Methane hydrate in the global organic carbon cycle
Keith A. Kvenvolden
U.S. Geological Survey, 345 Middlefield Road, MS 999, Menlo Park, CA 94025, USA
ABSTRACT
The global occurrence of methane hydrate in outer continental
margins and in polar regions, and the magnitude of the amount
of methane sequestered in methane hydrate suggest that
methane hydrate is an important component in the global
organic carbon cycle. Various versions of this cycle have
emphasized the importance of methane hydrate, and in the
latest version the role of methane hydrate is considered to be
analogous to the workings of an electrical circuit. In this circuit
Introduction
Methane hydrate occurs naturally as a
solid composed of rigid cages of water
molecules that enclose molecules of
mainly methane, often accompanied
by very small amounts of ethane and
carbon dioxide. The physical chemistry of methane hydrate is well
understood (Sloan, 1998), and the
occurrence on Earth of methane
hydrate is controlled by an interrelationship between temperature, pressure, composition of the gas mixture,
and ionic strength of the water (Kvenvolden, 1993). These factors limit its
occurrence on Earth to: (i) oceanic
sediment of continental and insular
slopes and rises of active and passive
margins at water depths greater than
300 m; (ii) deep-water sediment of
inland lakes and seas, also where water
depths are greater than 300 m; and
(iii) polar sediment of both continents
and continental shelves. Thus, methane hydrate occurs worldwide but
in restricted geographic regions. An
updated global inventory (Kvenvolden
and Lorenson, 2001) reports on methane hydrate recovered from 19
places worldwide and 77 places where
the presence of methane hydrate has
been inferred from geophysical, geochemical, and geological evidence
(Fig. 1). Therefore, its global distribution suggests that methane hydrate
could be an important component of
the global organic carbon cycle.
Correspondence: Keith A. Kvenvolden,
U.S. Geological Survey, 345 Middlefield
Road, MS 999, Menlo Park, CA 94025,
USA. Tel.: +1 650 329 4196; fax: +1 650
329 5441; e-mail: kkvenvolden@usgs.gov
302
the methane hydrate is a condenser and the consequences of
methane hydrate dissociation are depicted as a resistor and
inductor, reflecting temperature change and changes in earth
surface history. These consequences may have implications for
global change including global climate change.
Terra Nova, 14, 302–306, 2002
To be an important component in
the global organic carbon cycle, however, this methane-hydrate reservoir
needs to contain significant amounts
of methane carbon. Unfortunately, all
estimates of the methane content of
methane hydrate are highly speculative, but they suggest that the methane
carbon content is very large (Kvenvolden, 1999). When naturally occurring methane hydrate was first
recognized, the amount of methane
in the hydrate form was thought to be
‘enormous’ (Cherskiy and Makogon,
1970), with estimates reaching as
high as 4 million Gt (Gt ¼ giggigaton ¼ 1015 g) of methane carbon.
By the end of the 1980s, estimates of
hydrate methane were greatly constrained when Kvenvolden (1988) and
MacDonald (1990), working independently, estimated the global methane-carbon content of methane
hydrate to be about 10 000 Gt, when
expressed to one significant figure,
giving rise to a ‘consensus’ estimate
at the time.
In the 1990s revised estimates were
made using the power of general
circulation models (GCMs). Best estimates of 14 000 Gt (in situ model of
Gornitz and Fung, 1994) and of
24 000 Gt Harvey and Huang (1995)
were reported. Other global estimates
made during the same period were
3000 Gt (Holbrook et al., 1996),
1,000–10 000 Gt
(Dickens
et al.,
1997), 8000 Gt (Makogon, 1997),
and 500 Gt (Ginsburg and Soloviev,
1995). Thus, the best estimates range
from a low of 500 Gt to a high of
24 000 Gt of methane carbon. The
‘consensus’ value of 10 000 Gt of
methane carbon in methane hydrate
worldwide remains about midway
between the more recently estimated
extremes. Therefore, if this intermediate estimate is at all valid, then
methane hydrate must be an important element in the global organic
carbon cycle, because such a reservoir
of methane carbon is indeed enormous when compared to the sizes of
the other organic carbon reservoirs.
Organic carbon cycles
The major features of the global
carbon cycle were considered in detail
by Bolin (Bolin et al., 1979; Bolin,
1983, 1986). From these considerations, Fig. 2(A) was drawn (Kvenvolden, 1988) to emphasize the
organic carbon portion of the global
carbon cycle and the magnitude of
methane hydrate reservoir relative to
the other organic carbon pools. One
of the striking observations from this
depiction of the organic carbon cycle
was that all of the organic carbon
reservoirs, except one, were smaller
than the reservoir of organic carbon in
methane hydrate, estimated to be
about 10 000 Gt of methane carbon.
This value is twice as large as the fossil
fuel (oil, coal, and natural gas) reservoir containing 5000 Gt of carbon.
This value of 5000 Gt deserves
some scrutiny. For example, Bolin
et al. (1979) provided the first estimate
of > 5000 Gt of fossil fuel carbon,
but with time the (>) designation was
dropped (Bolin, 1983, 1986). Much
earlier Hunt (1972) had estimated that
the fossil fuel reservoir contained
16 000 Gt of carbon, and this value
was carried into the textbook by
Mason and Moore (1982). Mean 2002 Blackwell Science Ltd
K. A. Kvenvolden • Global methane hydrate
Terra Nova, Vol 14, No. 5, 302–306
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Fig. 1 Worldwide locations of known and inferred gas hydrate in aquatic (oceans,
seas, lakes) sediment and in polar continental sediment. Open circles indicate
locations where samples of natural gas hydrate have been recovered. Filled circles
indicate locations of inferred presence of natural gas hydrate. Filled squares show
regions for potential gas-hydrate occurrence in Russia. Modified from Kvenvolden
(2000).
while, Holser et al. (1988) estimated
11 000 Gt of carbon which included
the organic carbon of oil shales. Given
the lack of information on how any of
these values were actually obtained
and exactly what fossil fuel carbon
sources were included, the choice of
5000 Gt of carbon for the fossil fuel
reservoir in Fig. 2(A) is conservative.
The comparison between the magnitude of the gas hydrate reservoir and
the fossil fuel reservoir can be misleading, however. This comparison
assumes that all of the methane in
the gas hydrate reservoir can be
used as a source of fuel, which is
undoubtedly not true. The portion of
the hydrate methane reservoir that can
be used for fuel has yet to be estimated.
The features of Fig. 2(A) have been
rearranged in Fig. 2(B) to show methane hydrate as a central component in
the global organic carbon cycle that
affects in different ways the lithosphere, biosphere, hydrosphere, and
atmosphere. The methane hydrate is
both a sink and a source for methane.
There is evidence that methane from
dissociating gas hydrate enters the
ocean (MacDonald et al., 1994; Suess
et al., 1999). Whether sufficient me 2002 Blackwell Science Ltd
thane from methane hydrate reaches
the atmosphere where its greenhouse
gas properties could influence climate
change is still an unresolved issue.
RecentlyDickens (1999) introduced
the idea that methane hydrate acts as
a large capacitor in an organic carbon cycle modelled by Walker and
Kasting (1992). The capacitor stores
and releases methane, whereas a
capacitor in an electric circuit stores
and releases electrons. This idea has
evolved so that fluxes of methane to
and from the capacitor have been
estimated for the late Paleocene thermal maximum (Dickens, 2001a),
leading to the conclusion that
methane hydrate is likely a factor in
global change.
Global change
Support for the idea that methane
hydrate is an important factor in
global change comes from the carbon
isotopic record of both marine and
terrestrial records. Dickens et al.
(1997) showed that marine carbonate
and organic matter deposited during
the latest Paleocene thermal maximum worldwide is characterized by a
remarkable )2.5& excursion in d13C.
They suggested that methane released
from oceanic methane hydrate offers a
plausible explanation, because the
average carbon isotopic composition
of hydrate methane ()60&) provides
the necessary carbon isotopic composition to lead to the negative carbon
isotopic excursions. If all of this methane carbon was utilized in affecting
the carbon budget of the oceans
(Dickens, 2001b), then none reached
the atmosphere where its presence is
required to influence global climate
change. Recent evidence, however,
from the terrestrial record found within the Aptian Stage of the Lower
Cretaceous suggests that methane
from dissociated methane hydrate
may have reached the atmosphere,
giving rise to a striking )5& excursion
in d13C as measured in vascular landplant tissue at three localities (Jahren
et al., 2001).
Although our understanding of the
role of methane hydrate in global
change – including global climate
change – is incomplete, the circumstantial evidence available thus far is
highly suggestive. Worldwide marine
and terrestrial records, however, will
be needed from the same intervals of
geological time that all show the
appropriate negative carbon isotopic
excursions. Such consistency in data
would seriously implicate methane
hydrate as an important factor in
Earth surface processes and in global
change.
Tracking methane in the water column above currently dissociating gas
hydrate would provide further
evidence for the fate of methane in
the ocean/atmosphere system. However, the fate of methane hydrate in
the global methane budget is poorly
understood because so little is known
about how much methane from dissociating methane hydrate actually
reaches that system (Valentine et al.,
2001). Microbial aerobic methane
oxidation in the water column
(Valentine et al., 2001) and anaerobic
methane oxidation in the surface sediment (Orphan et al., 2001) limit the
amount of methane that enters the
water and the atmosphere. However,
if the rate of methane release from
methane hydrate exceeds the rate of
oxidation, methane from this source
should reach the atmosphere, all other
factors being equal, and likely affect
global climate change.
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Terra Nova, Vol 14, No. 5, 302–306
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Fig. 2 (A) The global organic carbon cycle adapted from Moore and Bolin (1987) showing the addition of a compartment for the
gas hydrate reservoir. Numbers are given in gigatonnes (1015 g) of methane carbon. From Kvenvolden (1988). (B) The
compartments of (A) are rearranged to show methane hydrate as a central component in the global organic carbon cycle. Units (Gt
or gigatonnes) of methane carbon are the same as in (A). The methane hydrate lies within the shallow and deep biosphere, is found
in the shallow lithosphere, and can occur at the interface between the lithosphere/biosphere and the hydrosphere in aquatic
systems. Biochemical oxidation of hydrate methane occurs in the lithosphere/biosphere whereas photochemical oxidation occurs in
the atmosphere. Vents and seeps depict the release of methane to the hydrosphere and atmosphere from non-hydrate as well as
hydrate sources.
Resonant circuit analogue
In order to attempt to capture in a
simplistic way the possible role of
methane hydrate in global change,
including global climate change, the
idea of methane hydrate as a capacitor
(Dickens 2001a,b) is advanced further.
304
The role of methane hydrate in the
global organic carbon cycle is visualized as being analogous to the workings of a coupled electric circuit
composed of a battery, condenser
(capacitor), resistor, inductor, and
timer (Fig. 3). The battery is the
source of methane, the condenser
stores and releases methane, and the
resistor and inductor represent in a
crude way the possible consequences
resulting from the release of methane.
The clock controls the time for switching back and forth between the two
circuits and is likely influenced by
some feedback. While the variable
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K. A. Kvenvolden • Global methane hydrate
Terra Nova, Vol 14, No. 5, 302–306
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Fig. 3 Electrical resonant circuit analogue for methane hydrate in the global organic carbon cycle. This simplistic drawing brings
attention to the possible interrelated consequences resulting from methane hydrate formation and decomposition throughout
geological time. The workings of the inductor, resistor and feedback circuit are analogous to the various global changes that may
be influenced by methane hydrate.
switch closes on the battery side
(opens on the inductor side) the condenser or methane hydrate stores
methane. Then one switch closes and
the other opens, releasing methane
from the methane hydrate condenser
to the resistor and inductor, leading to
possible interrelated consequences
such as changes in ocean chemistry,
temperature, sea level and glacial history. The coupled switches regulate
the flow of methane in the two
circuits. It is not certain yet the extent
to which the methane hydrate causes
these changes or is affected by these
changes, but the switches continue to
open and close through geological
time as methane hydrate stores and
releases methane.
There is a need to discover the
feedback controls of this system and
the directions in which they operate.
Although this representation (Fig. 3) is
simplistic and ignores possible feedback consequences of changing surficial conditions, it tries to illustrate the
dynamic nature of the interrelated
processes that methane hydrate can
effect. Many of these processes have
been addressed previously. For exam 2002 Blackwell Science Ltd
ple, McIver (1982) recognized a possible connection between gas hydrate
and submarine geohazards; Nisbet
(1990) and Paull et al. (1991) considered issues of glacial cycles, global
temperature, sea level, and climate
change; and Dickens et al. (1997)
noted changes in ocean chemistry (the
carbon isotopic record). Evidence for
an interrelationship among methane
hydrate and these various processes is
often suggestive, but not necessarily
convincing. Indeed, methane hydrate
may be a significant factor in global
changes at and near to the Earth’s
surface, but a firm understanding of its
role, as suggested in the resonant circuit analogue (Fig. 3), requires much
additional multidisciplinary research.
Acknowledgments
I thank B. W. Rogers of the USGS for
preparing the figures for this paper, and
I am grateful to W. G. Abraham, Varian
Associates (retired) for his help in designing the resonant circuit analogue. Finally,
I acknowledge with thanks the reviewers
(Martin Hovland, Euan Nisbet, and
Anonymous) for their very useful comments.
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Received 16 October 2001; revised version
accepted 26 February 2002
2002 Blackwell Science Ltd