Massive methane release triggered by seafloor erosion offshore southwestern Japan

Vast amounts of methane hydrate exist beneath continental margins, but whether this methane releases from sediment on a large scale and affects the oceans and atmosphere remains unclear. Analysis of newly acquired three-dimensional seismic images and drilling data from a large gas hydrate province reveal a recently eroded v-shaped depression. The depression sharply cuts through a relic bottom simulating reflection (BSR) and hydrate- laden sediments. The shape of the relic BSR indicates that the seafloor depression was once a large anticline that has recently been eroded and released an estimated 1.51 x [10.sup.11] [m.sup.3] of methane. We hypothesize that erosion of the seafloor via bottom-water currents unroofed buoyant hydrate- laden sediments and subhydrate overpressured free gas zones beneath the anticline. Once triggered, gas-driven erosion created a positive feedback mechanism, releasing gas and eroding hydrate-bearing sediment. We suggest that erosive currents in deep-water methane hydrate provinces act as hair triggers, destabilizing kilometer-scale swaths of the seafloor where large concentrations of underlying overpressured methane exist. Our analysis suggests that kilometer- scale degassing events are widespread, and that deep-water hydrate reservoirs can rapidly release methane in massive quantities. doi: 10.1130/G31491.1