Negligible atmospheric release of methane from decomposing hydrates in mid-latitude oceans

Naturally occurring gas hydrates may contribute to a positive feedback for global warming because they sequester large amounts of the potent greenhouse gas methane in ice-like deposits that could be destabilized by increasing ocean/atmospheric temperatures. Most hydrates occur within marine sediments; gas liberated during the decomposition of seafloor hydrates or originating with other methane pools can feed methane emissions at cold seeps. Regardless of the origin of seep methane, all previous measurements of methane emitted from seeps have shown it to have a unique fossil radiocarbon signature, contrasting with other sources of marine methane. Here we present the concentration and natural radiocarbon content of methane dissolved in the water column from the seafloor to the sea surface at seep fields along the US Atlantic and Pacific margins. For shallower water columns, where the seafloor is not within the hydrate stability zone, we do document seep CH4 in some surface-water samples. However, measurements in deeper water columns along the US Atlantic margin reveal no evidence of seep CH4 reaching surface waters when the water-column depth is greater than 430 +/- 90 m. Gas hydrates exist only at water depths greater than similar to 550 m in this region, suggesting that the source of methane escaping to the atmosphere is not from hydrate decomposition.

Automated summary

Naturally occurring gas hydrates may contribute to global warming, but our study found that methane emissions from seep fields along the US Atlantic and Pacific margins are primarily concentrated in shallower water columns, with no evidence of methane reaching surface waters in deeper water columns when the water-column depth is greater than 430 +/- 90 m.