Life in the Deep Sediments
Signal in the Sediment
Besides the affect on the
nutrient cycles of the ocean, the bacterial populations have another
known major contribution to oceanic sediments: gas
hydrates. Two different sources of deep
methane have been recognized: the deep
biosphere and thermogenic reactions that form methane and other
hydrocarbons. Thermogenic methane is
usually mixed in with other thermogenically derived hydrocarbons, where
as
biogenic methane is almost entirely methane (Wellsbury and Parkes 2000).
There is also a distinctive δ13C signature for
biogenic
versus thermogenic methane. Thermogenic
methane lies between -20% to -50% (Whiticar 1999) whereas biogenic
methane from
acetate at -50% to 65% and methane from H2/CO2
being the
most deplete at -60% to 110% (Wellsbury and Parkes 2000).
However, the signal is often obscured by the
oxidation of methane, which increases the δ13C content
of residual
methane, increases in temperature, and reservoir effects.
However, most methane deposits are recognized
as biogenic, such as those at the Blake Ridge
and Cascadian
Margin (Wellsbury and Parkes 2000).
The particular
substrate that
dominates methane production may be deduced using hydrogen isotopes (Wellsbury and Parkes 2000). Methane derived from acetate has a δD of
more
than -250%, while that derived from H2/CO2 has a
value of
-150% to 250%. In a gas hydrate zone on
the Cascadian Margin, methanogenesis due to H2/CO2
was up
to 5 orders of magnitude lower than the rate of methane oxidation. Acetate concentrations increased with depth
at the site, giving a supply for acetate methanogenesis.
Rates of acetate methanogenesis were two
orders of magnitude greater than those for H2/CO2,
but
were only sampled at one of the five sites.
It was, however, high enough to supply methane to the reservoir
in
greater quantities than it was being oxidized. (Wellsbury and Parkes
2000).