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GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L03801, doi:10.1029/2006GL028090, 2007
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Ice-core record of methyl chloride over the last glacial–Holocene
climate change
Takuya Saito,1,2 Yoko Yokouchi,1 Shuji Aoki,3 Takakiyo Nakazawa,3 Yoshiyuki Fujii,4
and Okitsugu Watanabe4
Received 6 September 2006; revised 29 November 2006; accepted 14 December 2006; published 1 February 2007.
[1] Methyl chloride (CH3Cl) concentration was measured
in air trapped in a deep ice core from Dome Fuji, Antarctica
covering the last glacial – present interglacial (Holocene)
change. The record shows that the CH3Cl concentration was
relatively constant, being similar to the present levels,
during the pre-industrial Holocene. In contrast, the CH3Cl
concentration was significantly high and variable in the last
glacial period, possibly due to impurity-related production
of CH3Cl in ice sheet. Under the assumption that the
production was the sole cause of the excess CH3Cl, the
atmospheric CH3Cl concentration during the last glacial was
estimated using simultaneously measured calcium data for
the ice core to have been enhanced by 30% compared with
the pre-industrial Holocene concentration. Because the
major sink of CH3Cl was stronger during the last glacial
than during the Holocene, the enhancement of CH3Cl
during the last glacial was likely due to the glacial period
source being enhanced. Citation: Saito, T., Y. Yokouchi,
S. Aoki, T. Nakazawa, Y. Fujii, and O. Watanabe (2007), Ice-core
record of methyl chloride over the last glacial – Holocene climate
change, Geophys. Res. Lett., 34, L03801, doi:10.1029/
2006GL028090.
1. Introduction
[2] Methyl chloride (CH3Cl) is the most abundant halocarbon in the atmosphere and mainly released from natural
sources, including tropical plants [Yokouchi et al., 2000;
2002], ocean [Moore et al., 1996], wood-rotting fungi
[Harper, 1985], coastal salt marshes [Rhew et al., 2000],
and biomass burning [Lobert et al., 1999]. Atmospheric
CH3Cl acts as a carrier of ozone-destroying chlorine into the
stratosphere and is responsible for about 15% of chlorinecatalyzed ozone destruction in the stratosphere [Montzka
and Fraser, 2003]. Since contribution of anthropogenic
halocarbons, such as chlorofluorocarbons (CFCs), to the
ozone destruction will decline due to the Montreal Protocol
and its amendments, CH3Cl will become relatively more
important.
[3] The relative contribution of CH3Cl to stratospheric
ozone chemistry has been suggested on the basis of the
analyses of the firn air in polar snow pack to have been
much greater before the anthropogenic halocarbons became
1
National Institute for Environmental Studies, Tsukuba, Japan.
Japan Society for the Promotion of Science, Tokyo, Japan.
Center for Atmospheric and Oceanic Studies, Graduate School of
Science, Tohoku University, Sendai, Japan.
4
National Institute of Polar Research, Tokyo, Japan.
2
3
Copyright 2007 by the American Geophysical Union.
0094-8276/07/2006GL028090$05.00
widely used in the mid-twentieth century [Butler et al.,
1999]. The authors reported that CH3Cl levels during the
early part of the twentieth century were about 90% of current
levels. Recent analyses of firn air by Trudinger et al. [2004]
have also showed an increase in CH3Cl concentration
between about 1930 and 1980 followed by relatively stable
concentration through the 1980s and early 1990s. Model
calculations by Trudinger et al. [2004] suggested that the
variation has mainly been caused by variation of biomass
burning emission. A longer atmospheric history of CH3Cl
has been reconstructed from measurements of CH3Cl in firn
air and air bubbles from an Antarctic ice core [Aydin et al.,
2004].
[4] For the earlier and longer periods of time, CH3Cl
concentrations might have changed greatly with the past
climate changes, such as glacial-interglacial changes. However, no corresponding long-term records have been reported.
In this study, we present the first ice-core record of CH3Cl
over the last glacial – present interglacial (Holocene) change
in a core from Dome Fuji station, Antarctica.
2. Experiment
[5] A 2503 m long ice core was collected during 1995–
1996 from Dome Fuji, East Antarctica by the Japanese
Antarctic Research Expeditions [Dome-F Deep Coring
Group, 1998]. The site is located about 1,000 km from the
coast at the highest point of the East Dronning Maud Land
Plateau (77°1900100S, 39°4201200E; elevation, 3810 m a.s.l.).
The annual mean air temperature is 58°C and the annual
mean accumulation rates is 32 kg m2 yr1. The age of ice
has been determined by using an inverse dating method
combining an ice flow model and a history of the accumulation rate [Watanabe et al., 2003a]. The age of air in the
Dome Fuji ice core has been determined by subtracting the
age difference between air and its surrounding ice from
the age of ice [Kawamura et al., 2003]. The age difference
was calculated with a dynamic firn densification/heat transfer model and reported to be 2000 years in the Holocene
and up to 4800 years between 10 and 80 kiloyears before
present (kyr B.P.). Such a large difference between air and
ice ages is attributed to cold temperature and low accumulation rate at the site.
[6] Ice-core samples used for this study were obtained
from the depth interval 125 – 1285 m, corresponding to the
time period 1 – 78 kyr BP. The samples were analysed as
described in detail by Saito et al. [2006]. Briefly, air
contained in the ice-core samples (300 g) was liberated
by milling the ice in a stainless steel chamber under vacuum
at a temperature below 20°C. The extracted air (30 ml)
was collected in a stainless steel tube cooled to about
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Figure 1. Measured CH3Cl concentrations and d 18O
values of ice [Watanabe et al., 2003b] over the past 80 kyr
deduced from the Dome Fuji ice core (open circles,
Holocene data; filled circles, last-glacial data). Dash line
indicates the CH3Cl concentrations after correction for
production in the ice (see text and Figure 3 for details).
265°C and analysed by cryogenic pre-concentration/gas
chromatography/mass spectrometry. CH3Cl was quantified
with 500 pptv standard gases, which were prepared gravimetrically. The CH3Cl measurements were corrected for
system blank determined by the analyses of pure air that
were processed through the whole analytical procedure. The
analytical precision of the method deduced from duplicate
ice core analyses was estimated to be better than ±20 pptv
[Saito et al., 2006].
3. Results and Discussions
[7] The reconstructed ice-core record of the CH3Cl concentration over the past 80 kyr is presented in Figure 1, as
well as the d18O values of the ice, a proxy for palaeotemperature [Watanabe et al., 2003b]. The main feature of our
measurements is that CH3Cl concentrations were significantly high and scattered during the last glacial period
(20– 80 kyr BP) with a maximum near 4600 pptv during
the last glacial maximum (LGM; around 21 kyr BP). By
contrast, CH3Cl concentrations during the pre-industrial
Holocene (the last 11 kyr) were relatively constant, with
an average value of 504 ± 37 pptv.
[8] The high and scattered CH3Cl values were observed
only during the last glacial. Ice formed during that period
characteristically contains high concentrations of dust and
impurities such as calcium (Ca2+). The impurities are related
to the production of CO2, CO, and N2O in the ice sheet
[Anklin et al., 1995; Haan and Raynaud, 1998; Flückiger et
al., 1999]. Thus, the question arises as to whether the
CH3Cl record might have been affected by the variations
of the impurities in the ice. In Figure 2, CH3Cl and Ca2+ are
plotted as a function of depth in the Dome Fuji ice core
[Watanabe et al., 2003a]. Surprisingly, the depth profiles for
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CH3Cl and Ca2+ are similar: CH3Cl peaks at 590, 650, 780,
and 1100 m depth largely correspond to elevated concentrations of Ca2+ at the corresponding depths, with the
exception of the CH3Cl peak at 500 m. Considering that
air enclosed in ice core is younger than its surrounding ice
[Schwander and Stauffer, 1984], this good correlation
suggests that a portion of the CH3Cl in the last-glacial
ice-core section might have been produced in situ in the ice.
In the last-glacial section of the ice core, the age difference
between the enclosed air and the surrounding ice was
estimated to be about 4 kyr, which corresponds to a depth
difference of around 50 m [Kawamura et al., 2003]. In situ
production is not likely the case for the Holocene data,
because impurities levels are much lower in the interglacial
ice and in that ice-core section the CH3Cl concentrations do
not correlate with variations of Ca2+.
[9] CH3Cl may be produced in situ in glacial ice either
biologically or chemically. Biological production requires
microbes to be entrapped in the ice. The results of recent
biological analyses of polar ice cores have indicated that
micro-organisms such as fungi are present [Castello et al.,
2005]. In forest ecosystems, CH3Cl is microbially produced
by wood-rot fungi [Watling and Harper, 1998]. However,
fungal artefacts in the CH3Cl record are unlikely because
such fungi probably cannot survive cold temperatures. A
potential chemical production of CH3Cl is reaction among
chloride, an electron acceptor such as Fe (III), and organic
matter such as humic substances. This reaction is responsible for CH3Cl production in soil [Keppler et al., 2000] and
could potentially take place in the ice sheet because microbial mediation is not required; moreover, the reactants have
been transported from their source regions to the ice sheets
as well as Ca2+ [Watanabe et al., 2003a; Marino et al.,
2004; Grannas et al., 2006]. If we assume that the impurityrelated production was the sole cause of the excess CH3Cl,
the CH3Cl concentration during the last glacial is estimated
to be around 700 pptv based on the CH3Cl data (n = 7) for
the samples with Ca2+ concentrations as low as those in
interglacial ice (Figure 3). However, we cannot rule out the
possibility that the post-depositional reactions would have
caused the slightly higher CH3Cl level even in ice with low
Figure 2. Comparison of variations of CH3Cl and Ca2+
[Watanabe et al., 2003a] as a function of depth. Mean Ca2+
values (open circles) over the depth intervals corresponding
to those of the CH3Cl data points are also shown.
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Figure 3. Correlation between CH3Cl and Ca2+ concentrations (open circles, Holocene data; filled circles, last-glacial
data). The Ca2+ concentrations are the depth-averaged values
from Figure 2.
Ca. Further investigations are needed to fully assess this
issue.
[10] Our results thus suggest that the atmospheric CH3Cl
concentration during the last glacial was 30% higher than
that during the pre-industrial Holocene. The change in the
CH3Cl concentration between the last glacial and the
Holocene should reflect changes in the sources and sinks
of CH3Cl. The major CH3Cl sink, reaction with OH radicals
in the atmosphere, has been estimated to have been
enhanced by 20% during the LGM compared with the
pre-industrial Holocene value [Martinerie et al., 1995].
Thus, a source 1.5 times larger during the last glacial is
required to explain the 30% increase in the glacial CH3Cl
concentration.
[11] Current important sources of CH3Cl are found in
forested areas: tropical plants [Yokouchi et al., 2002], woodrot fungi [Harper, 1985], leaf litter [Hamilton et al., 2003],
and biomass burning [Lobert et al., 1999]. During the LGM,
the land coverage of tropical plants was reduced by 40%
compared with that during the Holocene by the effect of low
CO2 concentration, a drier and colder climate [Adams and
Faure, 1997]. The decline in the areal coverage of tropical
forest, as well as in the coverage of subtropical and
temperate forests, would have resulted in a corresponding
reduction of wood-rot fungi and leaf litter. The reduced
terrestrial biomass during the LGM would also have led to a
reduction in the total amount of biomass burnt, by about
30% [Thonicke et al., 2005]. However, a proportional
reduction in CH3Cl emissions from these sources is unlikely,
because CH 3 Cl production strongly depends on the
chloride content of the plants [Watling and Harper, 1998;
Hamilton et al., 2003; Lobert et al., 1999; Saini et al.,
1995]. Chloride in plants is originally supplied by atmospheric sea-salt deposition to soils. Polar ice-core records
[De Angelis et al., 1987] have shown that sea-salt deposition was substantially increased during the last glacial.
Recent modelling work [Reader and McFarlane, 2003]
has shown that the glacial enhancement of sea-salt deposition occurred not only in polar regions but also in tropical
and temperate regions: the surface sea-salt level in those
regions was higher during the LGM by a factor of roughly 3
(1.5 –10) compared with the present. Such a strong sea-salt
influx to the continents and the arid climate would have
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increased soil salinity, thereby increasing CH3Cl production
efficiency. Assuming a simple proportionality among
the sea-salt level, forested area, and CH3Cl production,
the high-salinity conditions would more than make up
for the reduction of forested areas, even if the forested
areas were reduced by half. Therefore, strong forest-related
sources might have been responsible for the enhanced CH3Cl
level during the last glacial. Emission from coastal areas by
salt marshes [Rhew et al., 2000] and from the ocean [Moore
et al., 1996] might also have changed in response to the
climate change, but relevant data are lacking. Alternatively,
unknown sources may have been responsible for the high
CH3Cl level.
[12] Compared with the glacial – interglacial concentration change, the variation of CH3Cl during the Holocene
was small (504 ± 37 pptv), consistent with the relatively
stable climate during the Holocene [Alverson et al., 2003].
The Holocene level is similar to the current CH3Cl concentration over Antarctica [Butler et al., 1999, and references
therein] and to the concentration during the past 300 years
in an ice core from Siple Dome, Antarctica [Aydin et al.,
2004]. This finding suggests that atmospheric CH3Cl has
remained constant throughout the last 11 kyr, although the
relative contributions of each source to the atmospheric
concentration may have been different during the Holocene
from those at present. Although Aydin et al. [2004] found
cyclic variability of CH3Cl concentration over the last
300 years with a period of about 110 years, we cannot assess
the variation due to low time-resolution in our measurements.
[13] The last glacial to Holocene change in the CH3Cl
concentration might have affected the stratospheric ozone
chemistry. Past variation of the stratospheric ozone over the
last glacial – Holocene change has been estimated by one
model [Martinerie et al., 1995] to have been small, mainly
because of the compensating effects of CO2 and N2O on the
ozone-destruction rate. However, the model used a low
value (400 pptv) for the CH3Cl concentration during the
pre-industrial Holocene and the LGM as input data. For a
more accurate assessment of past stratospheric ozone, a
modelling study that takes into account the results of the
present study is required, as well as more ice-core measurements of CH3Cl and other naturally occurring ozonedepleting gases, such as CH3Br.
[14] Acknowledgments. We thank the members of the Japanese
Antarctic Research Expeditions for the ice-core drilling and K. Kawamura
of Scripps Institution of Oceanography for valuable discussion and for
providing the gas age data of the Dome Fuji core.
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