million barrels of oil into the Gulf of Mexico (GoM) over 87 days. Sediment and water sampling efforts were concentrated SW of the DWH and in coastal areas. Here we present geochemistry data from sediment cores collected in the aftermath of the DWH event from 1000 - 1500 m water depth in the DeSoto Canyon, NE of the DWH wellhead. Cores were analyzed at high-resolution (at 2 mm and 5 mm intervals) in order to evaluate the concentration, composition and input of hydrocarbons to the seafloor. Specifically, we analyzed total organic carbon (TOC), aliphatic, polycyclic aromatic hydrocarbon (PAHs), and biomarker (hopanes, steranes, diasteranes) compounds to elucidate possible sources and transport pathways for deposition of hydrocarbons. Results showed higher hydrocarbon concentrations during 2010-2011 compared to years prior to 2010. Hydrocarbon inputs in 2010-2011 were composed of a mixture of sources including terrestrial, planktonic, and weathered oil. Our results suggest that after the DWH event, both soluble and highly insoluble hydrocarbons were deposited at enhanced rates in the deep-sea. We proposed two distinct transport pathways of hydrocarbon deposition: 1) sinking of oil-particle aggregates (hydrocarbon-contaminated marine snow and/or suspended particulate material), and 2) advective transport and direct contact of the deep plume with the continental slope surface sediments between 1000-1200 m. Our findings underline the complexity of the depositional event observed in the aftermath of the DWH event in terms of multiple sources, variable concentrations, and spatial (depth-related) variability in the DeSoto Canyon, NE of the DWH wellhead. - Introduction Sediments serve as repository systems for a large range of organic matter sources and hydrocarbons that can be used to assess historical impacts to the environment. In the northern Gulf of Mexico (GoM), the composition of organic matter deposited in deep-sea sediments is Integrated Modeling and Analysis of the Gulf Ecosystem (C-IMAGE) and the Deep Sea to Coast Connectivity in the Eastern Gulf of Mexico (Deep-C) consortia. Competing Interests: The authors declare that that the funding source (Gulf of Mexico Research Initiative) does not alter the authors' adherence to PLOS ONE policies on sharing data and materials. controlled by physical sorting of particles (regional hydrodynamics) and the off-shore movement of the less dense material from terrigenous sources transported by the Mississippi River [1,2]. In shallow coastal areas (e.g. estuaries) the terrigenous pool is primarily composed of fresh vascular plant detritus whereas in deeper areas off-shore it is comprised of highly altered organic matter from angiosperm grassland soils [3,4]. Compared to sediments in shallow areas, terrigenous derived matter in deep-sea sediments is typically recalcitrant [3]. Marine-derived organic matter content in sediments is greater in areas associated with high rates of primary productivity in the surface waters [4]. Other sources of sedimentary organic carbon are erosion of sedimentary rocks from the Mississsipi River drainage basin, and fossil fuels from natural oil seeps and petroleum exploration [3,5,6]. It is estimated that an average of 95,500 tons oil enters the GoM annually from natural seeps (73%), oil and gas extraction activities (3%), transportation activities (4%), and oil combustion byproducts (~16%) [7]. In comparison, the Deepwater Horizon (DWH) spill in 2010 released 4.9 million barrels of oil into the Gulf of Mexico (~699,700 metric tons) [8], an amount over seven times the average annual input of oil into the GoM [9]. The DWH blowout was unique not only for its size but also its depth at 1500 m below the sea surface. The released oil partitioned into oil drops, gas bubbles, and gas hydrates with accompanied partitioning of petroleum hydrocarbons into aqueous, gas and particulate phases. A mixture of soluble and insoluble petroleum hydrocarbons reached the sea surface, where it evaporated (5% of the leaked mass), formed sheens and slicks (10% of the leaked mass), was mechanically recovered (20% of the leaked mass) or burned (6% of the leaked mass) [10]. In addition, a lateral plume at ~1000 1200 m depth formed (35% of the leaked mass) rich in water-soluble gases and compounds [1014]. The final fate of the hydrocarbons from the subsurface plume is unknown, although dissolution and biodegradation have been proposed as important processes [11,14,15]. Following the blowout of the DWH drilling rig, an unusually large marine snow event was observed [16,17]. The large depositional event may have occurred by marine snow formation from surface oil slicks (containing ~0.5 million barrels of liquid oil) and the subsurface deep plume (containing ~1.7 million barrels of liquid oil). Despite the broad accounts of chemical composition of hydrocarbons at sea surface and depth in the GoM in 2010 [10] and their fate in the environment [1828], a comprehensive discussion of possible transport pathways of oil into the deepwater sediment environment and a hydrocarbon inventory for 2010 is not available. The primary objectives of this study are to contribute to the overall understanding of hydrocarbon geochemistry in deepwater sediments by providing information on the concentration and composition of sediments samples collected from the DeSoto Canyon, NE of the DWH; and to interpret these data within the context of the possible sources and transport pathways of hydrocarbons to the deep sea during the period of the study. Materials and methods Sample collection Sediment cores were collected in December 2010 and February 2011 on board Florida of Institute of Oceanogrpahys (FIO) R/V WeatherBird II during oil spill response cruises WB1111 and WB1114. Thr (...truncated)