Resuspension, Redistribution and Deposition of Deepwater Horizon Recalcitrant Hydrocarbons to Offshore Depocenters

What is the fate of the sediments deposited immediately after the 2010 Deepwater Horizon oil spill?

Has the seafloor petrocarbon been degraded? Or resuspended and advected? Let's look downhill!

The research team proposes a geospatial, sedimentological, and geochemical approach to study the fate of the Deepwater Horizon (DwH) marine oil snow (MOS) deposited during the Marine Oil Snow Sedimentation and Flocculent Accumulation (MOSSFA) event on the seafloor in offshore waters of the Gulf of Mexico (GoM).

The central hypothesis of the proposed research is that recalcitrant compounds from the deposited oil still remain on the seafloor ~7 years after the DwH spill, however, their spatial distribution and concentration do not correspond with the distribution of the surface oil slick or the subsurface plume due to resuspension and redistribution following initial deposition on the seafloor.

Natural heterogeneity of bottom topography and circulation processes are key drivers transporting materials to deeper areas in the GoM by erosion and deposition of contaminated sediments beyond the surface extent of the once existing oil slick or the subsurface plumes(s). The results obtained in this effort will illuminate the fate of the oil released during the DwH oil spill, which is important for developing new mitigation strategies for future submerged spills in the GoM and worldwide. The seafloor depositional environment is highly diverse, which should result in a non-homogeneous distribution of material delivered from the overlying water column. Seafloor sediment is affected by currents, bottom morphology, and physical forcing events of different temporal and spatial scales that rework deposited material within the Bottom Nepheloid Layer (BNL).

We hypothesize that the area of seafloor that was affected by deposition of oil from the spill is larger than the area of the surface slick extending into “downhill” areas on the seafloor (>1,500m) that have not been considered as having been affected by oil derived hydrocarbons. Through down-gradient transport, material will follow pathways based on the highly variable seafloor morphology, with its hills, slopes and canyons, allowing for erosion and deposition beyond the spatial extent of the once existing oil slick or subsurface plumes(s), and beyond the distributions mapped in the year following the spill, thus potentially affecting an area much larger than originally described and has been sampled to date.

The proposed research will transform our understanding of the sinks, distribution, and transport processes of sedimentary hydrocarbons by investigating the connectivity of geomorphological features (erosional channels, lee depocenters, and isolated valleys), and the sedimentological and physical oceanographic processes (resuspension, advection and re-sedimentation) affecting oil-derived hydrocarbon distribution and deposition within the BNL in the deep sea. This research will build on and synthesize information from seafloor topographic and petrocarbon mapping and sediment studies by ECOGIG, C-IMAGE and Deep-C consortia as well as deep sea current measurements and model data provided by the GISR and ECOGIG-II consortia. This project will address the impacts of hydrocarbon deposition in a much broader spatial scale than previously studied in the GoM through an integrated research program that will address GoMRI research theme 1.

The research team from four coastal Gulf of Mexico institutions brings together experience in geomorphology, sedimentology, physical oceanography and geochemistry, and integrates experienced sea-going research with premier shore-based analytical capabilities to explore the dynamics of deep sea sedimentation and subsequent redistribution of MOSSFA. An Ultra Short Base Line (USBL) guided Multicorer will be used to precisely sample the seafloor environment at >1500 m depths at locations predicted through watershed modeling of the seafloor. Existing data of sediment texture/composition, short-lived radioisotopes (210Pb, 234Th) and Barium (drilling mud) concentrations will be synthesized with new data to determine sediment source(s), transport mechanisms/pathways, and sediment focusing/accumulation rates in support of the redistribution hypothesis proposed. Petrocarbon content (14C) and organic geochemical analyses (aliphatics, aromatics, hopanes, and steranes) will be used to quantify the concentration and degree of weathering of hydrocarbon residues. Laboratory flume analyses will test the behavior of the surface layer to determine threshold current velocities necessary to re-suspend size specific particles and subsequently the behavior during transport of the resuspended material, which can be utilized in sediment transport models. This study will develop a spatial and temporal perspective of the MOS deposited on the seafloor to compare with the MOS projected to have formed in the water column.