Considerable methane fluxes to the atmosphere from hydrocarbon seeps in the Gulf of Mexico

Abstract

The flux of methane—a greenhouse gas—from submarine hydrocarbon seeps to the atmosphere is not well quantified. Direct measurements of methane concentrations and isotopic depth profiles in deepwater hydrocarbon plumes indicate that a significant amount of methane from deep-ocean sources could reach the surface ocean. 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.