Macondo oil in deep-sea sediments: Part 1 – sub-sea weathering of oil deposited on the seafloor

•Sediment cores within 8 km of the failed well contain variably weathered spill oil.•Weathering generally increases with distance from the well.•Accelerated dissolution and biodegradation affected dispersed oil in sub-sea plume.•Sub-sea weathering imparts distinct “fingerprint” to Macondo oil seafloor residues.

Chemical analysis of sediment cores collected up to 8 km from the Macondo well in 2010/2011 demonstrates the extent of weathering of the Macondo oil deposited in deep-sea sediments following the Deepwater Horizon disaster. On average, dissolution and biodegradation of the oil on the seafloor increased with distance from the well indicating that weathering occurred rapidly and overwhelmingly during the oil's transport as dispersed oil droplets within the deep-sea plume. Beyond about 5 km from the well, the oil deposited on the seafloor had lost most mass below C25, was relatively enriched in n-C25 + n-alkanes and C3– and C4-alkylated benz[a]anthracenes/chrysenes, the latter owing to 95% depletion of total PAHs. Biodegradation of C28 and C29 tricyclic terpanes, C34 and C35 17α(H),21β(H)-homohopanes, C27 13β(H),17α(H)-dia and C27 14β(H),17β(H)-steranes and dissolution of C26 to C28 triaromatic steroids occurred. The results provide a means to distinguish Macondo oil in deep-sea sediments from naturally-occurring seep oils and pervasive ambient background hydrocarbons.