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Z. Warhaft for helpful discussions and suggestions over
the course of this work.
24 October 2005; accepted 5 January 2006
Late Quaternary Atmospheric CH4
Isotope Record Suggests Marine
Clathrates Are Stable
One explanation for the abrupt increases in atmospheric CH4, that occurred repeatedly during
the last glacial cycle involves clathrate destabalization events. Because marine clathrates have
a distinct deuterium/hydrogen (D/H) isotope ratio, any such destabilization event should cause the
D/H ratio of atmospheric CH4(dDCH4) to increase. Analyses of air trapped in the ice from the second
Greenland ice sheet project show stable and/or decreasing dDCH4values during the end of the
Younger and Older Dryas periods and one stadial period, suggesting that marine clathrates were
stable during these abrupt warming episodes. Elevated glacial dDCH4values may be the result of a
lower ratio of net to gross wetland CH4emissions and an increase in petroleum-based emissions.
Over long time scales (greater than 10,000
years) atmospheric CH4changes have a sub-
stantial amount of variance concentrated in
the precessional bandwidth (19,000 and 23,000
years) (1, 2) that is considered to be an integral
part of tropical climate throughout the late
Pleistocene. One hypothesis that accounts for
this observation involves an energized hydro-
logic cycle during periods of elevated low-
latitude insolation. The invigorated hydrologic
cycle promotes an increase in wetland extent
driving a concomitant increase in CH4emis-
sions that raise atmospheric CH4levels during
warm periods. Embedded within the precession
signal are millennial- and century-scale varia-
tions that are tightly coupled to Greenland
temperature (3, 4). In general, increasing at-
mospheric CH4levels are synchronous with, or
slightly lag (by a few decades), the surface tem-
perature increase over Greenland (5). Assessing
the nature of these abrupt CH4events is im-
portant for understanding how ecosystems and
he ice core record of atmospheric CH4
changes covering the past 650,000
years exhibits two primary frequencies.
climate are connected and in estimating the
degree to which future CH4levels may con-
tribute to changes in Earth_s radiation budget.
abruptCH4increases. One hypothesis holds that
the terrestrial biosphere is capable of rapidly
increasing CH4emissions in response to abrupt
changes in the hydrologic cycle that are tele-
connected to surface temperatures over Green-
land (3, 4). The other explanation involves the
sudden release of marine clathrates situated
along the continental margin where episodic
destabilization events may have been triggered
by enhanced ventilation (warming) of upper
thermocline waters (6). The majority of the
released CH4ultimately travels across the air-
sea interface leading to atmospheric CH4
Model estimates of changes in the primary
CH4sink (tropospheric hydroxyl radical) dur-
ing the last glacial termination suggest that
the observed CH4variations must be due in
large part to changes in the sources as opposed
to changes in the rate of removal (7). The
isotopic composition of atmospheric CH4
therefore provides additional information on
the relative contribution of the various sources.
Variations in the D/H ratio of atmospheric CH4
(dDCH4) can be used to infer variable clathrate
contributions on the basis of their elevated dD
values compared with all terrestrial CH4
sources (Fig. 1). Methane clathrates within the
continental margin sediments are formed al-
most exclusively by CO2reduction or thermal
cracking of longer chain hydrocarbons, whereas
terrestrial CH4emissions are primarily acetic-
lastic in nature (8, 9). During CO2reduction, all
the methyl hydrogen atoms come directly from
porewater H2that is in isotopic equilibrium
with the porewater (10). The resulting dDCH4
values are lower than the porewater dDH2Odue
to a È180 per mil (°) biologically induced iso-
tope effect associated with CO2reduction (9, 11).
Marine clathrate dDCH4values from 13 near-
shore sites scattered throughout the Northern
Hemisphere are surprisingly constant (–189 T
27°; error is SD) given the diverse nature of
the geologic and sedimentologic settings and
the varying proportions of microbial and
thermogenic CH4at each site (12, 13). In con-
trast, CH4production in terrestrial ecosystems
is dominated by acetogenesis (acetate fer-
mentation) where three-fourths of the hydro-
gen atoms in the emitted methane originate
from the methyl group associated with the
acetate substrate. The remaining hydrogen
comes from the local water with the resulting
terrestrial dDCH4values generally ranging from
–250 to –380°, with the local dDCH4value
strongly influenced by the dD of precipitation
An atmospheric dDCH4record (Fig. 2) was
generated from the second Greenland ice sheet
project (GISP II) ice core using a previously
described technique with an external precision
of T4.2° (14). The general picture of dDCH4
variations associated with the deglaciation shows
a progressive decrease in dDCH4as the con-
centration of CH4increases, opposite to that
predicted by increasing clathrate contributions
due to warming associated with the termina-
tion. During the last glacial maximum (LGM),
dDCH4values were generally È5° higher than
the Bolling/Allerod values E15 to 13 thousand
years ago (ka)^ and È20° higher than early
Holocene values. There are three factors that
can be reasonably constrained as contributing
to the elevated dDCH4values during the LGM.
All three factors are temperature dependent, so
Department of Geosciences and the Earth and Environ-
mental Systems Institute, Pennsylvania State University,
University Park, PA 16802, USA. E-mail: sowers@geosc.
10 FEBRUARY 2006 VOL 311SCIENCEwww.sciencemag.org
estimates have been made on the basis of two
different tropical LGM temperature estimates
(Table 1). First, colder temperatures during this
period would have increased atmospheric dDCH4
through the temperature-dependent kinetic iso-
tope effect (KIE) associated with the primary
removal process, OH oxidation in the tropo-
sphere. The magnitude of this effect is þ3.4°
on the basis of the laboratory-determined
temperature dependence (15), assuming tropo-
spheric temperatures were 5-C colder during
the LGM. Secondly, dD changes in mean ocean
water arising from changes in continental ice
volume impart a direct effect on atmospheric
dDCH4by altering the dD of porewater H2that is
utilized by CO2reducing methanogens (oceanic
and terrestrial). A less direct effect occurs as
the oceanic dDH2Ochange is propagated through
the hydrologic cycle and incorporated in ter-
restrial organic hydrocarbons (16, 17), the pri-
mary substrate for the fermentative methanogens.
The mean ocean dDH2Ochange E7.5° (18)^ is
transferred through the LGM global hydro-
logic cycle (19, 20), causing changes in dDCH4
values for terrestrial CH4that range from –5 to
5°, depending on the assumed LGM tropical
temperatures (8, 9). Finally, a 10% decrease in
the ratio of C3- to C4-type plants during the
LGM (21) would have lowered atmospheric
dDCH4values by 0 to 1.9° relative to Holo-
cene values. Together these three factors
account for a small portion of the observed
20° dDCH4shift between the LGM and early
Holocene, implying that other factors must be
There are at least three additional factors
contributing to the atmospheric dDCH4change
associated with the termination that are difficult
to quantify. First, elevated dDCH4values during
the LGM may be the result of a decrease in the
ratio of net to gross (N/G) CH4production. It
has been fairly well documented through in-
hibitor studies that as much as 50% of the CH4
produced at depth in soils is consumed by
microbially mediated methane oxidation near
the soil-atmosphere interface (22). The dDCH4
values for the emitted CH4are strongly de-
pendent on the N/G ratio because of the large
KIE associated with methane oxidation E–95
to –285° (23)^. For example, lowering N/G
by 11% during the LGM (with the dDCH4of
gross CH4assigned as –300° and the KIE for
methane oxidation as –95°) would raise
atmospheric dDCH4by 10°. The sense of this
change is consistent with observations that the
methane-producing communities are more sen-
sitive to temperature changes than methane-
oxidizing communities (24).
Two additional factors contributing to the
elevated dDCH4values during the LGM in-
volve an increase in the relative proportion
Fig. 2. Results from the last glacial termination as recorded in the GISP II ice core. The upper
purple curve is the isotopic temperature (30). The red curve is from the current dDCH4analyses with
1s (4.2°) error bars. The bottom CH4concentration curve (green) is from Brook et al. (3). ppb,
parts per billion. The increased sample resolution associated with the abrupt CH4concentration
increases associated with the onset of the Bolling/Allerod and the end of the Younger Dryas periods are
shown in expanded view in Fig. 3. The present-day dDCH4value for the Southern Hemisphere (SH) is
shown as a horizontal dashed line for reference.
Table 1. Constrainable factors influencing dDCH4during the LGM.
DdDCH4(LGM to Holocene) (°)
Factor CLIMAP SST*5-C Tropical cooling
10% decrease in C3/C4ratio during LGM†
KIE for OH oxidation
0 to –5
3.4 to –1.6
*CLIMAP Climate: Long-Range Investigation, Mapping, and Prediction; SST, sea surface temperature.
ratio in C3and C4plants differs by È15° [C3plants have higher dD values (31)], then a 10% reduction in the C3/C4ratio of
wetland plants during the LGM (21) would have raised atmospheric dDCH4values by È1.9° relative to Holocene values. Assume
additional 5-C cooling during LGM yields no change in C3/C4ratio.
ocean water (18). General circulation model simulations suggest little change in dDH2Oprecipitation using CLIMAP SST (19, 20) but a
slight decrease in dDH2Oprecipitation for 5-C tropical cooling (19). Finally, assume dDCH4/dDH2O0 0:675 (8).
†Assuming the D/H
‡Seawater dDH2Oduring LGM 0 7.5° standard mean
Fig. 1. Characteristic dDCH4values for various
present-day CH4sources. All the data except the
value for the marine clathrates (13) are from
(26–28). Present day atmospheric dDCH4esti-
mates are È–90 T 5° (29). The enriched
atmospheric value is the result of a large KIE
(þ250°) associated with the primary sink
(tropospheric OH) (14). Error bars for each source
correspond to the tabulated range of values.
SMOW, standard mean ocean water.
www.sciencemag.orgSCIENCEVOL 31110 FEBRUARY 2006
of petroleum-based and/or biomass-burning Download full-text
CH4emissions, both of which have elevated
D/H ratios (Fig. 1). Model simulations of bio-
mass burning, however, suggest lowered CH4
emissions during the LGM (25). If, as recently
suggested (12, 13), CH4from petroleum seeps
contributed a larger proportion of global
sources during the LGM compared with early
Holocene periods, then we would expect higher
atmospheric dDCH4values during the LGM.
Assuming global CH4emissions during the
LGM were 111 Tg/year (3) and the character-
istic dDCH4value for the terrestrial biosphere
was –300°, then a 10° dDCH4signal can be
accounted for by increasing the fraction of CH4
emissions based on petroleum and/or biomass
burning by 9% during the LGM (compared
with early Holocene emissions).
The high-resolution dDCH4records during
the end of the Younger and Older Dryas
periods (11.5 and 14.7 ka, respectively) and
the onset of interstadial 8 (IS8) (38.5 to 38 ka)
provide important constraints for assessing
clathrate stability during these periods (Fig. 3).
With the exception of one short period of
increasing dDCH4between 15.2 and 15.0 ka, the
atmospheric dDCH4record from GISP II shows
relatively stable or slightly decreasing dDCH4
values during periods of increasing CH4con-
centration. This trend in not consistent with either
a gradual or an episodic release of clathrates,
suggesting that marine clathrates were stable
throughout the last glacial termination as well as
during periods of abrupt warming.
To estimate the magnitude of the atmospher-
ic dDCH4shift associated with a hypothetical
clathrate destabilization event, a simple one-
box model of the atmosphere was developed
using the CH4concentration history from the
end of the Younger Dryas period to constrain
total CH4emissions (3). The model consists of
two sources and a single sink term. Terrestrial
CH4emissions, the lifetime of atmospheric CH4,
the dDCH4value for terrestrial CH4emissions
(–300°), and the KIE associated with the
sink (þ165°) were all held constant through-
out the simulation. Then, beginning at model
year 11.64 ka, we introduced clathrate-derived
CH4(dDCH40 j189°) at a rate of 0.8 Tg/year
for the next 100 model years, after which
clathrate emissions were held constant at 80
Tg/year. The model predicted evolution of
CH4concentration and dDCH4are included in
Fig. 3 for comparison with the GISP II data
from the Younger Dryas. Assuming clathrate
CH4was the only new CH4source at the end
of the Younger Dryas, the predicted dDCH4
change was þ21°. The relatively constant
dDCH4values throughout the transition to ele-
vated CH4levels suggest little change in the
relative proportion of all individual emissions
with near-constant characteristic dDCH4values.
The transition from the Older Dryas to
Bolling period (15 to 14 ka) provides a very
different view of the factors influencing dDCH4
(Fig. 3). During the 300-year period immedi-
ately preceding the abrupt increase in atmo-
spheric CH4loading, dDCH4initially decreases
by 15° followed by a rapid 10° increase,
during which time atmospheric CH4levels
remained effectively constant. Obviously, many
more data are needed to document this os-
cillation but, with the limited data in hand, it
appears that a rapid shift in the characteristic
dDCH4values of various sources is needed in
the absence of substantial global emission
changes during a period of relative climate
The general trend of decreasing dDCH4
throughout the termination, combined with rel-
atively stable dDCH4values during periods of
rapidly increasing CH4, suggests that marine
clathrates are stable during this period and
specifically during abrupt warming events. The
elevated LGM dDCH4values are likely to be
related to a number of factors, the most im-
portant being decreased N/G ratios and an
increase in petroleum-based CH4emissions
during the glacial period. Further insight into
these factors will derive from future measure-
ments of d13CCH4that are in progress.
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Supporting Online Material
Materials and Methods
11 October 2005; accepted 17 January 2006
Fig. 3. Expanded views of three abrupt CH4concentration events recorded in the GISP II ice core.
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10 FEBRUARY 2006VOL 311SCIENCEwww.sciencemag.org