Given the volume of oil released by the spill, however, it

is difficult to say that most of the oil was not derived from the spill. There was certainly some weathering of oil which occurred between the seafloor and the surface, and again between the spill site and the shore. This would have been affected by the addition of the dispersant Corexit® at the wellhead and the surface. Local seeps would be the most likely source of additional crude, although the volume of input from seeps would have been negligible in comparison to the spill volume. High concentrations of compounds at the spill site as observed in this study were to be expected, given the volume of the spill. The continental shelf of the northern GOM is known to have hundreds to thousands of small seeps of oil and gas, but the volume of these seeps is negligible compared to the BP/DWH spill volume. In addition, results of the diagnostic CH5424802 datasheet ratios of biomarkers were positive, indicating that the source of the oil in our samples EPZ-6438 was from the BP/DWH spill. Comparing our results with those of other investigators, Aeppli et al. (2012) collected 146 samples in 2010 and 2011 offshore and on the beaches in this region. They focused, however, on the production of oxyhydrocarbons during the weathering process. PAHs were analyzed for a small sub-set of samples

(n = 10); PAHs were not the focus of their analysis. Carmichael et al. (2012) report oiled and non-oiled honeycomb styrofoam material in the GOM surface waters and along the coastal beaches. Naphthalene, fluorene, phenanthrene, and chrysene in the oiled material were

depleted relative to Macondo well oil by 98%, 72%, 43% and 0%, respectively. This highlighted the greater susceptibility of smaller two-ring PAHs to weathering as opposed to the larger multi-ring PAHs. This is consistent with observations made on other oil spills (see Reddy et al., 2011, for data on sub-surface partitioning about of n-alkanes and benzene). The distribution of compounds measured in the central region of the northern GOM and in nearby areas are generally consistent with known ocean currents in the region (see Sturges and Lugo-Fernandez, 2005 for a review). The spill site was S–SE of the mouth of the Mississippi River. The river plume is known to be influenced by near-shore coastal currents in the region which split near the mouth of the river, with most of the plume being drawn to the west and the remainder to the east. In addition, the Loop Current is known to produce eddies which impinge on the spill site, potentially carrying petroleum hydrocarbons offshore. Such eddies also break free, potentially carrying such compounds to the west along the edge of the continental shelf. Various impacts extended from June 2010 to at least March 2011. Most samples were collected post-capping (July 15, 2010); thus, geographic patterns of compounds in general represent post-spill distributions.