Abstract:
Seawater samples were collected in the Gulf of Mexico at MC118, located at 28° 51.129’N, 88° 29.51’W, from 12 - 20 April, 2015 aboard the E/V Nautilus. The seawater samples were collected directly from waters impacted by seafloor methane seeps. This was carried out using the SUPR sampler [Breier et al., 2014] mounted to the ROV Hercules. The SUPR sampler is an in-situ seawater pumping system and was developed to sample dynamic, high gradient, ocean geochemical features at areas such as seep sites. The inlet of the SUPR sampler was attached with tubing to the ROV arm and is pumped seawater directly into sampling bottles mounted to the ROV chassis. The samples were taken from waters visibly impacted by methane bubbles, so an adapter was placed on the inlet of the sampling tube to collect seawater but not bubbles. Once the ROV was recovered, water collected by the SUPR sampler was transferred to 500 mL glass vials which were immediately capped using a butyl rubber topper and aluminum crimp cap (Wheaton). A helium headspace was then introduced into each vial by removing 10 mL of water while simultaneously injecting 10 mL of helium. Samples for the analysis of δ13C-CH4 were preserved using a supersaturated solution of mercuric chloride (HgCl2) for later analysis in a land-based laboratory. Over the course of 12-24 hours the headspace equilibrates with the water sample and the resulting concentration of methane in the headspace is proportional to the solubility of methane and the dissolved methane concentration in the original water sample. As outlined in [Leonte et al., 2017], an aliquot of this headspace gas is removed from each vial for the purpose of isotopic ratio analysis. The raw data from the analysis of this headspace gas was originally presented in https://doi.org/10.7266/N7610XXW. However, since different methane isotopes have been shown to have slightly different solubilities, the isotopic ratio of the headspace gas is slightly different than the isotopic ratio of methane originally dissolved in the water sample. Here, we convert the isotopic ratios measured in the vial headspace (labeled below as δh) to the δ13C-CH4 originally dissolved in water (δi) before a headspace was inserted by considering the isotopic fractionation associated with gas dissolution (αd) and an isotopic mass balance between methane in the headspace and dissolved in water. The standard deviation was also corrected by considering the uncertainty in measurements of the fractionation factor, dissolved methane concentration, and the isotopic ratio of methane in the headspace gas. Additional details can be found in [Leonte et al., 2018]. This dataset supports the publication: Socolofsky, S. A.; Lavery, A.; Kessler, J.; Wang, B.; Breier, J. A.; Leonte, M.; Chan, E.; Raineault, N. (2016). Fate and transport of gas bubbles from Sleeping Dragon seep in the northern Gulf of Mexico, in New Frontiers in Ocean Exploration: the E/V Nautilus and NOAA Ship Okeanos Explorer 2015 Field Season, Oceanography 29(1), supplement, 88 pp., Bell, K. L. C., Brennan, M. L., Flanders, J., Raineault, N. A., and Wagner, K., eds.. Oceanography, 29, 01, 26-27. doi:10.5670/oceanog.2016.supplement.01
Suggested Citation:
Mihai Leonte, John Kessler. 2018. Mathematical conversion of δ13C-CH4 for methane dissolved in water using raw data collected in the northern Gulf of Mexico, April 12-20, 2015. Distributed by: GRIIDC, Harte Research Institute, Texas A&M University–Corpus Christi. doi:10.7266/n7-g26b-a359
Purpose:
Seawater samples were collected to analyze for dissolved δ13C-CH4. These measurements have a variety of uses, but the intention of the authors here were to use these analyses for determining the extent of bubble dissolution in the water column following seafloor release as well as microbial methane oxidation. The raw data collected from the mass spectrometry analyses were presented previously and can be found in https://doi.org/10.7266/N7610XXW. The raw data reports δ13C-CH4 values measured in the headspace of a water sample vial. In this dataset, those raw data are converted into the values of δ13C-CH4 that were originally dissolved in seawater, prior to headspace equilibration.
Data Parameters and Units:
Dive number, Sample ID, d13C-CH4 (per mille, ‰), Standard Deviation of d13C-CH4 (per mille, ‰), Converted d13C-CH4 (per mille, ‰), Converted Standard Deviation of d13C-CH4 (per mille, ‰). The dive information (latitude, longitude, and date) can be found under GRIIDC UDI R1.x137.000:0025 (doi: 10.7266/N7610XXW).
Provenance and Historical References:
Breier, J., Sheik, C., Gomez-Ibanez, D., Sayre-McCord, R., Sanger, R., Rauch, C., Coleman, M., Bennett, S., Cron, B., Li, M., 2014. A large volume particulate and water multi-sampler with in situ preservation for microbial and biogeochemical studies. Deep Sea Research Part I: Oceanographic Research Papers 94, 195-206. doi:10.1016/j.dsr.2014.08.008
Leonte, M., Kessler, J.D., Kellermann, M.Y., Arrington, E.C., Valentine, D.L., Sylva, S.P., 2017. Rapid rates of aerobic methane oxidation at the feather edge of gas hydrate stability in the waters of Hudson Canyon, US Atlantic Margin. Geochimica et Cosmochimica Acta 204, 375-387. doi:10.1016/j.gca.2017.01.009
Leonte, M., Wang, B., Socolofsky, S.A., Breier, J.A., and Kessler, J.D., 2018. Using carbon isotope fractionation to constrain the extent of methane dissolution into the water column surrounding a natural hydrocarbon gas plume in the Northern Gulf of Mexico. In Review at Geochemistry, Geophysics, Geosystems.
View Metadata