Spatial and Temporal Investigation of Carbon Isotopes in Particulate Organic Carbon in the Gulf of Mexico

The Deepwater Horizon (DwH) blowout released 5.0x1011g C from gaseous hydrocarbon into the water column and up to 6.0x1011g C from oil (Joye et al., 2011). Oil was visible on the surface, but <0.01% of the gaseous hydrocarbon escaped the water column (Kessler et al., 2011; Yvon-Lewis et al., 2011). Metabolically efficient methanotrophs consumed the gases, converting them into either CO2 or biomass. For this study, we analyzed carbon isotopes, δ13C and Δ14C, in suspended particulate organic carbon (POC) to detect the fate of some of the hydrocarbons. Suspended particulate organic carbon has a residence time of 5-10 years in the water column (Druffel and Williams, 1990) and is composed of floating biomass including mucus, plankton, bacteria, fecal pellets and other organic particles greater than 1μm. POC too large to float sinks and is then classified as sinking POC. Marine snow is an example of sinking POC. Suspended and sinking POC can be composed of the same types of biomass, however they can be formed from different carbon sources and different areas of the water column. The primary focus of this study is on suspended POC. We hypothesized that the DwH event caused depleted δ13C and Δ14C signatures in POC that we would see fade over time and with distance away from the blowout, horizontally and vertically in the water column. Our second hypothesis was that carbon isotope signatures should also differ between natural hydrocarbon seep sites and non-seep sites. We also wanted to determine the percent carbon in each sample from methane or oil derived carbon and the total amount of POC in our sampling area in the Gulf of Mexico composed of carbon from hydrocarbons. Our data set was compared to sediment carbon isotope data from the Gulf of Mexico to rule out one possible alternative hypothesis that could cause depleted carbon isotope signatures, sediment resuspension. To complete these objectives, POC samples were collected from Desoto Canyon and other seep and non-seep sites across the Northern Gulf of Mexico in 2010, 2012 and 2013. Seawater was filtered, collecting the POC on glass microfiber filters, which were then acid fumed, dried and analyzed for δ13C and Δ14C. The hydrocarbon plume from the DwH blowout was detected in 2010 POC samples. Over 2010, 2012 and 2013 sampling years, δ13C signatures ranged from −17.8 / to −35.2 /. Radiocarbon signatures ranged from 71.2 / to −755.5 /. Percent carbon from modern, methane and oil derived carbon for 29 deep POC samples was estimated to contain 58% ±; 19% modern, 33%±;19% methane and 9%±;5% oil. We found no yearly variation between signatures on POC samples collected in the deeper waters, which suggests that the Gulf of Mexico always has depleted carbon isotope signatures at deeper depths and the effect of the oil spill, although clearly detectable in 2010, was relatively minor on suspended POC. GC600, a well known and active seep, appears to have the same qualitative impact on the POC carbon isotope signatures as the blowout. The blowout released an estimated 4.6−6.0x1011g C from oil into the water column. Through direct recovery, skimming, or burning, 35.5% of the released oil was removed from the environment leaving 2.9−3.9x1011g C from oil behind that could possibly be incorporated into POC. Adding the 5.0x1011g C from gaseous hydrocarbons to the oil left in the water column, there is an estimated 7.9−8.87x1011g C released from the wellhead. We calculated the total POC reservoir in our sampling area of 9.4x1010 m2 of the Gulf of Mexico, to be 2.1x1012 to 4.4x1012g C. If 100% of the remaining hydrocarbons were assimilated into biomass, the hydrocarbons from the blowout would make up 20−42% of the total POC reservoir in the Gulf of Mexico. Du and Kessler (2012) estimate that of the 0.47x1012g of hydrocarbons released, 0.1x1012 g C were converted into biomass, which makes up only 2−5% of the total POC reservoir we calculated. As suggested above, our data suggest that the deep Gulf of Mexico always has depleted carbon isotope signatures, implying that the bacteria in the water column of the Gulf of Mexico acts as a buffer, reducing the impact of the blowout and natural seeps on global climate by preventing methane from reaching the atmosphere.