Methane Leakage from Pingo-like Features on the Arctic Shelf, Beaufort Sea, NWT, Canada

The Arctic shelf is currently undergoing dramatic thermal changes caused by the continuing warming associated with Holocene sea level rise. During this transgression comparatively warmer waters flooded over relatively cold Arctic permafrost areas. The resulting thermal pulse is still propagating down into the ground and should be decomposing gas hydrates. A cruise to look for gas venting from the Arctic seafloor was conducted in September 2003. Because the entire shelf is prohibitively large, the primary focus was offshore pingo-like features (PLF) in the Beaufort Sea, east of the Mackenzie River Delta. PLFs are rounded positive relief features commonly found on the Beaufort Sea shelf in water depths from 20 to 200 m. They are typically a few hundred meters in diameter and rise 10 to 35 m above the seafloor. Eighty-one vibracores were collected during an MBARI-USGS-GSC-DFO coring cruise, primarily from the crests, flanks, and moats of 8 PLF. Methane concentrations were systematically elevated in cores from the PLF, sulfate depletion occurred as little as one meter below the seafloor, and ROV surveys revealed that streams of gas bubbles are coming from the crests of some of these PLF. Samples show that this gas is predominately microbial methane (δ13C = -83.6±6.3‰ n=30; C1/C2 = 83,000±137,000 n=23). δ18O and δD values of pore water from the crests and flanks of the PLF are similar to benthic Arctic Shelf water of today, and quite distinct from terrestrial lakes, rivers, and relict ice, which suggests that PLFs are not hydrologically coupled with modern terrestrial aquifer systems. While it is premature to conclude that the methane gas venting from the tops of the PLF is related to gas hydrate decomposition, this gas is emerging from where we predicted it would occur if it was coming from decomposing gas hydrate.