Australia: The Land Where Time Began

A biography of the Australian continent 

Methane Leakage over Widespread Areas of the Seafloor on the Atlantic Margin of Northern US

Methane inputs to the atmosphere (McGinnis et al., 2006) are affected by methane emissions from the sea floor, as are acidification and de-oxygenation (Biastoch et al., 2011; Archer, Buffett & Brovkin, 2009) of the ocean, distribution of chemosynthetic communities and energy resources. Global methane flux derived from cold seeps has only been estimated for continental shelves (Hovland, Judd & Burke, 1993), at 8-65 Tg CH₄ yr^-1, though methane is being emitted by other parts of the marine continental margins. The Atlantic margin of the US has not previously been considered to be an area where widespread seepage was occurring, with only 3 methane seeps being recognised seaward of the shelf break. It has been predicted that there would be massive upper-slope seepage related to gas hydrate degradation for the southern part of this margin (Phrampus & Hornbach, 2012), though previously this process has been recognised only in the Arctic (Biastoch et al., 2011; Westbrook et al., 2009; ). In this study the authors¹ used multibeam water-column backscatter data to cover 94,000 km² of the sea floor, identifying about 570 gas plumes at water depths between 50 and 1,700 m between Cape Hatteras and Georges Bank on the passive margin of the northern US Atlantic coast. At water depths bracketing the updip limit for methane hydrate stability there are about 440 seeps that originate there. At this location contemporary seepage from the upper slope may be triggered by ongoing warming of intermediate waters, though emissions are implied to have continued for more than 1,000 years at some seeps by authigenic carbonates observed. The authors¹ suggest there could be 10s of thousands of seeps that have not yet been discovered based on extrapolation of the upper slope density of this margin to global passive margin systems.

Sources & Further reading

1. Skarke, A., et al. (2014). "Widespread methane leakage from the sea floor on the northern US Atlantic margin." Nature Geosci 7(9): 657-661.