The fate and distribution of oil residues in deep-sea sediments resulting from the sinking of marine snow in the northern Gulf of Mexico

The Deepwater Horizon (DWH) was the largest spill in US history, with ~4 million barrels of oil released, a discharge over seven times larger than the average annual input of oil (i.e., natural seeps, transportation, land, and air derived) into the Gulf of Mexico (GoM). Following the DWH blowout, the formation of an unexpected and extended sedimentation event of oil-associated marine snow (MOSSFA) demonstrated the importance of marine snow on the fate of contaminants in the oceans. Despite the numerous studies about DWH oil-residues across a broad range of environments in the GoM, a comprehensive evaluation of the fate of oil-associated marine snow at a large spatial and temporal scale is not available. We used a wide range of chemical data and diagnostic ratios (aliphatics, hopanes, steranes, triaromatic steroids, polycyclic aromatics) collected in 2011 and 2018 (ERMA-NRDA and REDIRECT projects) to assess the spatial extent and weathering of DWH oil-residues in sediments. Results indicate that 85% of the deep-sea area in 2011 was contaminated with DWH oil-residues up to a distance of 180 km from the DWH site. The sedimentary chemical data revealed the different hydrocarbon mixtures formed after released into the environment (e.g., oil slicks, submerged-plumes), indicating most (~98%) of the deposition originated from MOSSFFA event (sea surface and water column) and not from the submerged plumes. In 2018, DWH oil-residues were found up to ~100 km southeast of the spill site. Further comparisons will be discussed relative to the role of marine snow (as organic matter deposition) in the transport of oil-residues (containing DWH oil), persistent organic pollutants, and natural compounds to the seafloor in the northern GoM.