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FROZEN HEAT
42
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).
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
2.4
LIFE AT MARINE
METHANE SEEPS