14 Gas Hydrates in Marine Sediments
506
The formation of hydrogen sulfide constitutes an
energy source for chemoautotrophic organisms living
on the seafloor. The colonization of the seeps depends
on the local H 2 S-gradient generated by AOM (Barry
and Kochevar 1998; Sahling et al. 2002). The sulfideoxidizing bacterium Beggiatoa, is usually found
forming mats in areas with very high sulfide flux.
Calyptogena clams, typically colonize areas with lower
sulfide concentrations and surround the Beggiatoa
mats. Acharax clams live in burrows within the
sediment and are restricted to environments of very
low sulfide concentration (Fig. 14.22). In addition to
sulfide production, AOM increases carbonate alkalinity, which drives pervasive carbonate precipitation.
The high concentration of bicarbonate as respiration
product (equation 9), the presence of microbial
surfaces, and the exudation of organic polymers that
can bind calcium ions are all factors that support active
carbonate precipitation (Iversen and Jørgensen 1985).
Near-surface deposits of porous gas hydrate (Fig.
14.11) are ideal sites for AOM because sulfate can
migrate through the porous space to the inner parts of
the hydrates, where the microbial consortia can thrive.
The aragonite precipitates observed within the
sponge-like bubble structure of gas hydrates are
evidence for such microbial processes. In addition,
biomarker analyses of those layers show extremely
high amounts of components (e.g. isoprenoids
crocetane and pentamethylicosane) typical of those
produced by methane-consuming and sulfatereducing microorganisms (Elvert et al. 2001).
14.6 Concluding Remarks
Although the existence of gas hydrates has now been
known for decades, our understanding of their
potential impact on slope stability, the biosphere,
carbon cycling, and climate change is still in its infancy.
Laboratory and field studies at gas-hydrate-bearing
sites, including several drilling expeditions in the past
decade, have provided critical background data on
the conditions of gas hydrate stability, and provide
overall view of the composition and distribution of
gas hydrates in nature (e.g. Dickens 2003). These
results have sparked the development of models
relating hydrate dynamics to tectonic and slope
stability, and the possible impact of this system on
global climate (Dickens 2003; Davie and Buffett 2001;
Sloan 1998; Clennell et al. 1999).
Although the total amount of carbon trapped in
gas hydrate is poorly constrained, existing knowledge
suggest that these deposits may constitute a significant carbon reservoir, but a quantitative evaluation
of its resource potential depends on reliable global
and national inventories, and a better understanding
of the geologic factors that lead to highly
concentrated hydrate deposits.
Methane is a powerful greenhouse gas with a
Greenhouse Warming Potential (GWP) 23 times that
of CO 2 on a per-molecule basis. Sudden release of
methane from gas hydrate therefore has the potential
to affect global climate, and current hypotheses
attribute past climate variations to methane release
from gas hydrates in response to ocean warming and/
or sea level change (Paull et al. 1991; Kennett et al.
2002; Dickens et al. 1995; Haq 1998). However, these
hypotheses have yet to be confirmed and more research
is needed to evaluate hydrate response to environmental change; the fate of steady fluxes of methane
from hydrate reservoirs to the seabed, ocean surface
and the atmosphere; and the radiative forcing of
methane on climate change.
The impact of gas hydrate on seafloor stability is
important for evaluating the safety of offshore structures as well as for understanding its role in rapid
release of methane, which may affect climate change.
Since gas hydrate encases large volumes of methane,
when destabilized, these deposits may transform the
host sediment into a gassy, water rich fluid. However,
any buildup of overpressure from excess gas will
depend on the balance between hydrate dissociation
and pressure dissipation through possible permeability
barriers. Freshening of the pore water may trigger slope
instabilities through a possible „quick clay” behavior,
which in turns would depend on the clay mineralogy
of the sediment. Although massive landslide triggered
by gas hydrate destabilization has not been directly
observed, various investigators have shown that vast
stretches of the oceanic margins where there is
evidence for major large-scale slides and slumps
coincide with deep water gas hydrate horizons
(Mienert et al. 1998; Nisbet and Piper 1998; Paull et al.
2000). There are still gaps in our understanding of the
mechanisms through which decaying hydrate may
affect slope stability, on the triggering mechanism for
gas hydrate decay, and on the environmental response
to slope failure, in particular the possible generation
of tsunamis (Driscoll et al. 2000). There are ongoing
efforts to understand these phenomena and to develop
predictive models, for example, in the region of the
Storrega slide, off the coast of Norway (Bouriak et al.
2000; Bryn et al. 2003).
A full understanding of the complex interrelationships associated with the presence of gas
506
The formation of hydrogen sulfide constitutes an
energy source for chemoautotrophic organisms living
on the seafloor. The colonization of the seeps depends
on the local H 2 S-gradient generated by AOM (Barry
and Kochevar 1998; Sahling et al. 2002). The sulfideoxidizing bacterium Beggiatoa, is usually found
forming mats in areas with very high sulfide flux.
Calyptogena clams, typically colonize areas with lower
sulfide concentrations and surround the Beggiatoa
mats. Acharax clams live in burrows within the
sediment and are restricted to environments of very
low sulfide concentration (Fig. 14.22). In addition to
sulfide production, AOM increases carbonate alkalinity, which drives pervasive carbonate precipitation.
The high concentration of bicarbonate as respiration
product (equation 9), the presence of microbial
surfaces, and the exudation of organic polymers that
can bind calcium ions are all factors that support active
carbonate precipitation (Iversen and Jørgensen 1985).
Near-surface deposits of porous gas hydrate (Fig.
14.11) are ideal sites for AOM because sulfate can
migrate through the porous space to the inner parts of
the hydrates, where the microbial consortia can thrive.
The aragonite precipitates observed within the
sponge-like bubble structure of gas hydrates are
evidence for such microbial processes. In addition,
biomarker analyses of those layers show extremely
high amounts of components (e.g. isoprenoids
crocetane and pentamethylicosane) typical of those
produced by methane-consuming and sulfatereducing microorganisms (Elvert et al. 2001).
14.6 Concluding Remarks
Although the existence of gas hydrates has now been
known for decades, our understanding of their
potential impact on slope stability, the biosphere,
carbon cycling, and climate change is still in its infancy.
Laboratory and field studies at gas-hydrate-bearing
sites, including several drilling expeditions in the past
decade, have provided critical background data on
the conditions of gas hydrate stability, and provide
overall view of the composition and distribution of
gas hydrates in nature (e.g. Dickens 2003). These
results have sparked the development of models
relating hydrate dynamics to tectonic and slope
stability, and the possible impact of this system on
global climate (Dickens 2003; Davie and Buffett 2001;
Sloan 1998; Clennell et al. 1999).
Although the total amount of carbon trapped in
gas hydrate is poorly constrained, existing knowledge
suggest that these deposits may constitute a significant carbon reservoir, but a quantitative evaluation
of its resource potential depends on reliable global
and national inventories, and a better understanding
of the geologic factors that lead to highly
concentrated hydrate deposits.
Methane is a powerful greenhouse gas with a
Greenhouse Warming Potential (GWP) 23 times that
of CO 2 on a per-molecule basis. Sudden release of
methane from gas hydrate therefore has the potential
to affect global climate, and current hypotheses
attribute past climate variations to methane release
from gas hydrates in response to ocean warming and/
or sea level change (Paull et al. 1991; Kennett et al.
2002; Dickens et al. 1995; Haq 1998). However, these
hypotheses have yet to be confirmed and more research
is needed to evaluate hydrate response to environmental change; the fate of steady fluxes of methane
from hydrate reservoirs to the seabed, ocean surface
and the atmosphere; and the radiative forcing of
methane on climate change.
The impact of gas hydrate on seafloor stability is
important for evaluating the safety of offshore structures as well as for understanding its role in rapid
release of methane, which may affect climate change.
Since gas hydrate encases large volumes of methane,
when destabilized, these deposits may transform the
host sediment into a gassy, water rich fluid. However,
any buildup of overpressure from excess gas will
depend on the balance between hydrate dissociation
and pressure dissipation through possible permeability
barriers. Freshening of the pore water may trigger slope
instabilities through a possible „quick clay” behavior,
which in turns would depend on the clay mineralogy
of the sediment. Although massive landslide triggered
by gas hydrate destabilization has not been directly
observed, various investigators have shown that vast
stretches of the oceanic margins where there is
evidence for major large-scale slides and slumps
coincide with deep water gas hydrate horizons
(Mienert et al. 1998; Nisbet and Piper 1998; Paull et al.
2000). There are still gaps in our understanding of the
mechanisms through which decaying hydrate may
affect slope stability, on the triggering mechanism for
gas hydrate decay, and on the environmental response
to slope failure, in particular the possible generation
of tsunamis (Driscoll et al. 2000). There are ongoing
efforts to understand these phenomena and to develop
predictive models, for example, in the region of the
Storrega slide, off the coast of Norway (Bouriak et al.
2000; Bryn et al. 2003).
A full understanding of the complex interrelationships associated with the presence of gas