The proposed research aims to document organic geochemical and isotopic changes of petroleum over time following the April 20, 2010 Gulf of Mexico oil spill. The objectives of this study are to document the geochemical fate of petroleum in the wake of the marine oil spill using the analytical arsenal of organic geochemistry, in particular by using mass-spectrometric identification and quantification, and isotopic characterization. These techniques record the history of chemical change of spilled oil through time, and also forensically discriminate between residual oil from the BP spill and unrelated hydrocarbon pollutants that would otherwise go undetected. As oil washes ashore and is degraded in different littoral and terrestrial environments, the PIs will characterize the residual organic compounds as a function of exposure time to test whether published models for anaerobic biodegradation of crude oils in oil reservoirs fit the recorded trends of aerobic biodegradation from each sampling site. Their compositional and isotopic data from a variety of repeatedly visited sampling sites representing ecologically different environments will evaluate possible acceleration or retardation of oil degradation that result from environmental factors.
One doctoral and several undergraduate students will participate in fieldwork, subsequent sample preparations, and analyses, educating students that across boundaries of organic, stable isotope and environmental geochemistry. Results of this study will be communicated to the scientific community through publications and presentations at conferences and will serve as a public outreach project at Grand Isle State Park, LA. Their data and interpretations will be used to help construct either a display or an informational pamphlet to educate the general public and K-12 students. Knowing which coastal environments have superior geochemical self-cleaning capability will help optimize the deployment of limited resources during future cleanup efforts.
Introduction Responding to the Deepwater Horizon drilling platform explosion and subsequent months-long oil spill, we sought federal funding to sample the oil that washed ashore along the coasts of Louisiana in order to monitor its chemical and isotopic composition through time. A grant from the National Science Foundation was awarded (EAR-1045845, $198,282, collaborative between University of Massachusetts, Indiana University, and Tulane University) for the sampling and analysis of oils using bulk stable isotope composition, ramped-pyrolysis radiocarbon composition, and compound-specific isotope composition. Due to its accessibility, we chose Grand Isle as one of our sampling sites to be revisited and we were permitted to sample from the Grand Isle State Park. Sampling has occurred on several dates (Table 1), with more frequent sampling occurring in the beginning of the research period. Samples were generally stored at Tulane University for a short period under inert gas and at low temperature, divided for analysis, and shipped to Indiana University. Samples remaining at Tulane University were treated with acid to remove minerals and analyzed by ramped pyrolysis. Samples at Indiana University were extracted and bulk hydrogen and carbon isotopes were measured along with elemental hydrogen:carbon (H:C) ratios. Extracts were then sent to University of Massachusetts for organic compound specific analysis. Table 1: Sampling dates from Grand Isle State Park. Date Number of Samples Day after Spill 4 June 2010 13 46 30 June 2010 12 72 16 July 2010 15 88 20 August 2010 8 123 15 October 2010 12 149 22 March 2011 14 337 8 September 2011 11 507 23 November 2011 10 583 26 February 2012 14 678 Field Sampling We have repeatedly visited two sample sites in Grand Isle State Park (Figure 1) that allow us to sample high energy beach deposits and low energy lagoonal deposits. The sample sites displayed evidence of oil contamination at every sampling visit. On June 4, 2010, oil was apparent floating on the surface of the water and on the surface of sediments below the beach scarp (measured at approximately 8 cm at vertical face)(Figure 2). Observation of a violent thunderstorm during that afternoon displayed how oil could reach areas of the beach that were above the beach scarp and mix with sediment due to large wave action. Subsequent visits have involved digging trenches to find layers of oil that were deposited during large wash-up events like June 4, 2010, and subsequently buried (Figure 3). During sampling, we always visit the same two locations and we sample tarballs and other deposits that we observe in other areas of the state park. The results shown below summarize our results from Grand Isle State Park with the exception of samples taken on February 26, 2012, which have not yet been analyzed. Although levels of O2 are not measured in the field, site B (Figure 1) regularly contains hydrogen sulfide (presence determined by odor) which indicates low-oxygen conditions within the sediment allowing reduction of marine sulfate. Site A generally does not have the odor of hydrogen sulfide. Ramped Pyrolysis Results We employ ramped pyrolysis to determine changes in the overall diagenetic stability of the deposited oil. Details of this novel technique can be found in Rosenheim et al. [2008]. In brief, during pyrolysis, the temperature of highest CO2 production is measured and named Tmax. This parameter is a general indicator for the overall chemical stability of the sample. Additionally, CO2 evolved from the reaction is trapped over several temperature intervals and measured for 14C content. Because oil is devoid of 14C, this is a powerful tracer of oil in the environment. Figure 4 shows changes in Tmax through time that relate to either increasing stability of the Macondo oil in this environment or mixing of natural organic material with the oil. Isotopic results (still pending) will be used to differentiate the two processes. If the signal of increased Tmax with time is due to degradation of the oil, there will not be large changes in the 14C content (near 0) nor the 13C content. If these trends are due to mixing with natural organic material, there will be observable isotopic changes. At this point we can be sure that the ramped pyrolysis method indeed records wholesale changes in the organic material through time at a site polluted with Macondo oil.
