Potential for redistribution of DwH-impacted bottom sediments to down-slope depocenters: Eastern Gulf of Mexico

Abstract

The 2010 Deepwater Horizon (DwH) oil spill was responsible for dispersing an estimated 4.9 million barrels of oil into the northeastern Gulf of Mexico. Much was transported rapidly to the seafloor during a sediment pulse, but the ultimate fate of the oiled sediments remains unknown. This study investigates the potential for bottom sediments within the initial DwH impact zone to be mobilized and redeposited in down-slope depocenters. Sediment cores were collected from 31 sites up to 250 km down-slope (SE) of the initial seafloor impact zone to search for evidence of sediment redistribution, and whether the sediment source area(s) included the initial impact zone. Twenty-seven of the 31sites contained sedimentary structures indicative of sediment redistribution, primarily by low density turbidity currents. Other evidence of sediment redistribution includes elevated benthic foraminiferal fracture percentage, rapid and pulsed accumulation rates and the paucity of extensive pelagic deposition. Sediments consist dominantly of siliciclastic muds, with subordinate carbonate sandy muds. The most likely source for siliciclastic muds is the Mississippi River Delta and adjacent continental margin as it is immediately up-slope from the study area. This area includes the DwH seafloor impact zone. DwH oil residues were detected in sediments deposited at different times (post 2010), at 13 core sites, up to 96 km SE of the impact zone, which is consistent with multiple redistribution episodes capable of transporting oiled sediments long distances. Results of this study are not only important for developing mitigation strategies for future contaminant spills, but as sediment redistribution processes can be quite destructive, information regarding process type and frequency can be vital for planning future activities on the sea floor.