Gulf of Mexico Recovery and Organic Matter Variability: A Tale of Two Sources

The Deepwater Horizon (DWH) blowout of 2010 released an estimated 4.5-4.9 million barrels of oil and 500,000t of methane into the Gulf of Mexico (GOM). Some of this oil rose to the surface, forming oil slicks, while an estimated 30% of the smaller particles and gasses formed a deep-water hydrocarbon plume between 1000-1200m (Ryerson et al., 2013; Valentine et al., 2010). The oil slicks at the surface covered a total area of 149,000km2 (MacDonald et al., 2015), but less than 0.01% of the gaseous hydrocarbons reached the surface (Kessler et al., 2011; Yvon-Lewis et al., 2011). After capping the wellhead and following natural and human remediation efforts, an estimated 11-30% of the oil was left unaccounted (Lehr et al., 2010). Studies of δ13C and Δ14C tracers in particulate organic carbon (POCsusp) in the water column and in sediments have shown the accumulation of fossil carbon in these pools. This dissertation explores the POC and sedimentary organic carbon pools using δ13C and Δ14C to characterize and track the recovery of these carbon pools following the DWH blowout. Due to the small particle size, residence time, and sensitivity to inputs, POCsusp provides a link between microbial processes in dissolved organic carbon and larger particles that pass carbon up the food web. Through this link we can evaluate the incorporation of hydrocarbons using δ13C and Δ14C. POCsusp was collected over 6 years from 43 sites across the Northern GOM. At the time of collection these sites were classified as seep or non-seep. We observed a wide range of natural variability in both δ13C (-17.8 to -35.4‰) and Δ14C (+71 to -755‰) throughout the water column. We found that deep-water POCsusp of the GOM was always more depleted than POCsusp from the euphotic zone. POCsusp collected from seeps was more depleted in Δ14C than non-seep sites. Endmember modeling indicated that in these particles, as much as 73% of the carbon was incorporated from oil. Four years following the blowout, we observed recovery in the Δ14C of deep-water POCsusp settling at a baseline of Δ14C=-164.4±18.9‰. We found the δ13C of POCsusp from the euphotic zone became more depleted over time, potentially due to the continuous incorporation of hydrocarbons. The deposition of oil in the sediments of the GOM has been estimated to be up to 14% of the total oil released (Valentine et al., 2014; Chanton et al, 2015), with marine oil snow as the primary mode of deposition. We employed inverse distance weighted interpolation to the surface sediment δ13C and Δ14C data. From these interpolations, we calculated the area affected by petrocarbon and followed its quantity through time. The area affected by petrocarbon decreased each year at a rate of -2x108 g/yr. Our maps indicated an east-west trend in depletion of both δ13C and Δ14C likely caused by the increasing importance of output from natural seeps and the Mississippi River. We also found significant differences between the sediment of the northern and southern GOM, with the north being much more depleted in δ13C and Δ14C than the south. Ramped pyrox paired with δ13C and Δ14C was used in previous studies of oil contaminated marsh sediments, showing the evolution of the thermostability and isotope signatures as the oil was transformed and the system recovered. We used ramped pyrox paired with δ13C and Δ14C measurement of the evolved fractions to explore the recovery of two DWH affected time-series sites, GIP07 and GIP17, and one site that had high PAH levels in 2010. We found differences in the thermographs and δ13C and Δ14C of the evolved CO2 between crude oil and the seep and control sediment. We observed shifts in the CO2 evolution over time from lower to higher-temperature at GIP17 (~16km from the wellhead), followed then by a loss of higher temperature peaks at GIP07 (~90km). At both sites we observed recovery going from bulk Δ14C=-491‰ in 2010 to almost background by 2015, Δ14C=-264‰ at GIP17. Our study supports ideas from Bagby et al. (2016) and Stout and Payne (2016) indicating a relationship between degradation rate and distance travelled in the water column. The further the hydrocarbons traveled in the water column, the faster they degraded before being sedimented. Following sedimentation, degradation rates were much slower than while the oil was in the water column. The level of contamination also affected the degradation rate, with high contamination recovering at slower rates (20‰ y-1) than sites with lower contamination (46‰ y-1).

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