Using carbon isotope fractionation to constrain the extent of methane dissolution into the water column surrounding a natural hydrocarbon gas seep in the northern Gulf of Mexico

Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Using carbon isotope fractionation to constrain the extent of methane dissolution into the water column surrounding a natural hydrocarbon gas seep in the northern gulf of Mexico. Geochemistry Geophysics Geosystems, 19(11), (2018); 4459-4475., doi:10.1029/2018GC007705.A gas bubble seep located in the northern Gulf of Mexico was investigated over several days to determine whether changes in the stable carbon isotopic ratio of methane can be used as a tracer for methane dissolution through the water column. Gas bubble and water samples were collected at the seafloor and throughout the water column for isotopic ratio analysis of methane. Our results show that changes in methane isotopic ratios are consistent with laboratory experiments that measured the isotopic fractionation from methane dissolution. A Rayleigh isotope model was applied to the isotope data to determine the fraction of methane dissolved at each depth. On average, the fraction of methane dissolved surpasses 90% past an altitude of 400 m above the seafloor. Methane dissolution was also investigated using a modified version of the Texas A&M Oil spill (Outfall) Calculator (TAMOC) where changes in methane isotopic ratios could be calculated. The TAMOC model results show that dissolution depends on depth and bubble size, explaining the spread in measured isotopic ratios during our investigations. Both the Rayleigh and TAMOC models show that methane bubbles quickly dissolve following emission from the seafloor. Together, these results show that it is possible to use measurements of natural methane isotopes to constrain the extent of methane dissolution following seafloor emission.This research was made possible by two grants from the Gulf of Mexico Research Initiative: Gulf Integrated Spill Response (GISR) Consortium (awarded to J. D. K. and S. A. S.) and Center for Integrated Modeling and Assessment of the Gulf Ecosystem (C‐IMAGE) II (awarded to S. A. S.). Additional support was provided by the U.S. Department of Energy (DE‐FE0028980; awarded to J. D. K.). Data are publicly available through the Gulf of Mexico Research Initiative Information & Data Cooperative (GRIIDC). Methane concentration and isotopic ratio data can be found at https://data.gulfresearchinitiative.org/data/R1.x137.000:0025, and TAMOC model scripts and results are found at https://data.gulfresearchinitiative.org/data/R1.x137.000:0026. The coversion of methane isotopic ratio data used in this manuscript can be found at https://data.gulfresearchinitiative.org/data/R1.x137.000:0028. We want to thank the captain and crew of the E/V Nautilus and the operators of ROV Hercules and Argus during the GISR G08 cruise and Nicole Raineault for their outstanding support at sea. Acoustically identifying the bubble flare was managed by Andone Lavery, and support for collecting gas and water samples was provided by John Bailey. We also want to thank Sean Sylva for analytical assistance on shore, Inok Jun for helping create the sampling schematics, and David Brink‐Roby for helping create the sample site map.2019-04-2