FROZEN HEAT | Volume 1

2.4 LIFE AT MARINE METHANE SEEPS

Sea floor cold seeps emit methane and sometimes other gases into the overlying water column. Some cold seeps are associated with gas hydrates, while others occur at water depths too shallow for gas hydrate to be stable. At these seeps, methane and other fluids are transported to the sea floor through conduits created by over pressurization, leakage of deep gas reservoirs, salt dome accommodation, mud volcano emplacement, and tectonic pro- cesses (Judd et al. 2002; Suess 2010). Methane seeps are often characterized by specialized life forms whose metabolism is based on chemosynthesis (Levin 2005; Suess 2010) (see Text Box 2.3), and these cold-seep environments are distinct from those associated with hydrothermal vents at mid-ocean ridges. The presence of near-surface hydrates at a methane seep tends to spread the methane release over a larger sea floor area, while also increasing the amount of methane dissolved in the pore water. This dissolved methane is more easily consumed by the chemosynthetic community than is the gaseous meth- ane that can bypass chemosynthetic communities by venting through focused gas channels outside the hydrate stability

zone (Treude and Ziebis 2010). Near-surface gas hydrates may also enhance the formation of carbonate pavements in the sediment, produced by anaerobic oxidation of methane (AOM) (Bohrmann et al. 1998). These carbonates, after erosion and exposure, become secondary habitats for deep-sea organisms (e.g. Paull et al. 1984). In this chapter, we will not discriminate between methane-seep life forms found in the presence or ab- sence of near-surface gas hydrates, because their adaptations and survival strategies are almost identical. Because methane seeps associated with gas hydrates were discovered 30 years ago (Paull et al. 1984), their investiga- tion is still in its infancy. Our knowledge of these systems – especially those located on continental margins – is slowly increasing with the advance of deep-sea technologies. Never- theless, we know these ecosystems can be relatively common features along certain continental margins and in tectoni- cally active areas of the sea floor. Investigations of terrestrial seep fossils (i.e., authigenic carbonates that are now exposed on land and believed to have formed along the sea floor at

Figure 2.4: Bacterial mats associated with gas hydrates. White and orange mats of sulphur-oxidizing bacteria cover sediments with near-surface gas hydrates at Hydrate Ridge, Northeast Pacific Ocean (Courtesy of Lisa Levin, Scripps Institution of Oceanography).

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