The evolution of oil contaminated sediments

The evolution of oil contaminated sediments
Deep sea coral covered in sedimented oil from the Deepwater Horizon accident. (c) ECOGIG & Pennsylvania State University

April 02, 2019

The 2010 Deepwater Horizon accident released an unprecedented amount of oil and gas into the Gulf of Mexico. A number of research studies determined that a portion of that oil sank to the bottom and was deposited onto the seafloor. Estimates as to exactly how much of the oil made it the sediments range from 0.5-14.4% of the total amount of oil spilled. However, these numbers most likely underestimate the total amount of oil contaminated sediments in the Gulf, as they fail to take into consideration the formation of marine oil snow over the total area of the Gulf of Mexico that was covered by surface oil slicks. In order to understand the severity of the impact on the benthic (sediment dwelling) communities, it is important to understand how degraded this oil was before reaching the seafloor.

Oil degrades due to exposure to oxygen, light and microbial activity (biodegradation). It is thought that biodegradation of the spilled oil in the water column prior to deposition on the seafloor may have lessened the severity of the impacts on the benthic communities - a recent study by ECOGIG researchers sought to understand exactly how degraded the oil was prior to reaching the sediment,  in order to accurately track the recovery of deep sea Gulf of Mexico sediments after the spill. The researchers used a method that combined two approaches - carbon isotopes and ramped oxidation. Carbon isotopes allowed them to estimate the contribution of different carbon sources to their samples. Ramped oxidation gave them an idea of the thermochemical stability of the compounds in the sediment - fresh crude oil burns at a lower temperature than weathered oil, and oil that is fresh is easier to degrade through microbial activity than weathered oil.

Researchers were able to determine that it took approximately four years for the oil deposited on the seafloor to dissipate. This finding was dependent on the proximity of the sediments to the wellhead and the distance that the oil traveled prior to being deposited on the seafloor - sediments farthest from the well head had returned to near background levels after four years, while sediments closer to the well head still had some oil residue remaining.

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You can find this entire scientific paper online here.

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