Study Uses Radioisotopes to Trace Marine Oil Snow Associated with Deepwater Horizon

A DIVA gridded contour map of the different values for 210Pbxs flux before and after the Deepwater Horizon incident. Polygons represent the spatial extent of differential 210Pbxs  flux greater than 3 dpm cm−2 yr−1 (blue), 1.5 dpm cm−2 yr−1 (black), and a minimum span between coring sites (red) in the western study area and greater than 1 dpm cm−2 yr−1 (black) and minimum span between coring sites (red) in the eastern study area. The gray overlay is the cumulative total extent of surface petroleum coverage. The star represents the location of the Macondo wellhead. Figure 5 in the study, provided by Patrick Schwing.

A DIVA gridded contour map of the different values for 210Pbxs flux before and after the Deepwater Horizon incident. Polygons represent the spatial extent of differential 210Pbxs flux greater than 3 dpm cm−2 yr−1 (blue), 1.5 dpm cm−2 yr−1 (black), and a minimum span between coring sites (red) in the western study area and greater than 1 dpm cm−2 yr−1 (black) and minimum span between coring sites (red) in the eastern study area. The gray overlay is the cumulative total extent of surface petroleum coverage. The star represents the location of the Macondo wellhead. Figure 5 in the study, provided by Patrick Schwing.

Researchers used naturally occurring radioisotopes to quantify the footprint of sedimented marine oil snow on the Gulf of Mexico seafloor following the Deepwater Horizon incident. The team estimated that the total spatial extent of marine oil snow to be between 12,805 and 35,425 km2. Sediment dated after the spill indicated a significant increase in radioisotope flux (the amount of radioisotopes moving from the water column into the sediments) compared to sediment dated before the spill. Increased fluxes were primarily centered around the spill site, suggesting that these fluxes were the result of sinking marine oil snow and not natural variations. The researchers published their findings in Environmental Science & Technology: Constraining the spatial extent of marine oil snow sedimentation and flocculent accumulation following the Deepwater Horizon event using an excess 210Pb flux approach.

Microbes and phytoplankton secreted exopolymeric substances as they came into contact with Deepwater Horizon oil, which created oily aggregate material that sank to the seafloor (see Study Explains Pathways for Oiled Marine Snow Formation and Study Shows Oil Promoted Formation of a Different Kind of Marine Snow). This deposition event is known informally as the “dirty blizzard” and formally as the marine oil snow sedimentation and flocculent accumulation or MOSSFA (see Study Summarizes Current Knowledge on Marine Oil Snow During and After Deepwater Horizon).

The scientists in this study used gamma spectrometry to analyze radioisotope (210Pb) flux in sediment cores collected from 32 sites during 2010 – 2013 (the authors provided a video of their high-resolution extrusion method). The team related radioisotope flux in sediment sections dated from 1900 to 2013 to water depth and distance from the wellhead.

All core measurements indicated an increase in radioisotope flux between 1900-1950, but the most significant increase occurred in sediment dated from 2010 – 2013. Increased radioisotope fluxes were predominantly in sediment collected in western study sites (stretching on an axis 230 km southwest to 140 km northeast of the wellhead) and in eastern sites (on an axis stretching 70 km northeast to southwest near the Desoto Canyon). Water depth was the primary determinant of radioisotope flux before the spill whereas sinking marine oil snow primarily controlled radioisotope flux afterwards.

The study’s spatial coverage measurements agree well with other chemical tracers of MOSSFA and areas of seafloor impact assessments (see Study Examines Sediment East of Deepwater Horizon for Oil-Associated Marine Snow, Study Documents Initial Impacts and Recovery of Benthic Foraminifera after Deepwater Horizon, and Study Reveals Oil Spill Changed Oxygen Conditions in Gulf Sediment). Additionally, the results are in line with the sea-surface extent of the oil spill (among the authors’ references are two Natural Resource Damage Assessment reports: Spatial Extent (“Footprint”) and Volume of Macondo Oil Found on the Deep-Sea Floor Following the Deepwater Horizon Oil Spill and Characterization and Flux of Marine Oil Snow into the Visca Knoll (Lophelia Reef) Area due to the Deepwater Horizon Oil Spill).

“MOSSFA caused long-term (at least 3-5 years) changes to the chemical and physical setting on the seafloor of the northern Gulf of Mexico, which in turn had implications for the health of seafloor biological communities,” explained study author Patrick Schwing. “It is therefore important to identify the areas that were impacted by MOSSFA.”

Schwing noted that to their knowledge, this is the first application of short-lived radioisotopes, specifically 210Pb, to trace the spatial extent of marine oil snow deposition on the seafloor. “We are continuing to use 210Pb to trace the MOSSFA material, as it has been advectively transported across the continental shelf and down the continental slope. We hope to identify the ultimate depositional centers of the MOSSFA material.”

Data are publicly available through the Gulf of Mexico Research Initiative Information & Data Cooperative (GRIIDC) at doi:10.7266/N73N21FW.

The study’s authors are P.T. Schwing, G.R. Brooks, R.A. Larson, C.W. Holmes, B.J. O’Malley, and D.J. Hollander.

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This research was made possible in part by a grant from the Gulf of Mexico Research Initiative (GoMRI) to the Center for the Integrated Modeling and Analysis of Gulf Ecosystems II (C-IMAGE II) and Deepsea to Coast Connectivity in the Eastern Gulf of Mexico (Deep-C) consortium and to Eckerd College and the University of South Florida for their project Assessing the Impact of the Deepwater Horizon Oil Spill on Sediments and Benthic Communities on the West Florida Shelf and Slope.

The Gulf of Mexico Research Initiative (GoMRI) is a 10-year independent research program established to study the effect, and the potential associated impact, of hydrocarbon releases on the environment and public health, as well as to develop improved spill mitigation, oil detection, characterization and remediation technologies. An independent and academic 20-member Research Board makes the funding and research direction decisions to ensure the intellectual quality, effectiveness and academic independence of the GoMRI research. All research data, findings and publications will be made publicly available. The program was established through a $500 million financial commitment from BP. For more information, visit http://gulfresearchinitiative.org/.

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