THE GLOBAL FLUX OF METHANE FROM SHALLOW SUBMARINE SEDIMENTS

The seepage of methane through the seabed of the world's continental shelves is considered. Methane may be generated by both bacterial and thermogenic processes acting on the complex organic matter stored in the seabed. Because these processes are ubiquitous, occurrences of gas and gas seepages are probably widespread. Although numerous observations of methane seepages from shallow marine sources have been reported, there are few documented flux measurements. Whereas a prolific 400 g CH4 m-2 year-1 has been calculated for an 18 km2 area in the Santa Barbara Channel, California, a more modest 26 g CH4 m-2 year-1 has been estimated for a site in the North Sea. Descriptions of these and other areas of gas seepage, from a variety of geological and oceanographic environments, are presented. A rough estimation of the global flux suggests that between 8 and 65 Tg of methane passes through the seabed of the continental shelves annually. If a significant proportion of this passes through the water column and into the atmosphere, the oceans may represent a more significant source of methane than has been suggested previously. As much of this methane is C-14-depleted, it may at least partially account for the shortfall of fossil methane in current atmospheric methane budgets.