ENVIRONMENTAL-EFFECTS OF SUBMARINE SEEPING NATURAL-GAS

It is suspected that most shallow reservoirs of natural gas vent to the surface to some degree. This seepage may be through diffusion of dissolved gas or by a flow of gas bubbles which entrain interstitial water during the rise through the sediments to the surface. Methane bubbles dissolve other gases, notably hydrogen sulphide and carbon dioxide, during their ascent. Under suitable temperature-pressure conditions gas hydrates may be formed close to or at the seabed. Black suphide-rich sediments and mats of sulphur oxidizing bacteria are frequently observed close to the sediment surface at seep sites, indicating a sharp oxic/anoxic boundary. Animal species associated with these gas seeps include both species which obtain nutrition from symbiotic methane-oxidizing bacteria and species with symbiotic sulphur-oxidizing bacteria. It is suspected that at some microseepage an enhanced biomass of meiofauna and macrofauna is supported by a food chain based on free-living and symbiotic sulphur-oxidizing and methane-oxidizing bacteria. The most common seep-related features of sea floor topography are local depressions, including pockmark craters. Winnowing of the sediment during their creation leads to an accumulation of larger detritis in the depressions. Where the depressions overlie salt diapirs they may be filled with hypersaline solutions. In some areas dome-shaped features are associated with seepage and these may be colonized by coral reefs. Other reefs, "hard-grounds", columnar and disc-shaped protrusions, all formed of carbonate-cemented sediments, are common on the sea floor in seep areas. Much of the carbonate appears to be derived from carbon dioxide formed as a result of methane oxidation. The resulting hard-bottoms on the sea floor are often colonized by species not found on the neighbouring soft-bottoms. As a result seep areas may be characterized by the presence of a rich epifauna.