The Thawing Arctic: Mapping U.S. Beaufort Sea Offshore Permafrost / Gas Hydrates (?)
USGS Coastal & Marine Science Center at Woods Hole
Gas hydrates are crystalline compounds that store significantly more methane than free gas with the same volume. Only stable within low-temperature and moderate-pressure conditions, gas hydrates occur in deep marine (>300 - 500 m water depth) settings and areas of thick permafrost, such as the Arctic. In nature, gas hydrates exist close to their pressure-temperature equilibrium, making them susceptible to warming-driven dissociation. Throughout the Holocene circum-Arctic shelves have experienced rapid sea-level rise and temperature increases with some offshore locations warming by as much as 16 °K. These shelves now host degrading submerged permafrost and possibly substantial deposits of dissociating gas hydrates. Nearshore gas hydrate can only exist within and subjacent to permafrost, and thus, areas at the permafrost-to-no permafrost transition zone are those most likely degassing.
On the U.S. Beaufort margin, subsea permafrost extent has never been systematically mapped, and the best insights about permafrost and associated gas hydrate have been based on a limited number of offshore boreholes and numerical studies, with sometimes contrasting predictions of the permafrost’s seaward extent. We use 5,000 km of multichannel seismic (MCS) data acquired by industry and the USGS between 1977 and 1992 to map a velocity anomaly diagnostic of ice bonded coarse-grained sediments along 700 km of the US Beaufort coastline. We integrate these data with recently collected (2010, 2011)swath bathymetry, high-resolution seismic data and methane flux data. High-velocity (>2.3 km/s) refractions, which are evident in prestack MCS shot records, reveal laterally continuous layers of shallow, ice-bonded, coarse-grained sediments beneath the inner continental shelf. The permafrost layer refractions (PLR) occur in less than 5% of the examined tracklines, and calculated PLR depths range from ~ 5 to 470 m below seafloor. The PLR end within 30 km of shore and do not extend beyond the 20 m isobath, implying that older assumptions about the present-day offshore extent of ice-bearing permafrost on this shelf should be revised to reflect the considerable thawing that has likely occurred since the onset of shelf inundation in the Late Pleistocene.
Laura Brothers is the National Energy Technology Laboratory (NETL)/ National Research Council Methane Hydrate Fellow. Working at the Woods Hole USGS Coastal & Marine Science Center she currently researches subsea permafrost degradation and seafloor fluid escape.