Late Quaternary Atmospheric CH4 Isotope Record Suggests Marine Clathrates Are Stable

Article (PDF Available)inScience 311(5762):838-40 · February 2006with18 Reads
DOI: 10.1126/science.1121235 · Source: PubMed
Abstract
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 (δDCH4) to increase. Analyses of air trapped in the ice from the second Greenland ice sheet project show stable and/or decreasing δDCH4 values 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 δDCH4 values may be the result of a lower ratio of net to gross wetland CH4 emissions and an increase in petroleum-based emissions.
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24 October 2005; accepted 5 January 2006
10.1126/science.1121726
Late Quaternary Atmospheric CH
4
Isotope Record Suggests Marine
Clathrates Are Stable
Todd Sowers
One explanation for the abrupt increases in atmospheric CH
4
, 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 CH
4
(dD
CH
4
) to increase. Analyses of air trapped in the ice from the second
Greenland ice sheet project show stable and/or decreasing dD
CH
4
values 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 dD
CH
4
values may be the result of a
lower ratio of net to gross wetland CH
4
emissions and an increase in petroleum-based emissions.
T
he ice core record of atmospheric CH
4
changes covering the past 650,000
years exhibits two primary frequencies.
Over long time scales (greater than 10,000
years) atmospheric CH
4
changes 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 CH
4
emis-
sions that raise atmospheric CH
4
levels 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 CH
4
levels are synchronous with, or
slightly lag (by a few decades), the surface tem-
perature increase over Greenland (5). Assessing
the nature of these abrupt CH
4
events is im-
portant for understanding how ecosystems and
climate are connected and in estimating the
degree to which future CH
4
levels may con-
tribute to changes in Earth_s radiation budget.
There are two competing explanations for the
abrupt CH
4
increases. One hypothesis holds that
the terrestrial biosphere is capable of rapidly
increasing CH
4
emissions 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 CH
4
ultimately travels across the air-
sea interface leading to atmospheric CH
4
increases.
Model estimates of changes in the primary
CH
4
sink (tropospheric hydroxyl radical) dur-
ing the last glacial termination suggest that
the observed CH
4
variations 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 CH
4
therefore provides additional information on
the relative contribution of the various sources.
Variations in the D/H ratio of atmospheric CH
4
(dD
CH
4
) can be used to infer variable clathrate
contributions on the basis of their elevated dD
values compared with all terrestrial CH
4
sources (Fig. 1). Methane clathrates within the
continental margin sediments are formed al-
most exclusively by CO
2
reduction or thermal
cracking of longer chain hydrocarbons, whereas
terrestrial CH
4
emissions are primarily acetic-
lastic in nature (8, 9). During CO
2
reduction, all
the methyl hydrogen atoms come directly from
porewater H
2
that is in isotopic equilibrium
with the porewater (10). The resulting dD
CH
4
values are lower than the porewater dD
H
2
O
due
to a È180 per mil (°) biologically induced iso-
tope effect associated with CO
2
reduction (9, 11).
Marine clathrate dD
CH
4
values 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 propor tions of microbial and
thermogenic CH
4
at each site (12, 13). In con-
trast, CH
4
production 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 dD
CH
4
values generally ranging from
–250 to –380°, with the local dD
CH
4
value
strongly influenced by the dD of precipitation
(8, 9).
An atmospheric dD
CH
4
record (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 dD
CH
4
variations associated with the deglaciation shows
a progressive decrease in dD
CH
4
as the con-
centration of CH
4
increases, opposite to that
predicted by increasing clathrate contributions
due to warming associated with the termina-
tion. During the last glacial maximum (LGM),
dD
CH
4
values 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 dD
CH
4
values 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.
psu.edu
REPORTS
10 FEBRUARY 2006 VOL 311 SCIENCE www.sciencemag.org
838
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 dD
CH
4
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
dD
CH
4
by altering the dD of porewater H
2
that is
utilized by CO
2
reducing methanogens (oceanic
and terrestrial). A less direct effect occurs as
the oceanic dD
H
2
O
change 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 dD
H
2
O
change E7.5° (18)^ is
transferred through the LGM global hydro-
logic cycle (19, 20), causing changes in dD
CH
4
values for terrestrial CH
4
that range from –5 to
5°, depending on the assumed LGM tropical
temperatures (8, 9). Finally, a 10% decrease in
the ratio of C
3
-toC
4
-type plants during the
LGM (21) would have lowered atmospheric
dD
CH
4
valuesby0to1.9° relative to Holo-
cene values. Together these three factors
account for a small portion of the observed
20° dD
CH
4
shift between the LGM and early
Holocene, implying that other factors must be
considered.
There are at least three additional factors
contributingtotheatmosphericdD
CH
4
change
associated with the termination that are difficult
to quantify. First, elevated dD
CH
4
values during
the LGM may be the result of a decrease in the
ratio of net to gross (N/G) CH
4
production. It
has been fairly well documented through in-
hibitor studies that as much as 50% of the CH
4
produced at depth in soils is consumed by
microbia lly mediated methane oxidation near
the soil-atmosphere interface (22). The dD
CH
4
values for the emitted CH
4
are 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 dD
CH
4
of
gross CH
4
assigned as –300° and the KIE for
methane oxidation as –95°) would raise
atmospheric dD
CH
4
by 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 dD
CH
4
values 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 dD
CH
4
analyses with
1s (4.2°) error bars. The bottom CH
4
concentration curve (green) is from Brook et al.(3). ppb,
parts per billion. The increased sample resolution associated with the abrupt CH
4
concentr ation
increases associated with the onset of the Bolling/Aller od and the end of the Younger Dryas periods are
shown in expanded view in Fig. 3. The present-day dD
CH
4
value for the Southern Hemisphere (SH) is
shown as a horizontal dashed line for reference.
Table 1. Constrainable factors influencing dD
CH
4
during the LGM.
DdD
CH
4
(LGM to Holocene) (°)
Factor CLIMAP SST* 5-C Tropical cooling
10% decrease in C
3
/C
4
ratio during LGM –1.9 0
KIE for OH oxidation 0 3.4
DSea level 5.1 0 to –5
Total change þ3.2 3.4 to –1.6
*CLIMAP Climate: Long-Range Investigation, Mapping, and Prediction; SST, sea surface temperature. Assuming the D/H
ratio in C
3
and C
4
plants differs by È15° [C
3
plants have higher dD values (31)], then a 10% reduction in the C
3
/C
4
ratio of
wetland plants during the LGM (21) would have raised atmospheric dD
CH
4
values by È1.9° relative to Holocene values. Assume
additional 5-C cooling during LGM yields no change in C
3
/C
4
ratio. Seawater dD
H
2
O
during LGM 0 7.5° standard mean
ocean water (18). General circulation model simulations suggest little change in dD
H
2
O
precipitation using CLIMAP SST (19, 20)buta
slight decr ease in dD
H
2
O
precipitation for 5-C tropical cooling (19). Finally, assume dD
CH
4
/dD
H
2
O
0 0:675 (8).
Fig. 1. Characteristic dD
CH
4
values for various
present-day CH
4
sources. All the data except the
value for the marine clathrates (13)arefrom
(26–28 ). Presen t day atmospheric dD
CH
4
esti-
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.
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www.sciencemag.org SCIENCE VOL 311 10 FEBRUARY 2006
839
of petroleum-based and/or biomass-burning
CH
4
emissions, both of which have elevated
D/H ratios (Fig. 1). Model simulations of bio-
mass burning, however, suggest lowered CH
4
emissions during the LGM (25). If, as recently
suggested (12, 13), CH
4
from petroleum seeps
contributed a larger proportion of global
sources during the LGM compared with early
Holocene periods, then we would expect higher
atmospheric dD
CH
4
values during the LGM.
Assuming global CH
4
emissions during the
LGM were 111 Tg/year (3) and the character-
istic dD
CH
4
value for the terrestrial biosphere
was –300°, then a 10° dD
CH
4
signal can be
accounted for by increasing the fraction of CH
4
emissions based on petroleum and/or biomass
burning by 9% during the LGM (compared
with early Holocene emissions).
The high-resolution d D
CH
4
records 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 dD
CH
4
between 15.2 and 15.0 ka, the
atmospheric dD
CH
4
record from GISP II shows
relatively stable or slightly decreasing dD
CH
4
values during periods of increasing CH
4
con-
centration. This trend in not consistent with either
agradualoranepisodicreleaseofclathrates,
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 dD
CH
4
shift associated with a hypothetical
clathrate destabilization event, a simple one-
box model of the atmosphere was developed
using the CH
4
concentration history from the
end of the Younger Dryas period to constrain
total CH
4
emissions (3). The model consists of
two sources and a single sink term. Terre s tr ia l
CH
4
emissions, the lifetime of atmospheric CH
4
,
the dD
CH
4
value for terrestrial CH
4
emissions
(–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
CH
4
(dD
CH
4
0 j189°) at a rate of 0.8 Tg/year
for the next 100 model years, after which
clathrate emi ssi ons were held constant at 80
Tg/year. The model predicted evolution of
CH
4
concentration and dD
CH
4
are included in
Fig. 3 for comparison with the GISP II data
from the Younger Dryas. Assuming clathrate
CH
4
was the only new CH
4
source at the end
of the Younger Dryas, the predicted dD
CH
4
change was þ21°. The relatively constant
dD
CH
4
values throughout the transition to ele-
vated CH
4
levels suggest little change in the
relative proportion of all individual emissions
with near-constant characteristic dD
CH
4
values.
The transition from the Older Dryas to
Bolling period (15 to 14 ka) provides a very
different view of the factors influencing dD
CH
4
(Fig. 3). During the 300-year period immedi-
ately preceding the abrupt increase in atmo-
spheric CH
4
loading, dD
CH
4
initially decreases
by 15° followed by a rapid 10° increase,
during which time atmospheric CH
4
levels
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
dD
CH
4
values of various sources is needed in
the absence of substantial global emission
changes during a period of relative climate
stability.
The general trend of decreasing dD
CH
4
throughout the termination, combined with rel-
atively stable dD
CH
4
values during periods of
rapidly increasing CH
4
, suggests that marine
clathrates are stable during this period and
specifically during abrupt warming events. The
elevated LGM dD
CH
4
values 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 CH
4
emissions
during the glacial period. Further insight into
these factors will derive from future measure-
ments of d
13
C
CH
4
that are in progress.
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Supporting Online Material
www.sciencemag.org/cgi/content/full/311/5762/838/DC1
Materials and Methods
Table S1
References
11 October 2005; accepted 17 January 2006
10.1126/science.1121235
Fig. 3. Expanded views of three abrupt CH
4
concentration events recorded in the GISP II ice core.
The isotopic temperature record (30) and atmospheric CH
4
concentration record (3) are plotted for
reference. The red curves are from the current dD
CH
4
analyses with 1s errors at each measured depth.
The results from the one-box clathrate-only model are shown for the Younger Dryas simulation with
black curves. All data in all three panels are plotted on the same y axes for comparison.
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10 FEBRUARY 2006 VOL 311 SCIENCE www.sciencemag.org
840
    • "No quantitative 393 estimate of this CH 4 source is to date possible, but a rise in temperature will 394 enhance microbial formation and permafrost thawing, hence emissions of 395 biogenic CH 4 from the deep subsea permafrost of the ESAS are expected to 396 play an increasingly important role for the radiative forcing of the Earth in the 397 future. 398 Variations in CH 4 isotopic signatures in air trapped in polar ice cores 399 have been studied to investigate the cause(s) of the CH 4 increase observed 400 during past warming events (Sowers, 2006, Fischer et al., 2008, Bock et al., 401 2010). The authors measured a shift towards lighter CH 4 stable isotope 402 values together with a temperature increase and they concluded that a rise in 403 wetland CH 4 emissions is the most likely explanation. "
    [Show abstract] [Hide abstract] ABSTRACT: Methane (CH4) is a strong greenhouse gas emitted by human activity and natural processes that are highly sensitive to climate change. The Arctic Ocean, especially the East Siberian Arctic Shelf (ESAS) overlays large areas of subsea permafrost that is degrading. The release of large amount of CH4 originally stored or formed there could create a strong positive climate feedback. Large scale CH4 super-saturation has been observed in the ESAS waters, pointing to leakages of CH4 through the sea floor and possibly to the atmosphere, but the origin of this gas is still debated. Here, we present CH4 concentration and triple isotope data analyzed on gas extracted from sediment and water sampled over the shallow ESAS from 2007 to 2013. We find high concentrations (up to 500μM) of CH4 in the pore water of the partially thawed subsea permafrost of this region. For all sediment cores, both hydrogen and carbon CH4 isotope data reveal the predominant presence of CH4 that is not of thermogenic/natural gas origin as it has long been thought, but resultant from microbial CH4 formation using as primary substrate glacial water and old organic matter preserved in the subsea permafrost or below. Radiocarbon data demonstrate that the CH4 present in the ESAS sediment is of Pleistocene age or older, but a small contribution of highly 14C-enriched CH4, from unknown origin, prohibits precise age determination for one sediment core and in the water column. Our data suggest that at locations where bubble plumes have been observed, CH4 can escape anaerobic oxidation in the surface sediment. CH4 will then rapidly migrate through the very shallow water column of the ESAS to escape to the atmosphere generating a positive radiative feedback.
    Full-text · Article · Sep 2016
    • "The release of CH 4 from these seeps shows strong spatial and temporal variability over a range of time-scales (from hours to years; Boles et al., 2001; Leifer and Boles, 2005; Tryon et al., 1999), and represents a potentially underestimated source of atmospheric CH 4 . Moreover, the potential destabilization of CH 4 -rich clathrate deposits under various ocean warming scenarios has prompted significant research effort in recent years (Archer, 2007; Solomon et al., 2009; Sowers, 2006). There has also been increased interest in other in situ sources of CH 4 in oxygenated marine surface waters, including the cleavage of methyl-groups from larger molecules, such as methylated sulfides (Damm et al., 2010; Florez-Leiva et al., 2013) and methylphosphonate (Metcalf et al., 2012; Karl et al., 2008). "
    [Show abstract] [Hide abstract] ABSTRACT: Coastal upwelling systems are important marine sources of methane (CH4) and nitrous-oxide (N2O). Current understanding of the controls on CH4 and N2O distributions in these coastal waters is restricted by limited data availability. We present the first multi-year measurements of CH4 and N2O distributions from the seasonally upwelling shelf waters of British Columbia, Canada, a coastal end-member of the north Pacific oxygen minimum zone (OMZ). Our data show significant seasonal differences in CH4 and N2O distributions and fluxes driven predominantly by upwelling. Methane is supplied to the water column primarily from sediments (especially near methane seeps), and is transported to the surface mixed layer by upwelling. A positive correlation between CH4 concentrations and salinity indicates limited inputs from Fraser River estuary waters to the study site. Shelf waters receive N2O from a deep, off-shelf N2O maximum in the OMZ core, and from nitrification in the water column and possibly sediments. Both the physical transport of N2O and its apparent in situ production are enhanced under upwelling conditions. N2O yields from nitrification, estimated from changes in N2O and nitrate + nitrite (NO3− + NO2−) along isopycnals, ranged from 0.04–0.49%, with the highest values observed under low ambient O2 concentrations. Sea–air fluxes ranged from − 4.5–21.9 μmol m− 2 day− 1 for N2O and 2.5–34.1 μmol m− 2 day− 1 for CH4, with the highest surface fluxes observed following summer upwelling over the broad continental shelf of southern Vancouver Island. Our results provide new insight into the factors driving spatial and inter-annual variability in marine CH4 and N2O in high productivity coastal upwelling regions. Continued time-series measurements will be invaluable in understanding the longer-term impacts of climate-driven variability on marine biogeochemical cycles in these dynamic near-shore waters.
    Full-text · Article · Feb 2016
    • "The transition from more humid to arid conditions at the start IETM (Schmitz et al., 2001)l e d to enhanced terrestrial erosion and could also have released methane from wetlands. Methane emissions from wetlands may exceed those from gas hydrates hosted in marine sediments, as suggested by isotopic analysis of methane within ice core records (Sowers, 2006). Recent work actually argues that the likely amount of carbon mass input at the onset CIE (4000–7000 PgC) required a major alternative source of carbon in addition to any contribution from methane hydrates (Dunkley-Jones et al., 2010). "
    [Show abstract] [Hide abstract] ABSTRACT: Previous studies propose that submarine landslides and turbidity currents may become more likely due to future rapid global warming. Determining whether global warming increases likelihood assists in assessment of landslide-triggered tsunami hazards and risk to seafloor structures. Other studies propose that landslides helped to trigger past rapid climate change due to sudden release of gas hydrates. Two deep-water turbidite records show prolonged hiatuses in turbidity current activity during the Initial Eocene Thermal Maximum (IETM) at ∼55 Ma. The IETM represents a possible proxy for future anthropogenically-induced climate change. It is likely that our records mainly represent large and fast moving disintegrative submarine landslides. Statistical analysis of long term (>2.3 Myr) records shows that turbidity current frequency significantly decreased after the IETM. Our results indicate that rapid climate change does not necessarily cause increased turbidity current activity, and do not provide evidence for landslides as a primary trigger for the IETM.
    Full-text · Article · Jun 2015
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