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 250 200 240 180 230 800 220 700 210 200 600 190 500 260 240 220 260 250 200 240 180 230 800 N2O (ppbv) 220 260 280 270 CO2 (ppmv) 240 N2O (ppbv) 260 300 -60 -80 -100 -120 -140 220 700 210 200 600 190 500 CH4 (ppbv) 280 270 CO2 (ppmv) sea level (m) 300 -60 -80 -100 -120 -140 CH4 (ppbv) sea level (m) 474 -380 400 -35.0 400 -42.5 o -33 -36 -440 -39 -40.0 -42.5 A 60 50 40 30 Age (kyr BP) 20 -460 B 20 18 16 14 12 10 o -37.5 O ( /oo) -35.0 18 -40.0 -30 -420 O ( /oo) o o 18 -32.5 300 18 300 -37.5 D ( /oo) O ( /oo) -400 -42 -45 QSR2010 age (kyr BP) 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 www.clim-past.net/7/473/2011/ P. Köhler et al.: Abrupt rise in CO2 at the onset of the Bølling/Allerød 6 PRE lognormal PRE CO2 firn model o Probability ( /oo) 5 4 3 2 B/A lognormal 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. www.clim-past.net/7/473/2011/ 475 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 476 P. Köhler et al.: Abrupt rise in CO2 at the onset of the Bølling/Allerød 290 280 270 0 250 240 100 230 200 220 210 300 200 190 400 -360 500 600 o D ( /oo) -380 width E of age distribution PDF (yr) CO2 (ppmv) 260 700 -400 -420 -440 20 18 16 14 12 10 8 6 4 2 0 QSR2010 age (kyr BP) 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 www.clim-past.net/7/473/2011/ 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). www.clim-past.net/7/473/2011/ 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 SD age (kyr BP) 15 14 260 -6.4 C -6.5 -6.6 -6.7 CO2 (ppmv) 255 -6.8 250 -6.9 -7.0 245 T 050 yr T 100 yr T 150 yr T 200 yr T 250 yr T 300 yr M 050 yr 240 235 230 225 220 245 -7.2 -7.3 -7.4 -7.5 -6.4 B D -6.5 240 o -6.6 -6.7 235 13 CO2 (ppmv) -7.1 o EDC (Monnin 2001) EDC (Lourantou 2010) TD (Smith 1999) SD (Ahn 2004) CO2 ( /oo) 265 A CO2 ( /oo) 270 13 478 -6.8 230 -6.9 225 15 14 QSR2010 and TD age (kyr BP) 15 14 -7.0 QSR2010 age (kyr BP) 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 www.clim-past.net/7/473/2011/ 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 References Ahn, J. and Brook, E. J.: Atmospheric CO2 and climate from 65 to 30 ka B.P., Geophysical Research Letters, 34, L10703, doi:10.1029/2007GL029551, 2007. Ahn, J. and Brook, E. J.: Atmospheric CO2 and climate on millennial time scales during the last glacial period, Science, 322, 83–85, doi:10.1126/science.1160832, 2008. 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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. 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