Scientists have discovered the mechanism that transported contaminants from the Deepwater Horizon oil spill to the bottom of the Gulf of Mexico.
Pollutants in the water column and seafloor had been thought to originate from natural oil seeps in the Gulf. But a study published yesterday in the Proceedings of the National Academy of Sciences used fingerprinting techniques to identify the presence of two key drilling mud components: barium and olefin compounds.
"It's kind of like a smoking gun for the source of the contaminants," said lead author Beizhan Yan, an environmental chemist at Columbia University's Lamont-Doherty Earth Observatory.
Yan said he was shocked to find that oil spill contaminants could remain in subsurface water for five months. Before this study, he would have guessed the material would stay for just weeks, Yan said. His research suggests that between August and September 2010 -- about four to five months after the Deepwater Horizon catastrophe -- a bloom of phytoplankton carried a "dirty blizzard" of oil remnants, black carbon and drilling mud to the seafloor.
"Normally we don't think of oil as sinking," said co-author Uta Passow, a biological oceanographer with the Marine Science Institute at the University of California, Santa Barbara. "People in the past have not really ever considered oil coming to the seafloor, especially very, very deep. We now know how the oil gets down there in large amounts and affects the communities that live there."
Once the spill matter reaches the sea bottom, it can enter the food web, potentially harming deepwater fish and corals, the study says.
"With time, the ecological impact will decrease," Yan said. The hydrocarbons will degrade, and new sediment layers will cover the contaminants, he said.
Other papers, including one by UCSB marine chemist David Valentine, indicate that the material can stay on the ocean floor for years (EnergyWire, Oct. 28, 2014).
The study by Yan and his co-authors builds on research by Pennsylvania State University biology professor Charles Fisher that found oil-damaged coral communities more than 13 miles from the well that led to the Deepwater Horizon explosion.
Fisher said at the time that his findings suggested that the oil plume could have traveled farther and deeper than initially thought (E&ENews PM, July 28, 2014).
BP PLC, the operator of the Macondo well, responded to Fisher's study, saying that the coral studied was located near the Biloxi salt dome, an area of natural hydrocarbon seepage.
"What I find most interesting about the current study is that it provides a mechanism that allows us to better understand some of the impact that we found further away from the site of the Deepwater Horizon spill," Fisher said. "In our study, we found one site, and BP had some questions about that, which we addressed in a response to them, but then we also found several other sites, one of which was even more heavily impacted."
Fisher and Passow are collaborating on a laboratory experiment to expose deep sea corals to artificially prepared marine snow containing hydrocarbons. Most of the spill-affected corals Fisher has studied have displayed patchy impacts, he said. In some spots, his team has witnessed branches starting to break and fall off. In others, there are signs of limited recovery, he said.
The Gulf of Mexico Research Initiative -- a group that investigates the impacts of oil, dispersed oil and dispersant in the Gulf -- will fund Fisher and Passow's project. GoMRI supported Yan and Passow's lab analyses.
Yan and Passow also received two National Science Foundation Rapid Response Research grants to collect their samples. The team used a sediment trap about 4.5 miles from the Macondo well.
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