Cold seeps

Abstract

Along the ocean margins, sediments experience chemical, microbial, and physical transformations that result in the generation and expulsion of reduced gases and water at the seafloor, in what is known as cold seepage. Methane is generated by microbial or thermochemical processes in the organic-rich marine sediments. Hydrogen sulfide is a by-product of microbial oxidation of methane under anaerobic conditions. Water is produced at greater depths by dehydration reactions of smectite and to a lesser extent opal. Accumulation of the excess water and gas creates overpressures that can fracture the overlying geological formations and drive fluid flow. Sediment compaction, generation of hydraulic heads, and tectonic compression also act as driving forces for fluids that eventually discharge at the seafloor. Whereas the majority of life in the deep ocean floor rely upon low levels of sinking organic matter, the chemosynthetic microbial communities at cold seeps take advantage of high fluxes of bioreactive reductants to support unique, oasis-type cold seep biomes in which the density of organisms is several orders of magnitude greater than in the surrounding regions. Authigenic carbonate and barite deposits, which can reach extremely large accumulations, characterize areas of cold seepage and constitute a geologic record of methane discharge at paleo-seep sites.