DEEP-WATER FORAMINIFERAL RESPONSE TO THE CATASTROPHIC DEEPWATER HORIZON WELL OIL SPILL, NORTHEASTERN GULF OF MEXICO

The Deepwater Horizon drilling rig offshore of the Mississippi Delta exploded April 20, 2010, resulting in the release of approx. 210 million gallons of oil at 1544m depth before the wellhead was capped on July 15, 2010. It is the world’s largest accidental marine oil spill. Four sediment cores taken with a multicoring system in Nov. and Dec. 2010 at depths of 202m, 400m, 504m and 1520m were studied for their foraminiferal content in relation to the oiled portions. The shallower sites were less contaminated, so research focused on the deepest site WB1110-MC-DSH10, nearest to the blown well.

Foraminiferal species in >300-specimen assemblages (mean of 477 specimens each) from 54 samples were identified at 2-mm intervals that continuously covered the contaminated portions of cores. In DSH10, samples covered the top 46mm, approx. twice the core depth of the oiled sediments as identified by core photographs, and by estimated percentages of tar pieces and coatings of >63-micron grains. Estimated tar content ranged up to 40%, with the highest values corresponding to the darkest sediments at the bottom of the contaminated interval. The diversity and abundance of the assemblages were measured. The most striking patterns, in DSH10, were (1) a shift in wall composition from <12% agglutinates below the contaminated interval to 23-60% within that interval, much higher than is typical of upper middle bathyal depths in the Gulf of Mexico; and (2) a correlation of percent tar with the proportion of stick-like agglutinated species (e.g. Rhabdammina linearis, Rhizammina algaeformis, Saccorhiza ramosa), with “sticks” decreasing along with tar in the upper 14mm of the contaminated interval. As more data are analyzed, an understanding of the impacts of petroleum pollution and related stressors on the deepwater marine ecosystem will facilitate the response, mitigation, and rehabilitation of deep-water areas affected by future oil spills.