Khl2011b

Khl2011b
Clim. Past, 7, 473–486, 2011
www.clim-past.net/7/473/2011/
doi:10.5194/cp-7-473-2011
© Author(s) 2011. CC Attribution 3.0 License.
Climate
of the Past
Abrupt rise in atmospheric CO2 at the onset of the Bølling/Allerød:
in-situ ice core data versus true atmospheric signals
P. Köhler1 , G. Knorr1,2 , D. Buiron3 , A. Lourantou3,* , and J. Chappellaz3
1 Alfred
Wegener Institute for Polar and Marine Research (AWI), P.O. Box 120161, 27515 Bremerhaven, Germany
of Earth and Ocean Sciences, Cardiff University, Cardiff, Wales, UK
3 Laboratoire de Glaciologie et Géophysique de l’Environnement, (LGGE, CNRS, Université Joseph Fourier-Grenoble),
54b rue Molière, Domaine Universitaire BP 96, 38402 St. Martin d’Hères, France
* now at: Laboratoire d’Océanographie et du Climat (LOCEAN), Institut Pierre Simon Laplace, Université P. et M. Curie
(UPMC), Paris, France
2 School
Received: 24 June 2010 – Published in Clim. Past Discuss.: 11 August 2010
Revised: 15 March 2011 – Accepted: 24 March 2011 – Published: 4 May 2011
Abstract. During the last glacial/interglacial transition the
Earth’s climate underwent abrupt changes around 14.6 kyr
ago. Temperature proxies from ice cores revealed the onset
of the Bølling/Allerød (B/A) warm period in the north and
the start of the Antarctic Cold Reversal in the south. Furthermore, the B/A was accompanied by a rapid sea level rise of
about 20 m during meltwater pulse (MWP) 1A, whose exact
timing is a matter of current debate. In-situ measured CO2 in
the EPICA Dome C (EDC) ice core also revealed a remarkable jump of 10 ± 1 ppmv in 230 yr at the same time. Allowing for the modelled age distribution of CO2 in firn, we show
that atmospheric CO2 could have jumped by 20–35 ppmv in
less than 200 yr, which is a factor of 2–3.5 greater than the
CO2 signal recorded in-situ in EDC. This rate of change in atmospheric CO2 corresponds to 29–50% of the anthropogenic
signal during the last 50 yr and is connected with a radiative
forcing of 0.59–0.75 W m−2 . Using a model-based airborne
fraction of 0.17 of atmospheric CO2 , we infer that 125 Pg
of carbon need to be released into the atmosphere to produce such a peak. If the abrupt rise in CO2 at the onset of
the B/A is unique with respect to other Dansgaard/Oeschger
(D/O) events of the last 60 kyr (which seems plausible if not
unequivocal based on current observations), then the mechanism responsible for it may also have been unique. Available
δ 13 CO2 data are neutral, whether the source of the carbon
is of marine or terrestrial origin. We therefore hypothesise
that most of the carbon might have been activated as a conse-
Correspondence to: P. Köhler
([email protected])
quence of continental shelf flooding during MWP-1A. This
potential impact of rapid sea level rise on atmospheric CO2
might define the point of no return during the last deglaciation.
1
Introduction
Measurements of CO2 over Termination I (20–10 kyr BP)
from the EPICA Dome C (EDC) ice core (Monnin et al.,
2001; Lourantou et al., 2010) (Fig. 1b) are temporally higher
resolved and more precise than CO2 records from other ice
cores (Smith et al., 1999; Ahn et al., 2004). They have an
uncertainty (1σ ) of 1 ppmv or less (Monnin et al., 2001;
Lourantou et al., 2010). In these in-situ measured data in
EDC, CO2 abruptly rose by 10 ± 1 ppmv between 14.74 and
14.51 kyr BP on the most recent ice core age scale (LemieuxDudon et al., 2010). This abrupt CO2 rise is therefore synchronous with the onset of the Bølling/Allerød (B/A) warm
period in the North (Steffensen et al., 2008), the start of the
Antarctic Cold Reversal in the South (Stenni et al., 2001), as
well as abrupt rises in the two other greenhouse gases CH4
(Spahni et al., 2005) and N2 O (Schilt et al., 2010). Furthermore, the B/A is accompanied by a rapid sea level rise of
about 20 m during meltwater pulse (MWP) 1A (Peltier and
Fairbanks, 2007), whose exact timing is matter of current debate (e.g. Hanebuth et al., 2000; Kienast et al., 2003; Stanford
et al., 2006; Deschamps et al., 2009).
However, atmospheric gases trapped in ice cores are
not precisely recording one point in time but average
over decades to centuries, mainly depending on their
Published by Copernicus Publications on behalf of the European Geosciences Union.
P. Köhler et al.: Abrupt rise in CO2 at the onset of the Bølling/Allerød
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Fig. 1. Climate records during MIS 3 and Termination I. From top to bottom: relative sea level, N2 O, CO2 , CH4 and isotopic temperature
proxies (δD or δ 18 O) from Antarctica (blue) and Greenland (red). (A) MIS 3 data from the Byrd (CO2 , CH4 , δ 18 O), GISP2 (Ahn and Brook,
2008) and Talos Dome ice cores (N2 O) (Schilt et al., 2010). Sea level from a compilation (magenta) based on coral reef terraces (Thompson
and Goldstein, 2007) and the synthesis (green) from the Red Sea method (Siddall et al., 2008). Age model of Byrd and GISP2 as in Ahn
and Brook (2008) and Talos Dome data on the TALDICE-1 age scale (Buiron et al., 2011). (B) Termination I data from the EDC (blue,
cyan: CO2 , CH4 , δD), Talos Dome (N2 O) and NGRIP (red: CH4 , δ 18 O) ice cores (Monnin et al., 2001; Stenni et al., 2001; NorthGRIPmembers, 2004; Lourantou et al., 2010; Schilt et al., 2010). Previous (Monnin et al., 2001) (blue) and new (Lourantou et al., 2010) (cyan)
EDC CO2 data. Sea level in from corals (green) on Barbados, U-Th dated and uplift-corrected (Peltier and Fairbanks, 2007), and coast line
migration (magenta) on the Sunda Shelf (Hanebuth et al., 2000). In (B) sea level is plotted on an individual age scale, N2 O on TALDICE-1
age scale of Talos Dome (Buiron et al., 2011), and EDC and NGRIP data are plotted on the new synchronised ice core age scale QSR2010
(Lemieux-Dudon et al., 2010). Vertical lines in (B) mark the jump in CO2 into the B/A as recorded in EDC.
Clim. Past, 7, 473–486, 2011
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P. Köhler et al.: Abrupt rise in CO2 at the onset of the Bølling/Allerød
6
PRE lognormal
PRE CO2 firn model
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EPRE=213yr
LGM lognormal
LGM CO2 firn model
EB/A=400yr
ELGM=590yr
1
0
0
500
1000
1500
2000
Time since last exchange with atmosphere (yr)
Fig. 2. Age distribution PDF of CO2 as a function of climate state,
here pre-industrial (PRE), Bølling/Allerød (B/A) and LGM conditions. Calculation with a firn densification model (Joos and Spahni,
2008) (solid lines, for PRE and LGM) and approximations of all
three climate states by a log-normal function (broken lines). For all
functions the expected mean values, or width E, are also given.
accumulation rate because of the movement of gases in the
firn above the close-off depth and before its enclosure in gas
bubbles in the ice. To infer the transfer signature of the true
atmospheric CO2 signal out of in-situ ice core CO2 measurements, the latter has to be deconvoluted with the ice-corespecific age distribution probability density function (PDF).
Based on a firn densification model (Joos and Spahni, 2008),
this age distribution PDF describing the elapsed time since
the last exchange of the CO2 molecules with the atmosphere
(Fig. 2) reveals for EDC a width of approximately 200 and
600 yr for climate conditions of pre-industrial times (PRE)
and the Last Glacial Maximum (LGM), respectively. These
wide age distributions implicate that the CO2 measured insitu, especially in ice cores with low accumulation rates (such
as EDC), differs from the true atmospheric signal when CO2
changes abruptly.
In the following we will deconvolve the atmospheric CO2
signal connected with this abrupt rise in CO2 measured insitu in the EDC ice core, allowing for the age distribution
PDF during the onset of the B/A. We furthermore use simulations of a global carbon cycle box model to develop and
test a hypotheses which might explain the abrupt rise in atmospheric CO2 .
2
2.1
Methods
Age distribution PDF of CO2
The age distributions PDF of CO2 or CH4 are functions of
the climate state and the local site conditions of the ice core.
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In Fig. 2, the age distributions PDF of CO2 in the EDC ice
core for pre-industrial (PRE) and LGM conditions based on
calculations with a firn densification model (Joos and Spahni,
2008) are shown. The resulting age distribution PDF for CO2
can be approximated with reasonable accuracy (r 2 = 90–
94%) by a log-normal function (Köhler et al., 2010b):
�
1
−0.5
y=
√ ·e
x · σ · 2π
�
ln(x)−µ 2
σ
(1)
with x (yr) as the time elapsed since the last exchange with
the atmosphere. This equation has two free parameters µ and
σ . For simplcity, we have chosen σ = 1, which leads to an
expected value (mean) E of the PDF of
E = eµ+0.5 .
(2)
The expected value E is described as width of the PDF in
the terminology of gas physics, a terminology which we will
also use in the following. E should not be confused with the
most likely value defined by the location of the maximum of
the PDF.
Our choice to use a log-normal function (Eq. 1) for the
age distribution PDF was motivated by the good representation of firn densification model output (r 2 ≥ 90%) and its
dependency on only one free parameter, which can be obtained from models. Other approaches using, for example, a
Green’s function are also possible (see Trudinger et al., 2002,
and references therein).
In the case of the CO2 jump at 14.6 kyr BP, one has to consider that the atmospheric records are much younger than the
surrounding ice matrix; indeed, the CO2 jump is embedded
between 473 and 480 m in glacial ice (Monnin et al., 2001;
Lourantou et al., 2010) with low temperatures and low accumulation rates. However, from a model of firn densification
which includes heat diffusion, it is known that the close-off
of the gas bubbles in the ice matrix is not a simple function of the temperature of the surrounding ice (Goujon et al.,
2003). Heat from the surface diffuses down to the close-off
region in a few centuries, depending on site-specific conditions. This implies that atmospheric gases during the onset of
the B/A were not trapped by conditions of either the LGM or
the Antarctic Cold Reversal, but by some intermediate state.
New calculations with this firn densification model (Goujon
et al., 2003) give a width of the age distribution PDF EB/A
of about 400 yr with a relative uncertainty (1σ ) of 14% at the
onset of the B/A (Fig. 3). The width E itself varies during
the jump into the B/A between 380 and 420 yr; we therefore
conservatively estimate EB/A to lie between 320 to 480 yr
with our best-guess estimate of EB/A = 400 yr in-between.
The performance of the applied gas age distribution PDF
(Eq. 1) is tested with ice core CH4 data for the time window
of interest (Appendix A, Supplement). In summary, this test
strongly suggests that the log-normal age distribution PDF
does not introduce a systematic bias in the shape of the signal
if applied onto a hypothetical atmospheric CO2 record. It is
Clim. Past, 7, 473–486, 2011
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P. Köhler et al.: Abrupt rise in CO2 at the onset of the Bølling/Allerød
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Fig. 3. Evolution of the width E of the age distribution PDF (±1σ )
during the last 20 kyr (red squares) calculated with a firn densification model including heat diffusion (Goujon et al., 2003). Green
diamonds represent the results for the LGM and pre-industrial climate with another firn densification model (Joos and Spahni, 2008).
Please note reverse y-axis. Top: EDC CO2 (Monnin et al., 2001;
Lourantou et al., 2010). Bottom: EDC δD data (Stenni et al., 2001).
All records are on the new age scale QSR2010 (Lemieux-Dudon
et al., 2010).
therefore justified to apply Eq. (1) to convolve the CO2 signal
which might be recorded in the EDC ice core.
2.2
Carbon cycle modelling
In order to determine how fast carbon injected into the atmosphere is taken up by the ocean, we used the carbon cycle box
model BICYCLE (Köhler and Fischer, 2004; Köhler et al.,
2005a, 2010b). The model version used here and its forcing
over Termination I are described in detail in Lourantou et al.
Clim. Past, 7, 473–486, 2011
(2010). Furthermore, we tried to determine of which origin
(terrestrial or marine) the carbon might have been by comparing the simulated and measured atmospheric δ 13 CO2 fingerprint during the carbon release event. Similar approaches
(identifying processes based on their δ 13 C signature) were
applied earlier for the discussion of the atmospheric δ 13 CO2
record over the whole Termination I (Lourantou et al., 2010)
and longer timescales (Köhler et al., 2010b). Here, we restrict the analysis to the question of whether the observed
signal might be generated by terrestrial or marine processes
only.
Briefly, BICYCLE consists of modules of the ocean (10
boxes distinguishing surface, intermediate and deep ocean
in the Atlantic, Southern Ocean and Indo-Pacific), a globally averaged terrestrial biosphere (7 boxes), a homogeneously mixed one-box atmosphere, and a relaxation approach to account for carbonate compensation in the deep
ocean (sediment-ocean interaction). The model calculates
the temporal development of its prognostic variables over
time as functions of changing boundary conditions, representing the climate forcing. These prognostic variables are
(a) carbon (as dissolved inorganic carbon DIC in the ocean),
(b) the carbon isotopes δ 13 C, �14 C, and (c) additionally in
the ocean total alkalinity, oxygen and phosphate. The terrestrial module accounts for different photosynthetic pathways
(C3 or C4 ), which is relevant for the temporal development
of the 13 C cycle.
Here, the model is equilibrated for 4000 yr for climate
conditions typical before the onset of the B/A. The Atlantic meridional overturning circulation (AMOC) is in an
off mode. Simulations with the AMOC in an on mode lead
to a different background state of the carbon cycle (atmospheric pCO2 is then 255 ppmv versus 223 ppmv in the off
mode), but the amplitudes in the atmospheric CO2 rise differ by less than 3 ppmv between both settings. Scenarios in
which the AMOC amplifies precisely at the onset of the B/A
warm period are not explicitly considered here, but are implicitly covered in the marine scenario. An amplification of
the AMOC would lead to stadial/interstadial variations typical for the bipolar seesaw. Such behaviour was found for
the onset of other D/O events in MIS 3 (Barker et al., 2010)
during which CO2 started to fall and not to rise as observed
for the B/A. Based on this analogy, our working hypothesis is that the main processes connected with changes in the
AMOC play a minor role for the abrupt rise in atmospheric
CO2 around 14.6 kyr BP (see Sect. 3.2 for details).
The simulated jump of CO2 is generated by the injection of a certain amount of carbon into the atmosphere,
while all other processes (ocean overturning, temperature,
sea level, sea ice cover, marine productivity, terrestrial biosphere) are kept constant. The size of the injection is deduced
from considerations on the airborne fraction and model simulations (see Sect. 3.1). The carbon is then brought with
a constant injection flux in a time window of a different
length (over either 50, 100, 150, 200, 250 or 300 yr) into the
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P. Köhler et al.: Abrupt rise in CO2 at the onset of the Bølling/Allerød
atmosphere. Our best guess injection amplitude of 125 PgC
corresponds to constant injection fluxes of 2.5 Pg C yr−1 (in
50 yr) to 0.42 Pg C yr−1 (in 300 yr) over the whole release period. The fastest injection (in 50 yr) with the largest annual
flux has been motivated by the abruptness in the climate signals recorded in the NGRIP ice core (Steffensen et al., 2008).
It is furthermore assumed that the injected carbon is either of
terrestrial or marine origin. These two scenarios differ only
in their carbon isotopic signature:
Terrestrial scenario: the δ 13 C signature is based on a study
with a global dynamical vegetation model (Scholze et al.,
2003), which calculates a mean global isotopic fractionation
of the terrestrial biosphere of 17.7‰ for the present day. We
have to consider a larger fraction of C4 plants during colder
climates and lower atmospheric pCO2 (Collatz et al., 1998),
as found at the onset of the B/A. This implies that about 20
and 30% of the terrestrial carbon is of C4 origin for present
day and LGM, respectively (Köhler and Fischer, 2004). The
significantly smaller isotopic fractionation during C4 photosynthesis (about 5‰) in comparison to C3 photosynthesis
(about 20‰) (Lloyd and Farquhar, 1994) therefore reduces
the global mean terrestrial fractionation to 16‰. With an atmospheric δ 13 CO2 signature of about −6.5‰, the terrestrial
biosphere has a mean δ 13 C signature of −22.5‰.
Marine scenario: in this scenario we assume that old carbon from the deep ocean heavily depleted in δ 13 C might
upwell and outgas into the atmosphere. Today’s values of
oceanic δ 13 C in the deep Pacific are about 0.0‰ (Kroopnick,
1985). From reconstructions (Oliver et al., 2010), it is known
that during the LGM deep ocean δ 13 C was on average about
0.5‰ smaller, thus δ 13 CLGM =−0.5‰. During out-gassing,
mainly in high latitudes, we consider a net isotopic fractionation of 8‰ (Siegenthaler and Münnich, 1981). This would
lead to δ 13 C=−8.5‰ in the carbon injected into the atmosphere if it were of marine origin.
The signals of simulated atmospheric CO2 and δ 13 CO2
plotted in the figures are derived by subtracting simulated
CO2 and δ 13 CO2 of a reference run without carbon injections
from our scenarios. The anomalies �(CO2 ) and �(δ 13 CO2 )
are then added to the starting point of the CO2 jump (δ 13 CO2
drop) into the B/A, which we define as 228 ppmv (−6.76‰)
at 14.8 kyr BP. In doing so, existing equilibration trends
(which will exist even for longer equilibration periods due
to the sediment-ocean interaction) are eliminated. The simulated atmospheric CO2 (δ 13 CO2 ) at the end of the equilibration period was 223 ppmv (−6.54‰). Our modelling exercise is therefore only valid for an interpretation of the abrupt
CO2 rise of 10 ppmv in the in-situ data of EDC. The mismatch in CO2 and δ 13 CO2 between simulations and EDC
data before 15 kyr BP and after 14.2 kyr BP, is therefore expected (Figs. 4b, 4d, 5b, 5d, 7).
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3
477
Results and discussions
3.1
Assessing the size of the carbon injection
We first estimate roughly the amount of carbon necessary to
be injected as CO2 into the atmosphere to produce a longterm jump of 10 ppmv using the airborne fraction f . The
long-term (centuries to millennia) airborne fraction f of CO2
can be approximated from the buffer or Revelle factor (RF)
of the ocean on atmospheric pCO2 rise. The present day
mean surface ocean Revelle factor (Sabine et al., 2004a) is
about 10. With
RF =
�pCO2 /pCO2
�DIC/DIC
(3)
and the content of C at the beginning of the B/A in the
atmosphere (CA = 500 Pg C ≈ 235 ppmv) and in the ocean
(CO = 37 500 Pg C = 75· CA ) it is
f=
�pCO2
1
=
= 0.118 .
75
�pCO2 + �DIC 1 + RF
(4)
Thus, the lower end of the range of the airborne fraction f
is about 0.12 (given by Eq. 4), while the upper end of the
range might be derived from modern anthropogenic fossil
fuel emissions to about 0.45 (Le Quéré et al., 2009). Please
note that f estimated with Eq. (4) assumes a passive (constant) terrestrial biosphere, while in the estimate of f from
fossil fuel emissions (Le Quéré et al., 2009), the terrestrial
carbon cycle is assumed to take up about a third of the anthropogenic C emissions. We take the range of f between
0.12 and 0.45 as a first order approximation and assume f
during the B/A to lie in-between. This implies that a longterm rise in atmospheric CO2 of 10 ppmv (equivalent to a rise
in the atmospheric C reservoir by 21.2 Pg C) can be generated
by the injection of 47 to 180 Pg C into the atmosphere.
We further refine this amplitude to 125 Pg C (equivalent to
f = 0.17) by using the global carbon cycle box model BICYCLE. The model then generates atmospheric CO2 peaks of
20–35 ppmv, depending on the abruptness of the C injection
(Fig. 4a). All scenarios with release times of 50–200 yr fulfil
the EDC ice core data requirements after the application of
the age distribution PDF (Fig. 4b). The acceptable scenarios
imply rates of change in atmospheric CO2 of 13–70 ppmv per
century, a factor of 3–16 higher than in the EDC data. Our
fastest scenario (release time of 50 yr) has a rate of change in
atmospheric CO2 , which is still a factor of two smaller than
the anthropogenic CO2 rise of 70 ppmv during the last 50 yr
(Keeling et al., 2009). For comparison, in the less precise
CO2 data points taken from the Taylor Dome (Smith et al.,
1999) and Siple Dome (Ahn et al., 2004) ice cores, the abrupt
rise in CO2 at the onset of the B/A is recorded with 15±2 and
19±4 ppmv, respectively (Fig. 4a), with changing rates in ice
core CO2 of ∼4–6 ppmv per century. This already indicates
that at 14.6 kyr BP, CO2 measured in-situ in EDC differed
markedly from the true atmospheric CO2 .
Clim. Past, 7, 473–486, 2011
P. Köhler et al.: Abrupt rise in CO2 at the onset of the Bølling/Allerød
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Fig. 4. Simulations with the carbon cycle box model BICYCLE for an injection of 125 Pg C into the atmosphere. Injected carbon was either
of terrestrial (T : δ 13 C=−22.5‰) or marine (M: δ 13 C=−8.5‰) origin. Release of terrestrial C occurred between 50 and 300 yr. Marine C
was released in 50 yr (grey), but is identical to the terrestrial release in A, B. (A) Atmospheric CO2 from simulations and from EDC (Monnin
et al., 2001; Lourantou et al., 2010) on the new age scale QSR2010 (Lemieux-Dudon et al., 2010), Siple Dome (Ahn et al., 2004) (SD, on
its own age scale on top x-axis) and Taylor Dome (Smith et al., 1999) (TD, on revised age scale as in Ahn et al., 2004). All CO2 data has
been synchronised to the CO2 jump. (B) Simulated CO2 values potentially be recorded in EDC and EDC data. The simulated values are
derived by the application of the gas age distribution PDF of the hypothetical atmospheric CO2 values plotted in (A), followed by a shift in
the age scale by the width EB/A = 400 yr towards younger ages. (C, D) The same simulations for atmospheric δ 13 CO2 , cyan dots are new
EDC δ 13 CO2 data (Lourantou et al., 2010). Only the dynamics between 15.0 and 14.2 kyr BP (white band) are of interest here and should
be compared to the ice core data.
The uncertainty in the size of the CO2 peak given by the
variability in the width EB/A of the age distribution PDF
and by the range in the airborne fraction f lead to slightly
different results. The differences in EB/A between 320 and
480 yr give for f = 0.17 variations in the atmospheric CO2
peak height of less than 1 ppmv from the standard case and
these results are still within uncertainties of the ice core data
(Fig. 5b). We show in Fig. 5a and 5c how the atmospheric
Clim. Past, 7, 473–486, 2011
CO2 and δ 13 CO2 would look like for the upper (f = 0.45)
and lower (f = 0.12) end-of-range values in the airborne
fraction f , if simulated with our carbon cycle box model using a release time of 100 yr. The signal potentially recorded
in EDC is achieved after applying the age distribution PDF
(Fig. 5b, 5d). Atmospheric CO2 rose by 10 ppmv only in
the 47 Pg C-scenario, which would potentially be recorded as
4 ppmv in EDC. In the 180 Pg C-scenario the CO2 amplitude
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P. Köhler et al.: Abrupt rise in CO2 at the onset of the Bølling/Allerød
in the atmosphere would be 42 ppmv, which is 13 ppmv
larger than the 29 ppmv in our reference case, leading to
a long-term CO2 jump of 16 ppmv in a hypothetical EDC
ice core. After the application of the age distribution PDF,
both extreme cases for f were not in line with the evidence
from the ice core data.
3.2
Fingerprint analysis and process detection –
the shelf flooding hypothesis
But what generated this jump of CO2 at the onset of the
B/A? Changes in the near-surface temperature and in the
AMOC had massive impacts on the reorganisation of the terrestrial and the marine carbon cycle (Köhler et al., 2005b;
Schmittner and Galbraith, 2008), respectively. This led
to CO2 amplitudes of about 20 ppmv during D/O events
(Ahn and Brook, 2008). At the onset of the B/A the temperature changes in the northern and southern high latitudes as recorded in Greenland and in the central Antarctic
plateau followed the typical pattern of the bipolar seesaw that
also characterised the last glacial cycle (EPICA-communitymembers, 2006; Barker et al., 2009): gradual warming in the
South during a stadial cold phase in the North switched to
gradual cooling at the onset of a abrupt temperature rise in
the North (Fig. 1a). These interhemispheric patterns were
identified for all D/O events during Marine Isotope Stage 3
(MIS 3) and for the B/A as D/O event 1 (EPICA-communitymembers, 2006) (Fig. 1). In contrast to all D/O events during MIS 3, in which CO2 started to decline at the onset of
Greenland warming (Ahn and Brook, 2008), CO2 abruptly
increased around 14.6 kyr BP. This temporal pattern strongly
suggests that changes in the AMOC are not the main source
of the detected CO2 jump at the onset of the B/A, since the
general trend of the CO2 evolution during the D/O events in
MIS 3 is, based on existing data, of opposite sign.
However, we have to acknowledge that the mean temporal
resolution �t of CO2 data obtained from various other ice
cores in MIS 3 is with �t = 150–1000 yr much larger than
for the CO2 record of EDC during Termination I (�t = 92 yr,
Table 1). For this comparison, one needs to consider that
those data with the highest temporal resolution (Byrd, �t =
150 yr, Neftel et al., 1988) are those with the highest measurement uncertainty (mean 1σ = 4 ppmv, for comparison
EDC: mean 1σ ≤ 1 ppmv). All other CO2 ice core records in
MIS 3 have �t > 500 yr. Furthermore, present day accumulation rates in these other ice cores are 2–5 times higher than
in EDC, implying an approximately 2–5 times lower mean
width E of the gas age distribution PDF in the other ice cores
(Spahni et al., 2003) and thus a smaller smoothing effect of
the gas enclosure (Table 1). Therefore, the possibility that
similar abrupt CO2 rises in the true atmospheric signal also
exist during other D/O events can not be excluded, although
the data evidence from the overlapping CO2 records of the
Taylor Dome and Byrd ice cores does not seem to allow such
dynamics for the time between 20–47 kyr BP (Table 1, Ahn
www.clim-past.net/7/473/2011/
479
and Brook, 2007). Furthermore, the rate of change in CO2 at
the onset of the B/A is not unique for the last glacial cycle. In
the time window 65–90 kyr, BP (belonging to MIS 4 and 5)
CO2 measured in-situ in the Byrd ice core (Ahn and Brook,
2008) rose several times abruptly by up to 22 ± 4 ppmv in
200 yr, sometimes synchronous with northern warming (similar as for the B/A), and sometimes not. It needs to be tested if
a similar mechanism as proposed here was also responsible
for these CO2 jumps. An ice core with higher resolution,
e.g. the West Antarctic Ice Sheet (WAIS) Divide Ice Core,
might help to clarify the magnitude and shape of the abrupt
rise in atmospheric CO2 during the onset of the B/A and its
uniqueness with respect to other D/O events in MIS 3. The
WAIS Divide Ice Core exhibits a present day accumulation
rate of 24 g cm−2 yr−1 (Morse et al., 2002), which is nearly
an order of magnitude larger than EDC and 50% larger than
Byrd (Table 1).
Our working hypothesis also implies that the changes in
the AMOC connected with the bipolar seesaw pattern observed for B/A and other D/O events during MIS 3 were
similar. Proxy-based evidence supports this assumed similarity: A reduction of the AMOC to a similar strength during various stadials (Younger Dryas, Heinrich Stadials 1 and
2) was deduced from 231 Pa/230 Th (McManus et al., 2004;
Lippold et al., 2009). These results were also supported by
reconstructed ventilation ages in the South Atlantic off the
coast of Brazil (Mangini et al., 2010). The magnitude of the
AMOC amplification during a stadial/interstadial transition
is more difficult to deduce from proxy data. However, Barker
et al. (2010) recently reconstructed ventilation changes in
the South Atlantic Ocean and found a deep expansion of the
North Atlantic Deep Water export during the B/A (following
Heinrich Stadial 1), similar to results during the D/O event 8
around 38 kyr BP (following Heinrich Stadial 4). Taken together the data-based evidence indicates that (a) the AMOC
was shut down in a very similar way during Heinrich Stadials, and (b) the magnitude and the characteristics of the
AMOC amplification at the B/A was not exceptional (Knorr
and Lohmann, 2007; Barker et al., 2010). Thus, the AMOC
amplification during the B/A seemed to be similar to some
D/O events in MIS 3 following Heinrich Stadials. Both indications support our assumption that changes in the AMOC
can not explain the majority of the abrupt rise in atmospheric
CO2 at the onset of the B/A. The robustness of our hypothesis with respect to the uniqueness of the event might also
be tested by future higher resolved CO2 data, as mentioned
above.
To constrain the origin of the released carbon further, we
investigate the two hypotheses, that the carbon was only of
either terrestrial or marine origin. Our two scenarios vary
only in the isotopic signature of the injected C (terrestrial:
δ 13 CO2 =−22.5‰, marine: δ 13 CO2 =−8.5‰). We compare
carbon cycle model simulations of the typical fingerprint of
these two hypotheses with new measurements of atmospheric
δ 13 CO2 from EDC (Lourantou et al., 2010). We find that the
Clim. Past, 7, 473–486, 2011
480
P. Köhler et al.: Abrupt rise in CO2 at the onset of the Bølling/Allerød
270
A
-6.5
-6.6
260
245
-7.0
240
-7.1
-7.2
235
-7.3
230
180 PgC
125 PgC
047 PgC
225
-7.4
-7.5
220
245
-6.4
B
D
-6.5
240
o
-6.6
-6.7
235
13
CO2 (ppmv)
o
-6.9
13
CO2 (ppmv)
-6.8
250
CO2 ( /oo)
-6.7
255
CO2 ( /oo)
265
-6.4
C
-6.8
230
E = 320 yr
E = 400 yr
E = 480 yr
225
15
14
QSR2010 age (kyr BP)
-6.9
15
14
-7.0
QSR2010 age (kyr BP)
Fig. 5. Influence of (i) the amount of carbon injected in the atmosphere and of (ii) the details of the gas age distribution on both the
atmospheric signal and that potentially recorded in EDC. The amount of carbon injected in the atmosphere (A, C) covers the range derived
from an airborne fraction f between 12 and 45% from 47 to 180 Pg C with our reference scenario of 125 Pg C in bold. Injections occurred
in 100 yr with terrestrial δ 13 C signature. In the filter function of the gas age distribution (B, D) the width E varies from 320 yr to 480 yr,
our best-estimated gas age width E at the onset of the B/A of 400 yr in the solid line, representing the range given by the firn densification
model including heat diffusion (Goujon et al., 2003), as plotted in Fig. 3. Only the dynamics between 15.0 and 14.2 kyr BP (white band) are
of interest here and should be compared with the ice core data.
small dip of −0.14 ± 0.14‰ in δ 13 CO2 measured in-situ in
EDC might be generated by terrestrial C released in less than
three centuries (Fig. 4c, 4d). The marine scenario leads to
changes in δ 13 CO2 of less than −0.03‰ (Fig. 4d). Within
the uncertainty in so-far-published ice core δ 13 CO2 of 0.10‰
(1σ ), this marine scenario seems less likely than the terrestrial one, but it can not be excluded. All together, this δ 13 CO2
fingerprint analysis shows that all terrestrial or marine scenarios seemed to be possible, but a further constraint is, based
on the given data so far, not possible. New measured, but up
to now unpublished δ 13 CO2 data does not seem to lead to
different conclusions (Fischer et al., 2010).
Clim. Past, 7, 473–486, 2011
Besides the similarity in the typical patterns of the bipolar
seesaw, the B/A and the other D/O events differ significantly
in the rate of sea level rise. While the amplitudes of sea level
variations are with about 20 m during MIS 3 and B/A comparable (Peltier and Fairbanks, 2007; Siddall et al., 2008), the
rates of change are not. It took one to several millennia for
the sea level to change during MIS 3 (rate of change of 1–
2 m per century, Siddall et al., 2008), but during MWP-1A
the sea level rose by more than 5 m per century accumulating 16 to 20 m of sea level rise within centuries (Peltier and
Fairbanks, 2007; Hanebuth et al., 2000). The exact magnitude but also the timing of the sea level rise during MWP-1A
www.clim-past.net/7/473/2011/
P. Köhler et al.: Abrupt rise in CO2 at the onset of the Bølling/Allerød
481
Table 1. Available high resolution ice core CO2 records over the last glacial cycle in comparison to the EPICA Dome C data covering
Termination I.
ice core
time window
#
mean �t
yr
present day
acc. rate∗
g cm−2 yr−1
CO2
mean 1σ
ppmv
units
kyr BP
–
EPICA Dome C
Taylor Dome
Siple Dome
Byrd
Byrd
Byrd
10–20
20–60
20–41
30–47
47–65
65–91
109
73
21
113
34
76
92
550
1000
150
530
342
3
7
12
16
16
16
≤1
≤1
2
4
2
2
reference
Monnin et al. (2001); Lourantou et al. (2010)
Indermühle et al. (2000)
Ahn et al. (2004)
Neftel et al. (1988) as published in Ahn and Brook (2007)
Ahn and Brook (2007)
Ahn and Brook (2008)
∗ Taken from the compilation of Ahn et al. (2004).
varied depending on site location and reconstruction method.
However, Sunda Shelf data (Hanebuth et al., 2000; Kienast
et al., 2003) and recent evidence from Tahiti (Deschamps
et al., 2009) point to a timing of MWP-1A at 14.6 kyr BP,
in parallel to the temperature rise and the abrupt rise in CO2
at the onset of the B/A. Sea level records (Thompson and
Goldstein, 2007) suggest that large shelf areas which were
exposed around 30 kyr BP were re-flooded within centuries
by MWP-1A. The terrestrial ecosystems had thus ample time
to develop dense vegetation and accumulate huge amounts of
carbon, which could thus be released abruptly. In contrast to
MWP-1A, the gradual sea level rise during MIS 3 allowed
for CO2 equilibration between atmosphere and ocean. This
difference between the B/A and other D/O events in MIS 3 in
both the rate of sea level rise and the return interval of shelf
flooding events (used for terrestrial carbon build-up) suggests
that other rapid CO2 jumps are probably not caused by the
process of shelf flooding.
We estimate from bathymetry (Smith and Sandwell, 1997,
version 12.1) that 2.2, 3.2 or 4.0×1012 m2 of land were
flooded during MWP-1A for sea level rising between −96 m
and −70 m by 16, 20 or 26 m, respectively. This covers
the different reconstructions published for MWP-1A (from
−96 m to −80 m, from −90 m to −70 m, or a combination
of both, Hanebuth et al., 2000; Peltier and Fairbanks, 2007).
It ignores differences in sea level rise due to local effects
such as continental uplift or subduction, glacio-isostasy and
the relative position with respect to the entry point of waters responsible for MWP-1A. About 23% of the flooded
areas (Fig. 6) are located in the tropics (20◦ S to 20◦ N).
To calculate the upper limit of the amount of carbon potentially released by shelf flooding during MWP-1A, we assume
present-day carbon storage densities typical for tropical rain
forests (60 kg m−2 ) for the tropical belt, and the global mean
(20 kg m−2 ) for all other areas (Sabine et al., 2004b). Depending on the assumed sea level rise mentioned above, we
estimate that up to 64, 94 or 116 Pg C (equivalent to 51 to
93% of the necessary C injection) might have been stored on
those lands flooded during MWP-1A with about 50% located
www.clim-past.net/7/473/2011/
in the tropical belt. This estimate includes a complete relocation of the carbon stored on the flooded shelves to the atmosphere without any significant time delay. The efficiency
of this “flooding-scenario” depends on the relative timing
of MWP-1A. Several studies have indicated a time window
between the onset of the B/A and the Older Dryas, i.e. between about 14.7 and 14 kyr BP (Stanford et al., 2006, 2011;
Hanebuth et al., 2000; Kienast et al., 2003; Peltier and Fairbanks, 2007), including scenarios that place MWP-1A right
at the onset of the B/A (Hanebuth et al., 2000; Kienast et al.,
2003; Deschamps et al., 2009).
To set the timing of the abrupt rise in atmospheric CO2
into the temporal context with MWP-1A one has to consider
that the recent ice core age model used here (Lemieux-Dudon
et al., 2010) is based on the synchronisation of CH4 measured
in-situ in various ice cores. Accounting for a similar age distribution PDF in CH4 than in CO2 , the abrupt CH4 rise at the
onset of the B/A is recorded in EDC about 200 yr later than
in the Greenland ice core NGRIP, which depicts the atmospheric CH4 signal with only a very small temporal offset,
due to its high accumulation rate (Appendix B, Supplement).
If corrected for this CH4 synchronisation artefact, the proposed atmospheric rise in CO2 then starts around 14.6 kyr
BP, in perfect agreement with the possible dating of MWP1A (Fig. 7).
The residual carbon needs to be related to other processes.
From the discussed comparison of the B/A with other D/O
events during MIS 3, it has emerged that processes directly
related to the bipolar temperature seesaw (e.g. enhanced
northern hemispheric soil respiration due to warming or vegetation displacements (Köhler et al., 2005b), marine productivity changes (Schmittner and Galbraith, 2008) connected
with changes in the AMOC) are unlikely candidates, because
they should also have been in operation during those other
D/O events and would then have led to a similar carbon release. However, it might certainly be possible that the amplification strength of the AMOC, and thus the bipolar seesaw,
varied between different D/O events and thus a minor fraction of the released carbon might have been related to such
Clim. Past, 7, 473–486, 2011
482
P. Köhler et al.: Abrupt rise in CO2 at the onset of the Bølling/Allerød
Fig. 6. Areas flooded during MWP-1A. Changes in relative sea level from −96 m to −70 m are plotted from the most recent update (version
12.1) of a global bathymetry (Smith and Sandwell, 1997) with 1 min spatial resolution ranging from 81◦ S to 81◦ N.
processes. The origin of the water masses responsible for
MWP-1A is debated (Peltier, 2005). If a main fraction of
the waters was of northern origin and released during a retreat (not a thinning) of northern hemispheric ice sheets, then
the release of carbon potentially buried underneath ice sheets
following the glacial burial hypothesis (Zeng, 2007) might
also be considered. This might, however, be counteracted by
enhanced carbon sequestration on new land areas available at
the southern edge of the retreating ice sheets. Both processes
are irrelevant for the retreating ice sheets in Antarctica. The
generation of new wetlands at the onset of the B/A, as corroborated by the isotopic signature of δ 13 CH4 points to a unique
redistribution of the land carbon cycle during that time (Fischer et al., 2008). Furthermore, a potential contribution from
the ocean might also be necessary. However, a quantification
of these processes is not in the scope of this study.
3.3
The impact of shelf flooding on the carbon cycle
Shelf flooding might have had an impact on the marine export
production. According to Rippeth et al. (2008), the flooding
of continental shelves would have increased the marine biological carbon pump. This hypothesis is based on recent
observations that shelf areas are sinks for atmospheric CO2
(e.g. Thomas et al., 2005a,b). Thus, increasing the area of
flooded shelves by sea level rise would according to Rippeth
et al. (2008) increase the marine net primary production and
might lead to enhanced export production and reduced atmospheric CO2 . The impact of shelf flooding on the marine exClim. Past, 7, 473–486, 2011
port production might therefore have increased the amplitude
of the atmospheric CO2 rise, which needs to be explained by
other processes.
To our knowledge, so far no study considers how carbon stored on land would be released in detail by flooding
events. Our first order approximation given here is therefore based on the assumption that all carbon stored on land is
released into the atmosphere within the given time window
of the carbon injection (50 to 200 yr). Our understanding of
shelf flooding is as follows: a rise in sea level with a rate of
more than 5 m per century typical for MWP-1A would be superimposed on sea level variability with higher frequencies
(e.g. tides). Short sea level high stands (e.g. spring tides)
successively threaten plants so far established on the flooded
land. Salt-intolerant species would be the first to suffer and
become locally extinct after sufficient exposure to salt-water
conditions, even after a temporal water retreat following sea
level high stands. Finally, all previously established plants
relying on freshwater conditions would die and decay. The
decay of foliage is abrupt (less than a 1 yr), while that of
hard wood might takes considerably longer (up to 10 yr in
recent Amazonian rain forest plots, Chao et al., 2009). Heterotrophic respired carbon of this dead vegetation is dominantly partitioned to the detritus and partially to the atmosphere and soil pools. Detritus itself has a turn over times of
a few years only. Most soil carbon pools have a turnover
time of less than one century. We therefore assume that
after the collapse of the vegetation, implying a stop to the
www.clim-past.net/7/473/2011/
P. Köhler et al.: Abrupt rise in CO2 at the onset of the Bølling/Allerød
240
atmosphere
atm @ gas age filter
potential EDC
220
270
245
EB/A
230
220
180
250
240
225
200
260
Monnin 2001
Lourantou 2010
14.8
CO2 (ppmv)
CO2 (ppmv)
250
800
240
CO2 jump 050y
CO2 jump 200y
230
700
EDC
220
600
210
200
15.0
14.5
14.0
13.5
500
190
QSR2010 age (kyr BP)
-2
Fig. 7. Influence of the gas age distribution PDF on the CO2 signal.
The original atmospheric signal (blue) leads to a time series (red)
with similar characteristics (e.g., mean values) after filtering with
the gas age distribution PDF with the width EB/A = 400 yr. To account for the use of the width of the gas age PDF in the gas chronology (R. Spahni, personal communication, 2010) the resulting curve
has to be shifted by EB/A towards younger ages to a time series
potentially recorded in EDC (black). This leads to a synchronous
start in the CO2 rise in the atmosphere (blue) and in EDC (black)
around 14.8 kyr BP on the ice core age scale QSR2010 (lower xaxis) (Lemieux-Dudon et al., 2010). Due to a similar gas age distribution PDF of CH4 the synchronisation of ice core data contains
a dating artefact which is for EDC at the onset of the B/A around
200 yr (Appendix B, Supplement). On the age scale corrected for
the synchronisation artefact (upper x-axis), the onset in atmospheric
CO2 falls together with the earliest timing of MWP-1A (grey band)
(Hanebuth et al., 2000; Kienast et al., 2003).
R of CO2, CH4, N2O (W m )
180
Greenland
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
-1.4
-1.6
-1.8
-2.0
-2.2
CH4 (ppbv)
255
235
260
400
300
RN2O
RCH4
RCO2
RGHG
20
18
16
14
12
10
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
-1.4
-1.6
-1.8
-2.0
-2.2
-2.4
-2.6
-2.8
-3.0
-2
14.6
Talos Dome
N2O (ppbv)
corrected age (kyr BP)
15.0 14.5 14.0 13.5
R of GHG (W m )
260
483
QSR2010 age (kyr BP)
input of carbon into the soil carbon pools, most soil carbon
is released into the atmosphere in less than a century. Our
estimate that 50% of the released carbon had originated in
the tropics would allow for an even faster release of terrestrial carbon into the atmosphere, because respiration rates
are temperature dependent and much faster (turnover times
much smaller) in the warm and humid tropics than in boreal regions. The soil carbon release is affected by rising sea
level and thus salt water conditions and depends on the temporal offset between the vegetation collapse and the start of
the long-term influence of salty water on the soil. Following
the spring tide idea above, this temporal offset might have
been substantial, e.g. some decades. All together, the carbon
released from flooded shelves might include nearly the complete standing stocks and should not be delayed by more than
a century.
www.clim-past.net/7/473/2011/
Fig. 8. Greenhouse gas records (Talos Dome N2 O, EDC CO2 ,
Greenland composite CH4 ) and their radiative forcing �R during
Termination I. See Captions to Fig. 1b for details. EDC CO2 and
Greenland composite CH4 are plotted on the QSR2010 age scale,
thus without considering a potential dating artefact in EDC CO2
due to CH4 synchronisation, Talos Dome N2 O is shown on the
TALDICE-1 age scale. Black lines are running means over 290 yr
(to reduce sampling noise) of resamplings with 10 yr equidistant
spacing. Talos Dome and Greenland gas records are temporally
higher resolved than EDC and should contain a much smaller effect
of the age distribution PDF proposed for CO2 in EDC. The two CO2
jump scenarios are the minimum and maximum injection scenarios from our BICYCLE simulations which are still in line with the
in-situ CO2 data in EDC. The 50-yr and 200-yr injection scenario
contains a constant injection flux of either 2.5 and 0.625 Pg C yr−1 ,
respectively, over the given time window. The calculated radiative
forcing �R uses equations summarised in Köhler et al. (2010a) including a 40% enhancement of the effect of methane (Hansen et al.,
2008).
Clim. Past, 7, 473–486, 2011
484
4
P. Köhler et al.: Abrupt rise in CO2 at the onset of the Bølling/Allerød
Conclusions
Our analysis provides evidence that changes in the true atmospheric CO2 at the onset of the B/A include the possibility of
an abrupt rise by 20–35 ppmv within less than two centuries.
This result depends in its details on the applied model and
the assumed carbon injection scenarios and needs further investigations into sophisticated carbon cycle-climate models,
because the radiative forcing of this CO2 jump alone is 0.59–
0.75 W m−2 in 50–200 yr (Fig. 8). The Planck feedback of
this forcing causes a global temperature rise of 0.18–0.23 K,
which other feedbacks would amplify substantially (Köhler
et al., 2010a). Based on the dynamical linkage between the
temperature rise, the changes in the AMOC and the timing
of MWP-1A we have provided a shelf flooding hypothesis
which might explain the CO2 jump at the onset of the B/A.
In the light of existing CO2 data, this dynamic is distinct
from the CO2 signature during other D/O events in MIS 3
and might potentially define the point of no return during the
last deglaciation. A new CO2 record from the WAIS Divide
ice core has the potential to clarify whether this abrupt rise
in atmospheric CO2 during the B/A is unique with respect
to other D/O events during the last 60 kyr, thus also testing
the robustness of our hypothesis. The mechanism of continental shelf flooding might also be relevant for future climate
change, given the range of sea level projections in response
to rising global temperature and potential instabilities of the
Greenland and the West Antarctic ice sheets (Lenton et al.,
2008). In analogy to the identified deglacial sequence, such
an instability might amplify the anthropogenic CO2 rise.
Supplementary material related to this
article is available online at:
http://www.clim-past.net/7/473/2011/
cp-7-473-2011-supplement.pdf.
Acknowledgements. We thank Hubertus Fischer for discussions
and for pointing us at the question of strong terrestrial carbon
changes during abrupt CO2 jumps. Johannes Freitag provided
us with insights to gases in firn and related difficulties in dating ice core gas records. Renato Spahni provided the gas age
distribution calculated with a firn densification model plotted
in Fig. 2 and in-depth details on gas chronologies. We thank
Luke Skinner, Mark Siddall and an anonymous reviewer for their
constructive comments. Work done at LGGE was partly funded by
the LEFE programme of Institut National des Sciences de l’Univers.
Edited by: L. Skinner
Clim. Past, 7, 473–486, 2011
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Manuscript prepared for Clim. Past
with version 3.2 of the LATEX class copernicus.cls.
Date: 24 March 2011
Supplemental Material for:
Abrupt rise in atmospheric CO2 at the onset of the Bølling/Allerød:
in-situ ice core data versus true atmospheric signals
P. Köhler1 , G. Knorr1,2 , D. Buiron3 , A. Lourantou3,* , and J. Chappellaz3
1
Alfred Wegener Institute for Polar and Marine Research (AWI), P.O. Box 120161, 27515 Bremerhaven, Germany
School of Earth and Ocean Sciences, Cardiff University, Cardiff, Wales, UK
3
Laboratoire de Glaciologie et Géophysique de l’Environnement, (LGGE, CNRS, Université Joseph Fourier-Grenoble), 54b
rue Molière, Domaine Universitaire BP 96, 38402 St. Martin d’Hères, France
*
now at: Laboratoire d’Océanographie et du Climat (LOCEAN), Institut Pierre Simon Laplace, Université P. et M. Curie
(UPMC), Paris, France
2
Appendix A Investigating the principle behaviour of the
log-normal function describing the age distribution
PDF during gas enclosure in ice cores
The log-normal function which was applied here as the age
distribution PDF of CO2 was fitted as described in the methods section of the main text to the output of a firn densification model (Joos and Spahni, 2008). Furthermore, the chosen
width E = 400 yr of this function for the onset of the B/A
warm period was determined by another firn densification
model, which includes heat diffusion (Goujon et al., 2003).
When this function (Eq. 1 in the main text) is applied onto
atmospheric CO2 time series derived from our carbon cycle
model simulations it then acts as a filter whose resulting output has the characteristics of a CO2 time series potentially
recorded in the EDC ice core.
A data-based investigation of the principle behaviour of
this filter function would provide an independent support for
this approach. We therefore test the behaviour of the filter
with ice core CH4 data, which exist for our time window of
interest not only from EDC (our ice core of interest with low
accumulation rates), but also from Greenland sites. These
Greenland sites have high accumulation rates and therefore
the process of gas enclosure has a smaller impact on the CH4
time series than in EDC.
A1
Target
The behaviour of the filter can be tested on the abrupt rise
in atmospheric CH4 which occurs in parallel to the abrupt
rise of atmospheric CO2 around 14.6 kyr BP. For this test
the CH4 record in an ice core with highest accumulation
rate (namely a composite record from Greenland (EPICAcommunity-members, 2006)) might serve as a substitute for
atmospheric CH4 . Applying the chosen filter function for
the EDC ice core gas enclosure to the Greenland ice core
Correspondence to: P. Köhler ([email protected])
CH4 record should provide a temporal behaviour (in terms of
slope or gradient m) similar to that recorded in CH4 in EDC
(Monnin et al., 2001). Performing the same test not only for
the Greenland composite CH4 record but also for other ice
cores might in principle extend the robustness of the filter,
but the lower accumulation rates in Antarctic ice cores (implying also a lower temporal resolution) have already large
effects on the recorded CH4 , which implies further difficulties in the interpretation.
A2
Requirements
It is essential that the gas enclosure characteristics in terms
of age distribution PDF are similar for CH4 and CO2 . This
seems to be the case based on the output of a firn densification model (Joos and Spahni, 2008).
A3
Limitations
There are certain limitations to this investigation:
1. Because the gas enclosure of CH4 in Greenland already
changed the true original atmospheric signal, a principle understanding of the filter is necessary. This can be
obtained by using an artificial CH4 time series as input
data to the filter. Thus, by filtering this artificial CH4
record for conditions typical for EDC (step a: width
E = 400 yr), for Greenland (step b: E = 60 yr) and then
finally by filtering the output for Greenland of step b:
a second time with conditions typical for EDC (step c:
E = 400 yr) we can generate two artificial time series
whose behaviour in terms of the slope m can be compared with that of the ice core data of Greenland and
EDC (see Appendix B of the Supplemental Material for
details on E for Greenland).
The difficulty of this comparison of the behaviour of the
filter for artificial and real input data is, that the comparison works best, if the artificial CH4 is as similar as
P. Köhler et al.: Abrupt rise in CO2 at the onset of the Bølling/Allerød
2
Table A1. Slope m of CH4 rise at onset of B/A warm event in ppbv per century
ice core data
ice core
Greenland
Greenland filtered to EDC (E = 400 yr)
EDC target
∆m difference (Greenland – EDC)
171 ± 15
28 ± 5
39 ± 4
132 ± 16
possible to the original atmospheric CH4 peak. However, the atmospheric CH4 peak is not precisely known,
and thus the slope m in the artificial data can only be
estimated to lie somewhere between infinity (instantaneous rise of CH4 ) and the slope calculated in the ice
core CH4 data with highest accumulation rate (Greenland, m = 171 ppbv per century).
2. This comparison is further complicated by a potential interhemispheric gradient in CH4 . Although data
analysis suggests a stronger interhemispheric gradient
in warm interstadials than in cold stadials (Dällenbach
et al., 2000), the change in the interhemispheric gradient
in CH4 in the very narrow time window of the transition
into the B/A warm period is not precisely known (Brook
et al., 1999).
3. Checking if the filtered Greenland CH4 time series fits
onto single CH4 points measured in EDC is not meaningful, because the filtering affects the age model of the
time series, which needs to be corrected accordingly.
4. For our problem at hand, which focuses on the transition into the B/A warm period, the dynamics of CH4
and the usability of the applied filter function at other
times with different climate background conditions, as
well as different accumulation rates and densification
characteristics (e.g. at the beginning and the end of the
Younger Dryas), is not relevant.
A4
Results
Within the given limitations and estimated uncertainties
(CH4 : 1σ = 10 ppbv measurement uncertainty (Monnin
et al., 2001); 20% uncertainty in the suggested widths E of
the age distribution PDF; ignoring errors and uncertainties
in the gas age models) the filter produces slopes in the
artificial CH4 peaks which are similar to those of the ice
core data (see Tab. A1 and Fig. A1 of the Appendix). The
artificial CH4 did not include an interhemispheric gradient in
CH4 and therefore the difference in the slope ∆m between
Greenland and EDC should be larger or similar in the
analysis of the ice core data than in the artificial data. This
difference in the slope depends on the original slope of the
assumed artificial atmospheric CH4 and this knowledge can
artificial atmospheric CH4 with different m
m = 2000 m = 400
m = 250
234 ± 50
181 ± 24
146 ± 15
32 ± 7
32 ± 6
33 ± 7
36 ± 8
36 ± 5
35 ± 7
198 ± 51
145 ± 25
111 ± 17
be used to suggest, that the slope m of the true atmospheric
CH4 was likely smaller than 400 ppbv/century, probably m
was between 200 and 350 ppbv/century.
We therefore conclude that the investigations on the principle behaviour of the age distribution PDF used here to mimic
the gas enclosure process does not introduce any systematic
bias to the gas records. The age distribution PDF operates
as a filter on atmospheric CH4 or CO2 records and produces
convoluted time series of the respective gas records which
are comparable in their slope or gradient with in-situ measurements in ice cores. The application of the filter on the
abrupt rise in CO2 during the onset of the B/A warm period
seems therefore to be justified. These investigations based
on ice core CH4 data are a very reliable support for the gas
enclosure characteristic assumed for the abrupt rise of CO2
into the B/A.
Appendix B Uncertainty in methane synchronisation of
ice cores
Recently, a consistent synchronisation of the ice cores
NGRIP, EPICA Dome C (EDC), EPICA DML (EDML) and
Vostok was published (Lemieux-Dudon et al., 2010) (named
here: QSR2010 age scale). This effort combined the use of
various different age markers from the ice matrix (e.g. volcanic horizons, magnetic reversals, 10 Be peaks) and the gas
phase to overcome shortcomings of previous age scales (Ruth
et al., 2007; Loulergue et al., 2007; Parrenin et al., 2007).
Here, especially the synchronous matching of abrupt changes
in CH4 was a prominent target to align ice core climate
records over the last glacial cycle, especially over Termination I. This new ice age scale was used within our study.
As described in the methods section of the main text gases
entrapped in ice cores have a typical age distribution PDF
which they derive during mixing in the firn before bubble
close-off. This age distribution PDF mainly depends on local accumulation rate and temperature, and can be calculated
with firn densification models (main text Fig. 2). These PDFs
are very similar for CO2 and CH4 for the EDC ice core (Joos
and Spahni, 2008). The effect of the age distribution PDF
is, that the ice cores do not record the true atmospheric signal, but one that is attenuated. The back calculation from the
P. Köhler et al.: Abrupt rise in CO2 at the onset of the Bølling/Allerød
Artificial CH4 variability, no age correction
3
Raw data + interpolated filtering, no age correction
750
CH4 (ppbv)
650
ATM, m=250 ppbv/century
E=400y ( 20%), EDC
E=060y ( 20%), Greenland
E=400y ( 20%) @ Greenland
used for slope calculation
EDC
Greenland
700
650
600
600
550
550
500
500
450
450
400
400
CH4 (ppbv)
700
750
Greenland filtered, E=400 80 yr
350
-1000
0
Time (yr)
1000
-15000
-14000
350
Time (yr BP)
Fig. A1. Left: Artificial methane peak, filtered with the log-normal function with various widths E including a relative uncertainty of
20%. An interhemispheric gradient in methane is not considered. Right: Ice core raw data from EDC (Monnin et al., 2001) and Greenland
composite (EPICA-community-members, 2006) and Greenland filtered data. The circled data points were taken to calculate the slope m in
the CH4 data as given in Table 1.
in-situ ice core measurements to the atmospheric signal is
not unique. Nevertheless, certain details of the atmospheric
signal can be prescribed based on the knowledge of gas enclosure in ice cores: (1) Fast changes in atmospheric records
are always more abrupt and larger than their in-situ measurements in ice cores. Due to typically higher accumulation
rates in Greenland this partly explains the interhemispheric
gradient in CH4 during D/O events. (2) Synchronisation
of ice cores along abrupt changes in in-situ measured CH4
have an embedded dating artefact which depends on the gas
age distribution PDF. The second point, the embedded dating
artefact, was not mentioned explicitly as a source of dating
errors in descriptions of gas chronologies and synchronisation attempts (Blunier et al., 2007; Loulergue et al., 2007;
Lemieux-Dudon et al., 2010) and is presumably not included
in their uncertainty estimates.
The pure application of the age distribution PDF of CH4
as a filter function on a true atmospheric CH4 peak shifts the
onset of a CH4 peak by the width E of the gas age distribution PDF towards older ages (Fig. B1A of Appendix). This
age offset is corrected for during the preparation of gas age
scales of ice cores (Spahni, personal communication) with
the effect, that the onset in the true atmospheric signal and
those recorded in ice cores occur simultaneously (Fig. B1B
of Appendix). However, this correction comes on the cost of
uncertainty in the timing of the peak and the transition.
During wiggle matching of different paleo records the
mid-transition points of abrupt changes are often taken as
reference tie-points, on which the respective transitions are
aligned to. The approach of mid-transition points is taken
here for the sake of argument, but we are aware, that more
sophisticated models might be used, which would nevertheless still have to cope with the synchronisation uncertainty
discussed here. In the case of CH4 synchronisation from
ice cores the in-situ measured mid-transition points differ
from the true atmospheric CH4 by about 58% of the width
E of the gas age distribution PDF of the relevant ice core
in the respective climate period of interest (Fig. B1D of Appendix). The alignments of various ice cores performed so
far (Lemieux-Dudon et al., 2010) synchronised the in-situ
measured CH4 data. A more precise approach would try to
use the underlying true atmospheric CH4 for synchronisa-
P. Köhler et al.: Abrupt rise in CO2 at the onset of the Bølling/Allerød
4
B/A, age uncorrected
B/A, age corrected
750
750
CH4 (ppbv)
650
A
E=400y, EDC
E=060y, NGRIP
700
650
600
600
550
550
500
500
450
450
400
400
350
-1000
-500
0
Time (yr)
500
1000
B/A synchronized
-500
0
Time (yr)
700
650
ATM @ EDC, m=250ppbv/century
ATM @ NGRIP, m=250ppbv/century
500
350
1000
CH4 synchronisation offset = f(E, m)
750
CH4 (ppbv)
B
E=400y, EDC
E=060y, NGRIP
2
y = 3.6+0.59*x, r = 99.98%
2
y = 16.3+0.57*x, r = 99.96%
C
E=400y, EDC
E=060y, NGRIP
m=100ppbv/century
m=400ppbv/century
D
400
350
300
600
EDC
250
550
~200 yr
200
500
EDC-NGRIP
150
offset at B/A
100
~200 yr
450
NGRIP
400
350
-1000
-500
0
Time (yr)
CH4 (ppbv)
700
ATM, m=250ppbv/century
500
0
50
mid-transition point offset (yr)
ATM, m=250ppbv/century
0
100 200 300 400 500 600 700
Width E of age PDF (yr)
Fig. B1. Effect of the age distribution PDF on an artificial time series of atmospheric CH4 , which includes a abrupt rise in CH4 by 200 ppbv.
(A) Peak attenuation for two different widths E of the age distribution PDF, which represent B/A conditions of the NGRIP (E = 60 yr) and
EDC (E = 400 yr) ice cores without age correction. (B) Same as in (A), but potential ice cores records are now aged corrected (shifted by
E to younger ages). Circles mark the mid-transition points defined as a CH4 concentration of 550 ppbv. (C) Potential synchronisation error
for the transition into the B/A. Here, the mid-transition points of the smoothed time series (potential recorded in ice cores) are synchronised.
The bold lines show the temporal settings of the atmospheric signals connected to the synchronised potential ice core CH4 . (D) Summary
on the synchronisation effect for different width E of the age distribution PDF and different slopes m, by which atmospheric CH4 changed
during its abrupt rise. Results are based in the difference of the mid-transition points in atmospheric CH4 and potential ice core records.
tion. Unfortunately, the true atmospheric signal is not precisely known and can only be approximated using assumptions on the rates of change and amplitudes which might
have been occurred in the atmosphere. However, if CH4 synchronisations rely on the ice core CH4 data they then have
a dating artefact which depends on the embedded age offset
between true atmospheric values and in-situ measurements.
For EDC a width E of the gas age distribution PDF of
400 yr was calculated here with a firn densification model
for the climate around 14.6 kyr BP. An estimate of the width
P. Köhler et al.: Abrupt rise in CO2 at the onset of the Bølling/Allerød
E at NGRIP based on firn densification models was not available. For the GRIP ice core (recent accumulation rate of 211
mm water equivalent per year (Chappellaz et al., 1997)) E is
estimated by a firn densification model to 25 yr for present
day climate (Spahni et al., 2003). NGRIP has a recent accumulation rate of 174 mm water equivalent per year (Andersen
et al., 2006), which is about a factor seven larger than at EDC
(Blunier et al., 2007). Using the inverse of the ratio of the accumulation rates as an estimate for the ratio of the width E of
the gas age distribution PDF leads to a E = 60 yr at NGRIP
during the onset of the B/A. Our estimate for E therefore
seems to be in a right order of magnitude and this approach
should illustrate the orders of magnitude for our problem at
hand.
We now apply how a abrupt rise in artificial CH4 with a
true atmospheric amplitude of 200 ppbv, which rises with a
slope m between 100 and 400 ppbv per century, would be
recorded in these two ice cores with the given gas enclosure
characteristics. If the age correction by the width E of the
gas age PDF is applied, the onset in CH4 in the atmosphere
and in all ice cores is dated to be simultaneously, but the
mid-transition points in the ice cores are recorded 240 and
45 years later than in the atmosphere under conditions typical
for the B/A for EDC and NGRIP, respectively (Fig. B1B,D
of Appendix). The wiggle matching alignment of CH4 of the
two ice cores is therefore at maximum as accurate as the difference of the dating of these mid-transition points from the
true atmospheric signal. The QSR2010 gas age scale used
here (Lemieux-Dudon et al., 2010) is at the onset of the B/A
due to the embedded CH4 synchronisation artefact about 200
years too old (Fig. B1C,D of Appendix). We need to correct
for this temporal offset to set our proposed atmospheric CO2
signal into context with the dating of MWP-1A. The correction for this temporal offset aligns our proposed true atmospheric CO2 shift to the occurance of MWP-1A (Fig. 7 of
main text).
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