Hydrocarbon composition and concentrations in the Gulf of Mexico sediments in the 3 years following the Macondo well blowout

In April of 2010, the Macondo well blowout in the northern Gulf of Mexico resulted in an unprecedented release of oil into the water column at a depth of approximately 1500 m. A time series of surface and subsurface sediment samples were collected to the northwest of the well from 2010 to 2013 for molecular biomarker and bulk carbon isotopic analyses. While no clear trend was observed in subsurface sediments, surface sediments (0–3 cm) showed a clear pattern with total concentrations of n-alkanes, unresolved complex mixture (UCM), and petroleum biomarkers (terpanes, hopanes, steranes) increasing from May to September 2010, peaking in late November 2010, and strongly decreasing in the subsequent years. The peak in hydrocarbon concentrations were corroborated by higher organic carbon contents, more depleted Δ14C values and biomarker ratios similar to those of the initial MC252 crude oil reported in the literature. These results indicate that at least part of oil discharged from the accident sedimented to the seafloor in subsequent months, resulting in an apparent accumulation of hydrocarbons on the seabed by the end of 2010. Sediment resuspension and transport or biodegradation may account for the decrease in sedimented oil quantities in the years following the Macondo well spill. [Display omitted] •Deep sea sediments were collected from 2010 to 2013 to the NW of the Macondo well.•Oil-derived biomarkers increased from May to September, peaking in November 2010.•Peak in hydrocarbon concentrations were corroborated by more depleted Δ14C values.•Fraction of oil introduced due to blowout had sedimented by late fall 2010.•Hydrocarbon concentrations from 2011-2013 can be used as a post-spill benchmark. Weathered Macondo hydrocarbons associated with marine oil snow reached the seafloor months after the accident, peaking in late fall of 2010.