On the self-sealing nature of marine seeps

Seeps in ocean and lacustrine environments are often ephemeral and self-sealing. Although the nature of the self-sealing processes varies from one area to the other, field studies carried out at seeps at the Tommeliten hydrocarbon field in the North Sea, suggest three phases to be common. The methane gas seeps at Tommeliten occur at a water depth of 74m, centrally in the North Sea above a buried salt diapir. The seeps were surveyed and sampled with the use of a remotely operated vehicle (ROV) in 1983. The gas is mainly methane, but contains small amounts of the heavier hydrocarbons ethane, propane, and butane, and is of thermogenic origin. These seeps were re-surveyed 15 years later, in 1998, and other features were also mapped out. There are three types of seepage phenomena in the area: (1) new, incipient seeps, where the gas comes directly from small vents in the sandy seabed. (2) bacterial mats (probably Beggiatoa sp.) where the gas accumulates in anoxic sediments below a thin layer of bacterial mats before occasionally venting through holes in the mats. (3) authigenic carbonate cemented bioherm-structures where no visible gas is evident. Sampling of carbonate nodules suggests that gas is migrating up to the lower part of the structures within the sediments, where it probably oxidizes and is utilized in various processes, including the production of carbonate cements. These modern results suggest that the formation of the bacterial mats may represent the first phase of natural sealing. The formation of the bioherms, which host numerous sessile and filter-feeding organisms, represents the final phase in the natural seep sealing process. In other regions of the oceans, where other gas compositions, migration rates, and sediment conditions prevail, other sealing processes may occur. (C) 2002 Elsevier Science Ltd. All rights reserved.