Minerals Report - International Seabed Authority

4.2.5. Benthic Biomass
The melting of hydrate at depth generates methane, small amounts
of hydrogen sulphide and ammonia, and freshwater. Oxidation of these
chemicals into carbon dioxide, sulphates and nitrates provides rich
nutrients supply for chemosynthetic bacteria 10 . The Pogonophorans group
of organisms in general, and Scalerolinum and Oligobrachia species in
particular have been shown to harbour methanotroph bacteria in areas of
increased methane discharge on the seafloor. The three main microbial
processes occurring during the diagenesis in sediments (organic matter)
can be explained by the following three chemical reactions 41,45 .
a) Sulphate reduction:
SO4 2- + 2H2O H2S + 2HCO3 -
b) Methane generation:
CH3COOH CH4 + CO2 and CO2 + 4H2 CH4 + 2H2O
c) Methane oxidation:
CH4 + 2O2 CO2 + 2H2O
The above reactions clearly indicate that in both methane oxidation
and methane generation processes, CO2 is evolved. The evolved CO2 reacts
with calcium in seawater precipitating it as carbonate slabs and nodules in
the vicinity of methane emanations. These carbonate precipitates are
enriched in the light carbon isotope.
Some recent works have indicated the occurrence of high benthic
biomass in methane rich areas, where methanotrophs form the base of the
benthic food web. Vestimentifera, tubeworms or clam colonies, on an
otherwise sparsely inhabited seafloor, may provide the visible clues for the
presence of buried hydrate deposits.
4.3. Gas escape features on the sea floor
In many parts of the world's ocean, high resolution acoustic
investigations (side scan imagery, shallow sub bottom profiling) of the sea
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