of the sediment column (review by Zonneveld et al.,
2010). Early organic matter decomposition is partially controlled by the rate of sedimentation and is higher in regions
of low sediment accumulation. Consequently, large methane concentrations are usually found in the rapidly accumulating sediments that underlie the high-productivity regions
of most continental margins. The sediments beneath the
low-productivity waters of the ocean gyres and those overlying recently formed oceanic crust do not harbor significant amounts of this simple hydrocarbon.
Sources
Our understanding of the origin and distribution of methane in marine sediments was pioneered by the work of
Claypool and Kaplan (1974), who described microbial
methane generation as the terminal catabolic product of
organic matter degradation. As such, it follows an ecological succession of microbial ecosystems, driven mainly by
the redox condition of the sediment. The remineralization
of organic material proceeds in a continuous sequence of
redox reactions that cascade from high to low free energy
yield until the electron acceptor source is depleted. The
sediment depths that harbor the various stages of this
redox ladder depend on the nature and magnitude of the
organic loading, sedimentation rate, seawater oxygenation, and sediment porosity. Methane production, commonly referred to as methanogenesis, is only significant
in environments where burial of organic matter is rapid
enough to survive remineralization by the more energy
favorable electron acceptors (O 2 , NO 3 , Fe, Mn, and
SO 4 ). Globally, microbial methanogenesis in marine and
freshwater systems is thought to generate ~85 Tg CH 4 /
year (1 Tg ¼ 10
12 g; Reeburgh et al., 1993).
Microbial methanogenesis is a multistep process,
performed exclusively by a diverse group of obligate
anaerobic archaea, generally known as methanogens.
These microorganisms can live in a wide range of temperature, salinity, and pH, but are limited in the substrates
they can utilize for growth. A detailed description of the
microbial methanogenic pathways in marine sediments is
given by Wellsbury et al., (2000). The main pathway for
microbial methane generation in organic-rich anoxic sediments is thought to proceed via autotrophic
methanogenesis, whereby advantage of the energy
released from the reduction of carbon dioxide by molecular hydrogen is taken. This is a multistep process, but the
net reaction can be written as
CO 2 þ 8 H
þ
þ 8 e
À
! CH 4 þ 2 H 2 O
ð1Þ
Heterotrophic methanogens are known to utilize simple
organic acids such as acetic acid to generate methane via
acetoclastic fermentation, which releases both methane
and CO 2 :
CH 3 COOH ! CH 4 þ CO 2
ð2Þ
Temperature is a critical factor in sub-seafloor methane
generation, as it may impact enzyme activity and
metabolic rates (Colwell et al., 2008). Usually deeper in
the sediment, thermal alteration of organic matter generates methane and higher order hydrocarbons within the
temperature range of 50–200
C. At typical oceanic geothermal gradients of 20–50
C/km, depending on the local
geothermal gradient, sediment depths larger than 1 km are
required to produce significant amounts of hydrocarbons
by thermochemical reaction, with low molecular weight
hydrocarbons (methane to butane) being produced at rates
that are proportional to temperature.
Even though a few hyperthermophilic methanogens
have been shown to produce methane at temperatures of
~122
C (Takai et al., 2008), the upper limit for subsurface
methanogenesis appears to be ~80
C (Wilhems et al.,
2001), which corresponds to depths of 1.500–3.000 m
below seafloor, depending on the local geothermal
gradient.
Several combinations of molecular and isotopic characteristics are commonly used to identify hydrocarbon
sources (Figure 1), as well as their post-genetic alterations.
Microbial methanogenesis generates mainly methane (C 1 )
and traces of ethane (C 2 ) and propane (C 3 ), resulting in relatively high C 1 /(C 2 + C 3 ) ratios (commonly > 1,000;
Figure 1a). In contrast, thermogenic gases are commonly
enriched in higher hydrocarbons, resulting in low
C 1 /(C 2 + C 3 ) ratios (<100). The now classic differentiation
scheme places microbial and thermogenic methane into
distinct regions within the d
13
C CH4 and dD CH4 field
(Figure 1b). In addition, carbon and hydrogen isotopes also
aid in elucidating autotrophic vs heterotrophic microbial
pathways. Differences in carbon isotopic fractionation
arise from the large kinetic effects involved in breaking
the C-O bond during CO 2 reduction (Eq. 1), which leads
to a more
13
C-depleted methane than the methane generated by heterotrophic methanogenesis (Eq. 2), in which
the methyl group remains intact. The hydrogen isotope signature may also provide information on the metabolic
pathways, as acetate fermentation yields methane with
a dD value lower than À250 %, whereas carbonate
fermentation leads to dD values ranging from À150 % to
À250 % (Whiticar et al., 1986).
Sediment reservoirs
Marine sediments contain an estimated 500–10,000 Gt
(Kvenvolden and Lorenson, 2001; Milkov, 2004) of methane carbon. Because of its low solubility, methane
dissolved in the pore fluids accounts for a negligible fraction of the total sedimentary reservoir. Another small fraction is adsorbed to solid phases (Reeburgh 2007).
Traditional natural gas reservoirs are locations characterized by the presence of a source rock (high organic carbon
needed for methane generation), a sedimentary trap
(normally high-porosity layers where methane gas can
accumulate), and a seal rock (which prevents methane
leakage from the reservoir). Perhaps the larger methane
component of the marine sediment pool occurs in the form
of methane hydrates. These compounds are ice-like
496
METHANE IN MARINE SEDIMENTS
2010). Early organic matter decomposition is partially controlled by the rate of sedimentation and is higher in regions
of low sediment accumulation. Consequently, large methane concentrations are usually found in the rapidly accumulating sediments that underlie the high-productivity regions
of most continental margins. The sediments beneath the
low-productivity waters of the ocean gyres and those overlying recently formed oceanic crust do not harbor significant amounts of this simple hydrocarbon.
Sources
Our understanding of the origin and distribution of methane in marine sediments was pioneered by the work of
Claypool and Kaplan (1974), who described microbial
methane generation as the terminal catabolic product of
organic matter degradation. As such, it follows an ecological succession of microbial ecosystems, driven mainly by
the redox condition of the sediment. The remineralization
of organic material proceeds in a continuous sequence of
redox reactions that cascade from high to low free energy
yield until the electron acceptor source is depleted. The
sediment depths that harbor the various stages of this
redox ladder depend on the nature and magnitude of the
organic loading, sedimentation rate, seawater oxygenation, and sediment porosity. Methane production, commonly referred to as methanogenesis, is only significant
in environments where burial of organic matter is rapid
enough to survive remineralization by the more energy
favorable electron acceptors (O 2 , NO 3 , Fe, Mn, and
SO 4 ). Globally, microbial methanogenesis in marine and
freshwater systems is thought to generate ~85 Tg CH 4 /
year (1 Tg ¼ 10
12 g; Reeburgh et al., 1993).
Microbial methanogenesis is a multistep process,
performed exclusively by a diverse group of obligate
anaerobic archaea, generally known as methanogens.
These microorganisms can live in a wide range of temperature, salinity, and pH, but are limited in the substrates
they can utilize for growth. A detailed description of the
microbial methanogenic pathways in marine sediments is
given by Wellsbury et al., (2000). The main pathway for
microbial methane generation in organic-rich anoxic sediments is thought to proceed via autotrophic
methanogenesis, whereby advantage of the energy
released from the reduction of carbon dioxide by molecular hydrogen is taken. This is a multistep process, but the
net reaction can be written as
CO 2 þ 8 H
þ
þ 8 e
À
! CH 4 þ 2 H 2 O
ð1Þ
Heterotrophic methanogens are known to utilize simple
organic acids such as acetic acid to generate methane via
acetoclastic fermentation, which releases both methane
and CO 2 :
CH 3 COOH ! CH 4 þ CO 2
ð2Þ
Temperature is a critical factor in sub-seafloor methane
generation, as it may impact enzyme activity and
metabolic rates (Colwell et al., 2008). Usually deeper in
the sediment, thermal alteration of organic matter generates methane and higher order hydrocarbons within the
temperature range of 50–200
C. At typical oceanic geothermal gradients of 20–50
C/km, depending on the local
geothermal gradient, sediment depths larger than 1 km are
required to produce significant amounts of hydrocarbons
by thermochemical reaction, with low molecular weight
hydrocarbons (methane to butane) being produced at rates
that are proportional to temperature.
Even though a few hyperthermophilic methanogens
have been shown to produce methane at temperatures of
~122
C (Takai et al., 2008), the upper limit for subsurface
methanogenesis appears to be ~80
C (Wilhems et al.,
2001), which corresponds to depths of 1.500–3.000 m
below seafloor, depending on the local geothermal
gradient.
Several combinations of molecular and isotopic characteristics are commonly used to identify hydrocarbon
sources (Figure 1), as well as their post-genetic alterations.
Microbial methanogenesis generates mainly methane (C 1 )
and traces of ethane (C 2 ) and propane (C 3 ), resulting in relatively high C 1 /(C 2 + C 3 ) ratios (commonly > 1,000;
Figure 1a). In contrast, thermogenic gases are commonly
enriched in higher hydrocarbons, resulting in low
C 1 /(C 2 + C 3 ) ratios (<100). The now classic differentiation
scheme places microbial and thermogenic methane into
distinct regions within the d
13
C CH4 and dD CH4 field
(Figure 1b). In addition, carbon and hydrogen isotopes also
aid in elucidating autotrophic vs heterotrophic microbial
pathways. Differences in carbon isotopic fractionation
arise from the large kinetic effects involved in breaking
the C-O bond during CO 2 reduction (Eq. 1), which leads
to a more
13
C-depleted methane than the methane generated by heterotrophic methanogenesis (Eq. 2), in which
the methyl group remains intact. The hydrogen isotope signature may also provide information on the metabolic
pathways, as acetate fermentation yields methane with
a dD value lower than À250 %, whereas carbonate
fermentation leads to dD values ranging from À150 % to
À250 % (Whiticar et al., 1986).
Sediment reservoirs
Marine sediments contain an estimated 500–10,000 Gt
(Kvenvolden and Lorenson, 2001; Milkov, 2004) of methane carbon. Because of its low solubility, methane
dissolved in the pore fluids accounts for a negligible fraction of the total sedimentary reservoir. Another small fraction is adsorbed to solid phases (Reeburgh 2007).
Traditional natural gas reservoirs are locations characterized by the presence of a source rock (high organic carbon
needed for methane generation), a sedimentary trap
(normally high-porosity layers where methane gas can
accumulate), and a seal rock (which prevents methane
leakage from the reservoir). Perhaps the larger methane
component of the marine sediment pool occurs in the form
of methane hydrates. These compounds are ice-like
496
METHANE IN MARINE SEDIMENTS