Abstract
The focus of this study was to determine the long-term fate of oil-residues from the 2010 Deepwater Horizon (DwH) oil spill due to remobilization, transport, and re-distribution of oil residue contaminated sediments to down-slope depocenters following initial deposition on the seafloor. We characterized hydrocarbon residues, bulk sediment organic matter, ease of resuspension, sedimentology, and accumulation rates to define distribution patterns in a 14,300 km2 area southeast of the DwH wellhead (1,500 to 2,600 m water depth). Oil-residues from the DwH were detected at low concentrations in 62% of the studied sites at specific sediment layers, denoting episodic deposition of oil-residues during 2010–2014 and 2015–2018 periods. DwH oil residues exhibited a spatial distribution pattern that did not correspond with the distribution of the surface oil slick, subsurface plume or original seafloor spatial expression. Three different regions were apparent in the overall study area and distinguished by the episodic nature of sediment accumulation, the ease of sediment resuspension, the timing of oil-residue deposition, carbon content and isotopic composition and foram fracturing extent. These data indicate that resuspension and down-slope redistribution of oil-residues occurred in the years following the DwH event and must be considered in determining the fate of the spilled oil deposited on the seafloor.
Highlights
A large percentage of the oil released during the 2010 Deepwater Horizon (DwH) spill in the northern Gulf of Mexico (GoM) was either chemically or naturally dispersed (Lehr et al, 2010; Lubchenco et al, 2012) and/or settled to the seafloor associated with the Marine Oil Snow Sedimentation and Flocculent Accumulation event (MOSSFA) (Passow et al, 2013; Daly et al, 2016)
Core analyses revealed a range of sediment textures, composition and accumulation rates indicating that sedimentation processes varied throughout the study area
All other analyses focused on the surficial sediments (0– 20 mm) to identify recent patterns that may be associated with deposition and redistribution of DwH contaminated sediments
Summary
A large percentage of the oil released during the 2010 Deepwater Horizon (DwH) spill in the northern Gulf of Mexico (GoM) was either chemically or naturally dispersed (Lehr et al, 2010; Lubchenco et al, 2012) and/or settled to the seafloor associated with the Marine Oil Snow Sedimentation and Flocculent Accumulation event (MOSSFA) (Passow et al, 2013; Daly et al, 2016). Increased short-term (months) sedimentation rates reflected the rapid transport of sediments to the seafloor associated with the MOSSFA event (Brooks et al, 2015; Romero et al, 2015, 2017; Yan et al, 2016; Larson et al, 2018). Layers of detectable oil-contaminated sediment were deposited on the seafloor directly below the extent of the surface slick or under the southwest deep sea plume (Valentine et al, 2014; Brooks et al, 2015; Chanton et al, 2015; Romero et al, 2015, 2017; Larson et al, 2018). A sampling bias toward seafloor locations under the surface slicks and the deep plumes (Camilli et al, 2010; Diercks et al, 2010) exists, and sediments were sampled at only few of the locations outside these areas (Schwing et al, 2017)
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