Resuspension, Redistribution and Deposition of Deep Water Horizon (DwH) Recalcitrant Hydrocarbons to offshore Depocenters (REDIRECT)
Arne Diercks, Gregg Brooks, Jeff Chanton and Isabel Romero
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, (1)) 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.
The GoM plays a vital role in the regional and national economy. The Northern Gulf is underlain by extensive hydrocarbon reservoirs and petroleum exploration, and extraction and refinement industries are the economic backbones of the region. The economic impact of Gulf commercial fisheries industry is still recovering from the 2010 DwH oil spill. Oil that reached the seafloor and that was incorporated in bottom sediments was not included as a term in the oil budget produced by the oil budget calculator group (5). This group estimated that 11-30% of the released oil was unaccounted for or listed as “other.” While the sedimentation of oil is often discussed, for example Jernelöv and Lindén (13) speculated that 25% of the 475,000 metric tons of oil released from the 1979 Ixtoc spill went to the seafloor, its role has never properly been assessed. Valentine et al. (3) estimated that 1.8 to 14.4% of the oil remained in the deep ocean reached the seafloor, while Chanton et al. (2) indicated that the amount was between 0.5 to 9.1% of the total oil released by the DwH spill. Both of these studies examined areas around the DwH wellhead and to the southwest. The downslope areas to the south and east have not been examined in any detail. The results obtained in this effort will provide an important input to our understanding of the fate of the oil released during the DwH oil spill. The wide-ranging sampling program proposed here, using state of the art sampling techniques, will provide measurements of spatial deposition, transport and distribution of oiled surface sediments to determine the long-term fate of the DwH oil in the GoM deep environment. These data will contribute to the overall objectives of the GoMRI research goals and will provide key data to model the long-term fate of the missing oil.
Resuspended material from the deep seafloor at 1,200m water depth. Individual particles are illuminated by two strobes arranged perpendicular to the camera.
Coring Sites visited during May 2018 Point Sur cruise.
Cores collected during May 2018 cruise. Note the clean overlying water and the undisturbed sediment - water interface. Perfect cores and perfect samples for scheduled flume analyses.
Science party of Point Sur cruise in May 2018.
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