Subseafloor to Sea-Air Interface Characterization of Methane Dynamics in the northern US Atlantic Margin Seep Province

Abstract:

Since the discovery of hundreds of northern US Atlantic margin (USAM) cold seeps in 2012 and 2013, the USGS Gas Hydrates Project has undertaken intensive studies of the along-margin gas hydrate/free gas distribution, the plumbing systems sustaining seeps, seafloor gas emissions, and sea-air methane flux. Interest in the USAM is motivated both by climate change (i.e., documented ocean warming may contribute to seepage) and energy resource (i.e., the amount of gas-in-place in hydrates on the USAM is about the same as that in the northern Gulf of Mexico) issues. USGS-led field efforts have included an April 2015 study to acquire high-resolution multichannel seismic data, coincident split-beam water column methane plume imaging data, and real-time sea-air methane flux measurements between Wilmington and Norfolk Canyons and a September 2015 cruise (with OSU, UCLA, and Geomar) to collect piston cores, multicores, heat flow data, subbottom imagery, CTDs, and coincident water column imagery from Block Canyon to the Currituck Slide. In April 2015, we discovered methane seeps not included in the previously-published database, but found that some known seeps were not active. New high-resolution multi-channel seismic data revealed clear differences between the deep gas distribution in mid-Atlantic upper slope zones that are replete with (up to 240 sites) and lacking in seeps. Based on sea-air flux measurements, even shallow-water outer shelf (~125 m water depth) seeps and a 900-m-high methane plume originating on the mid-slope do not contribute methane to the atmosphere. Using thermistors placed on piston core outriggers, we will in September 2015 acquire thermal data to identify zones of high fluid advection and to constrain background geotherms in areas where heat flow has never been measured. During that same cruise, we will collect a series of piston cores across the no-hydrate/hydrate transition on the upper slope to constrain fluid and gas dynamics in this zone.