The fluxes of the greenhouse gas methane from many individual sources to the atmosphere are not well constrained1. Marine geological sources may be significant2, but they are poorly quantified and are not included in the Intergovernmental Panel on Climate Change budget1. Previous results based on traditional indirect sampling techniques and modelling suggested bubble plumes emitted from marine seeps at depths greater than 200 m do not reach the surface mixed layer because of bubble dissolution and methane oxidation3,4,5. Here we report methane concentration and isotope-depth profiles from direct submersible sampling of deepwater (550–600 m) hydrocarbon plumes in the Gulf of Mexico. We show that bubble size, upwelling flows and the presence of surfactants inhibit bubble dissolution, and that methane oxidation is negligible. Consequently, methane concentrations in surface waters are up to 1,000 times saturation with respect to atmospheric equilibrium. We estimate that diffusive atmospheric methane fluxes from individual plumes are one to three orders of magnitude greater than estimates from shallow-water seeps6,7,8, greatly expanding the depth range from which methane seep emissions should be considered significant. Given the widespread occurrence of deepwater seeps, we suggest that current estimates of the global oceanic methane flux to the atmosphere1 may be too low.
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We thank the crew members and pilots of the R/V Seward Johnson and Johnson Sea-Link for their support in meeting the objectives of the 2002 and 2003 dive programs. We thank D. Valentine for use of his laboratory for the C1–C3 hydrocarbon analyses, and F. Kinnaman, B. Deck, R. Solem and T. Rust for technical assistance. We greatly appreciate the shipboard assistance from G. Robertson and the numerous UCSD and Texas A&M students during the 2002 and 2003 research expeditions. J. Kessler and J. Greinert are thanked for constructive comments, which helped improve the manuscript. Research financial support was provided by the US Department of Energy National Methane Hydrates R&D Program.
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Solomon, E., Kastner, M., MacDonald, I. et al. Considerable methane fluxes to the atmosphere from hydrocarbon seeps in the Gulf of Mexico. Nature Geosci 2, 561–565 (2009). https://doi.org/10.1038/ngeo574
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