Mathematical conversion of δ13C-CH4 for methane dissolved in water using raw data collected in the northern Gulf of Mexico, April 12-20, 2015

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