Temporal and Spatial Characterization of Macondo 252 Signatures in Gulf of Mexico Shelf and Slope Sediments: Evidence for Weathering, Biodegradation, and Transport

The long-term fate of hydrocarbons in the Gulf of Mexico (GOMx) following the 2010 Deepwater Horizon oil spill has yet to be fully characterized. Elemental (% C and % N), stable isotopes (δ13Corganic), and polycyclic aromatic hydrocarbon (PAH) molecular signatures were investigated in shelf and slope sediments collected in October 2010 and 2011 to gain insight into processes affecting the distribution and fate of spilled Macondo oil. Particulate organic carbon (POC) ranged between 1.55 and 2.22 wt. % in 2010, and between 0.55 and 2.06 % in 2011, while the corresponding δ13Corganic ranges were from -23.37 to -20.77 ‰ (vs. PDB (Pee Dee Belemnite)) in 2010, and -22.68 to -20.75 ‰ (vs. PDB) in 2011. Ranges of total polycyclic aromatic hydrocarbon (TPAH) concentrations were from 72.57 to 7,543.53 ng/g in 2010 and 25.55 to 16,582.77 ng/g in 2011.The significant findings of this study are: a) sediments in 2010 and 2011 both display a deviation from the background signature of GOMx seafloor sediments through elevated POC concentrations, elevated OC:N ratios, depleted δ13Corganic, elevated TPAH concentrations, and a PAH signature dominated by petrogenic source materials, b) sediments collected 19 months after the Deepwater Horizon event display evidence of biodegraded and weathered oil, through decreased POC concentrations and OC:N, enriched δ13Corganic values, decreased TPAH concentrations, and alkyl homologue distributions characteristic of a biodegraded or weathered petrogenic source of PAHs, and c) physical processes that have distributed Macondo oil as indicated by the strong relationship of δ13Corganic (in 2010 and 2011) with longitude, in accordance with previous investigations that have characterized a west, southwest trending subsurface hydrocarbon plume that extended west from the Macondo well.