Credit: GSA GEOLOGY

Methane is continually produced by microbes that decompose organic matter in sediments at the bottom of the sea. And thermal decomposition of organic matter buried deep beneath the ocean causes methane to seep up from depth. Both methane sources can become trapped in subsurface reservoirs, either as gas or locked into gas hydrate — if the pressure and temperature are just right. Today, huge reservoirs of methane are found, mainly along continental margins. However, slight changes in temperature or pressure have the potential to destabilize the reservoirs, either slowly through venting or catastrophically through seafloor failure. In the latter case, large amounts of methane could enter the oceans and — unless the gas is oxidized quickly — the atmosphere.

Catastrophic destabilization may have occurred about 50,000 years ago offshore from Japan, near the Nankai trough. Nathan Bangs and colleagues at the University of Texas imaged the sea bed and subsurface using three-dimensional seismic data and cores of sediment drilled from the ocean bottom (Geology 28, 1019–1022; 2010). The images reveal a large, sharp-edged, V-shaped notch that cuts 400 m through the ocean floor, down into hydrate-laden sediments.

In the same region, there are signs that strong currents at the bottom of the ocean could have eroded away tens of metres of sea floor. Taking this observation together with the evidence for hydrate destabilization, the researchers hypothesize that erosion of the sediments above a methane reservoir may have unroofed over-pressured gas and hydrate-laden sediments, and sparked a positive feedback loop.

Based on the volume of eroded material and the concentration of hydrates in the sediments, the eroded notch alone could have released 1.51 × 1011 m3 of methane, approximately 3% of the quantity found in the atmosphere at present.

Bottom currents strong enough to erode seafloor sediments are not uncommon. More surprises from the bottom of the ocean may therefore be in store.