Definitions after Judd et al. 2002.
and authigenic carbonate can form mounds, platforms or other structures. Much of the carbonate precipitation is now understood to be microbially mediated (Barbieri & Cavalazzi 2004). Mats of filamentous bacteria and bioherms (reefs or aggregations of clams, tubeworms or mussels) provide biological evidence of seepage. Indirect indicators include bright spots, acoustic turbidity, gas chimneys, scarps, gassy cores and possibly deep-water coral reefs (Table 1).
Significant methane reservoirs are generally found in areas of high organic content (i.e., in sediments underlying upwelling areas characterized by high primary productivity in the water column). When the supply of other oxidants becomes depleted in deeper sediments, CO2 becomes the most important oxidant for the decomposition of organic material coupled to methane production. In geologically active areas, methane-enriched fluids formed by the decomposition of organic matter in deeper sediment layers are forced upward and the advective flow provides a high supply of methane emanating as dissolved or free gas from the sea floor. Under low temperature and high pressure, methane hydrates are formed as ice-like compounds consisting of methane gas molecules entrapped in a cage of water molecules. An increase in temperature or decrease in pressure leads to dissolution of hydrate, yielding high methane concentrations that are dissolved in the surrounding and overlying pore waters or emerge to the overlying water. Methane may originate from decaying organic matter (e.g., sapropel) or by thermogenic degradation of organic matter, with fluid circulation within sediments bringing it to the surface (Coleman & Ballard 2001).
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