Potential Of The Antarctic Margins For Hydrates Formation

A peculiarity of Antarctica relative to other continents is the fact that the source of terrigenous organic carbon (terrestrial vegetation) was eliminated as the climate cooled and ice sheets began to form. However, this cooling was generally accompanied by an increased flux of marine biogenic sediments to the sea-floor. Presently the continental shelves of Antarctica are draped with sediments which have relatively high concentrations of siliceous biogenic material (Anderson et al., 1983), and the content of organic carbon is typically in the range of 1 to 3% (Dunbar et al., 1985). The continental rise sediments, however, contain a much lower amount of organic carbon, as little as 0.1% (Shipboard Scientific party, 1999) owing to a greater dilution of the biogenic component by terrigenous particles.

Extensive scientific seismic exploration surveys have accumulated considerable information to define the geological setting of most of the circum-Antarctic basins (Mclver, 1975; Ivanhoe, 1980; Mitchell & Tinker, 1980; Cameron, 1981; Behrendt, 1983b; Aleyeva & Kucheruk, 1985; Davey, 1985; Ivanov, 1985; St. John, 1984; Elliot, 1988; Cook & Davey, 1990; Collen & Barrett, 1990; Anderson et al., 1990, among others).

Scientific drilling has demonstrated that biogenic methane forms in situ in Late Cenozoic deep sea sediments of the Antarctic Offshore, although no data are available on in situ methane saturation with respect to the stability field of methane hydrates. In general, the action of ice sheets on the marine deposition is that of producing a large quantity of unsorted sediments on the continental shelf, in which a very fine grained matrix may become volumetrically important. Downslope re-distribution of sediments tends to sort the material in a proximal mud-dominated slope-rise environment (debris-flows, contourites, and channel-levee deposits), and a coarser grained more distal environment dominated by turbidites.

It could be inferred therefore that hydrocarbon gases formed in situ on the proximal continental margin do not find stratigraphic paths to migrate and concentrate in sufficient proportion to generate a BSR. In addition, the limited tectonic activity of the Antarctic margins, mainly old passive margins, does not create preferential structural paths (along deep faults) to allow deep gas to migrate in solution and concentrate at shallower depth within the hydrate stability zone. In particular, because the seismic appearance of a hydrates (the BSR) depends on the presence of a free gas bearing sediment layer below the hydrate stability zone, a widespread presence of hydrates only on the Antarctic margin would not be in conflict with the few observations of BSRs.

In many other places of the world's margin however, BSRs are found in very fine grained sedimentary successions and with limited or absent deep folding (such as the Blake Ridge as an example). More reasonably, therefore, we are inclined to conclude that the knowledge of the margin is still inadequate in terms of coverage, and adequacy of seismic investigastions to elaborate a firm and definitive statement on the occurrence of gas hydrates along the Antarctic margin.

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