Abstract:

The purpose of this study was to analyze the recovery of oil contaminated deep-sea sediment from by analyzing the carbon isotopic composition of bulk sedimentary organic carbon and the thermochemical stability of sediment from 5 sites in the northern Gulf of Mexico, using ramped pyrolysis/oxidation. There were clear differences between crude oil (low temperature), natural hydrocarbon seep sediment (medium temperature; Δ14C = -912‰) and our control site (medium temperature; Δ14C = -189‰), in both the thermographs and carbon isotope signatures. We observed recovery in the bulk Δ14C in sites further from the wellhead in ~4 years, while sites in closer proximity took 5-6 years to recover. The thermographs also indicated changes over time in the composition of the sediment, with shifts towards higher temperatures over time at sites near the wellhead, and losses of higher temperature CO2 peaks at more distal sites.

Suggested Citation:

Jeffrey Chanton. 2018. Sediment Ramped Pyrolysis and Isotope Data from the northern Gulf of Mexico, October 12, 2010 to June 19, 2015. Distributed by: Gulf of Mexico Research Initiative Information and Data Cooperative (GRIIDC), Harte Research Institute, Texas A&M University–Corpus Christi. doi:10.7266/N7KH0KWJ

Publications:

Rogers, K. L., Bosman, S. H., Lardie-Gaylord, M., McNichol, A., Rosenheim, B. E., Montoya, J. P., & Chanton, J. P. (2019). Petrocarbon evolution: Ramped pyrolysis/oxidation and isotopic studies of contaminated oil sediments from the Deepwater Horizon oil spill in the Gulf of Mexico. PLOS ONE, 14(2), e0212433. doi:10.1371/journal.pone.0212433

Purpose:

To determine the fate of oil as it weathered and was biodegraded on the seafloor.

Data Parameters and Units:

Accession # = NOSAMS (national ocean science ams) facility id number; Sample ID = FSU sample ID; Fraction = time series sample of CO2 evolution with increasing temperature; Latitude = decimal degrees North; Longitude = decimal degrees West; d13C 13C/12C isotope ratio in per mil; 14C radiocarbon content in per mill; CO2 umol = micromoles of CO2; evolve from temperature start to stop start T = degrees C; stop T = degrees C GIP07 2010 0-1 10-12-2010 GIP07 2011 0-1 10-21-2011 GIP07 2014 0-1 8-24-2014 GB480 2015 0-1 6-12-2015 BP444 2015 0-1 6-19-2015 BP444 2015 1-2 6-19-2015 BP444 2015 3-4 6-19-2015 GIP17 2010 0-1 10-17-2010 GIP17 2011 0-1 10-23-2011 GIP17 2015 0-1 6-6-2015 GC600 2014 0-1 4-4-2014

Methods:

Ramped pyrolysis/oxidation (RPO) is an approach used to detect the thermochemical stability of organic carbon. When paired with δ13C and Δ14C isotope analysis, the source of the carbon can be determined as a function of thermal stability. The thermochemical stability of a compound is based on the amount of energy needed to break the bonds in the compound, with higher stability requiring higher temperatures, while more labile bonds break at lower temperatures. The thermal stability of a compound is related to its lability, reactivity, and suitability as a substrate in microbially mediated reactions in the environment. The Programmable Temperature Control System (PTCS) at NOSAMS was used to serially oxidize these sediments in a controlled environment, following the instrument protocol for pyrolysis in Rosenheim et al. (2008). An aliquot of sediment, between ~80-110 mg depending on the C content, was loaded into a precombusted quartz tube, between layers of precombusted quartz wool, and inserted into the combustion oven, sealed away from atmosphere. A total gas flow of 35 mL/min of helium with 8% oxygen flowed through the sample as the temperature was consistently ramped up to 800-1000°C (5°C /min). At selected temperature intervals, the evolved CO2 was collected and cryogenically purified prior to being sealed in a sample ampoule. With each switch of the temperature interval, the CO2 was continuously collected in a secondary N2 (l) trap. Next, roughly 10% of CO2 was diverted during the graphitization process, to be analyzed for δ13C. The graphite was analyzed for Δ14C at NOSAMS. Hemingway et al. (2017) estimated the blank for a typical RPO analysis was 3.7 ± 0.6 µg C, with δ13C = -29.1± 0.1‰ and potentially Δ14C = -449 ± 41‰. The blank carbon correction for δ13C ranged between -0.02 to +0.15‰ and Fm ranged from -0.002 to +0.002 (Δ14C ~ 3-4 ‰). Due to the small size of these corrections, the data herein were not corrected. Rosenheim, B.E., Day, M.B., Domack, E., Schrum, H., Benthien, A., & Hayes, J.M. (2008). Antarctic sediment chronology by programmed-temperature pyrolysis: Methodology and data treatment. Geochemistry, Geophysics, Geosystems, 9(4): Q04005. doi: 10.1029/2007GC001816 Hemingway, J.D., Galy, V.V., Gagnon, A.R., & Grant, K.E. (2017). Assessing the Blank Carbon Contribution, Isotope Mass Balance, and Kinetic Isotope Fractionation of the Ramped Pyrolysis/Oxidation Instrument at NOSAMS. Radiocarbon, 59(1): 179-193. doi: 10.1017/RDC.2017.3

Instruments:

The Programmable Temperature Control System (PTCS) Accelerator Mass Spectrometer

Error Analysis:

The blank for a typical RPO analysis was 3.7 ± 0.6 µg C, with δ13C = -29.1± 0.1‰ and potentially Δ14C = -449 ± 41‰. The blank carbon correction for δ13C ranged between -0.02 to +0.15‰ and Fm ranged from -0.002 to +0.002 (Δ14C ~ 3-4 ‰). Due to the small size of these corrections, the data herein were not corrected. Replication of 17 bulk sediment samples averaged 6.5‰

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