Abstract
Sediment cores were collected from three sites (1000–1200 m water depth) in the northeastern Gulf of Mexico from December 2010 to June 2011 to assess changes in benthic foraminiferal density related to the Deepwater Horizon (DWH) event (April-July 2010, 1500 m water depth). Short-lived radioisotope geochronologies (210Pb, 234Th), organic geochemical assessments, and redox metal concentrations were determined to relate changes in sediment accumulation rate, contamination, and redox conditions with benthic foraminiferal density. Cores collected in December 2010 indicated a decline in density (80–93%). This decline was characterized by a decrease in benthic foraminiferal density and benthic foraminiferal accumulation rate (BFAR) in the surface 10 mm relative to the down-core mean in all benthic foraminifera, including the dominant genera (Bulimina spp., Uvigerina spp., and Cibicidoides spp.). Cores collected in February 2011 documented a site-specific response. There was evidence of a recovery in the benthic foraminiferal density and BFAR at the site closest to the wellhead (45 NM, NE). However, the site farther afield (60 NM, NE) recorded a continued decline in benthic foraminiferal density and BFAR down to near-zero values. This decline in benthic foraminiferal density occurred simultaneously with abrupt increases in sedimentary accumulation rates, polycyclic aromatic hydrocarbon (PAH) concentrations, and changes in redox conditions. Persistent reducing conditions (as many as 10 months after the event) in the surface of these core records were a possible cause of the decline. Another possible cause was the increase (2–3 times background) in PAH’s, which are known to cause benthic foraminifera mortality and inhibit reproduction. Records of benthic foraminiferal density coupled with short-lived radionuclide geochronology and organic geochemistry were effective in quantifying the benthic response and will continue to be a valuable tool in determining the long-term effects of the DWH event on a larger spatial scale.
Highlights
The Deepwater Horizon (DWH) event released over 4.9 million barrels of oil into the Gulf of Mexico from April to July of 2010 [1]
Two initial sites [PCB06 (29° 5.99’ N, 87° 15.93 W, 1043 m depth) and DSH08 (29° 7.25’ N, 87° 51.93’ W, 1143 m depth)] were chosen for benthic foraminifera analysis due to preliminary organic geochemistry results suggesting the presence of oil (University of South Florida, College of Marine Science’s baseline survey, 29) and each site was located at a water depth that was within the range of the documented primary hydrocarbon plume (1000–1300 m) [1,6]
Bulimina spp. density decreased from 5.4 indiv./cm3 at 10 mm to 0.2 indiv./cm3 at the surface, Uvigerina spp. density decreased from 3.8 indiv./cm3 at 10 mm to 0.5 indiv./cm3 at the surface, and Cibicidoides spp. decreased from 0.14 indiv./cm3 at 10 mm to 0.07 indiv./cm3 at the surface
Summary
The Deepwater Horizon (DWH) event released over 4.9 million barrels of oil into the Gulf of Mexico from April to July of 2010 [1]. An estimate of 60% of the oil reached the surface where it was subject to skimming, coastal deposition, evaporation, and incorporation into flocculent material [2]. Flocculent material consisting of algae, dispersant, clay particles, and microbes formed at the water surface with the aggregated oil and settled to the sea floor [3]. Ryerson et al [6] estimated that only 35% of the oil made it to the water surface and 35% was included in the subsurface intrusion. The estimates from Ryerson et al [6] and Thibodeaux et al [2] suggest that as much as 30–40% of the oil unaccounted for was likely deposited on the seafloor
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