Proposing a mechanistic understanding of
atmospheric CO 2 during the last 740,000 years
— a contribution to the EPICA challenge
P. K¨ ohler & H. Fischer
Alfred Wegener Institute for Polar and Marine Research in the Helmholtz Association
P.O. Box 12 01 61, D-27515 Bremerhaven, Germany, email: pkoehler@awi-bremerhaven.de, hufischer@awi-bremerhaven.de Paleo-records in Antarctic ice cores revealed strong glacial/interglacial variations in temperature, atmospheric dust as well as carbon dioxide. To date, the longest CO2record derived from the Vostok ice core goes back in time as far as about 410 kyrs showing that CO2concentrations vary between 280 and 180 ppmv for interglacials and glacials, respectively.
Latest measurements of dust and isotope temperatures on the new EPICA ice core from Dome C (EDC), cover the last 740 kyrs, i.e. four more glacial cycles which showed, however, reduced temperature amplitudes compared to the Vostok time span. This new archive offers the possibility to propose atmospheric CO2changes for the pre-Vostok era as called for in the EPICA challenge (Wolff et al., 2004, The EPICA challenge to the Earth System Modeling Community.
EOS 85: 363). Here, we contribute to this challenge using a box model of the isotopic carbon cycle based on process understanding previously derived for Termination I. Our Box model of the Isotopic Carbon cYCLE BICYCLE (K¨ohler et al.
Quantitative interpretation of atmospheric carbon records over the last glacial termination, submitted to GBC.) consists of ten ocean resvervoir in three high layers distinguishing Atlantic, Indo-Pacific, and Southern Ocean, a seven compartment terrestrial biosphere and considers also fluxes of dissolved inorganic carbon and alkalinity between ocean and sediments.
BICYCLE is forced by various ice core and marine sediment records to depict observed changes in temperature, sea level, lysocline dynamics, and aeolian iron input into the Southern Ocean. Our results show that major features of the Vostok period are reproduced while prior to Vostok our model predicts significantly smaller amplitudes in CO2variations. The main contributions (in decreasing order) to the variations in pCO2were given by changes in Southern Ocean vertical mixing, exchange fluxes between ocean and sediment, sea surface temperature, North Atlantic deep water formation, iron fertilisation, and Heinrich events. While most processes were reduced in their magnitude during the terminations of the pre-Vostok period, the absolute contribution of iron fertilisation changed only slightly. Thus, the relative importance of biological and biogeochemical processes is enhanced (approx. doubling their relative share) in the pre-Vostok period. The contribution of physical processes (ocean temperature, sea level, sea ice) to the pCO2rise during terminations stayed always below 25%, while ocean circulation contributed up to 75% during the Vostok era but less than 50% before.
Impact of different processes on G/IG changes in pCO2during the last eight terminations.
Impact on pCO2(ppmv) (one process at a time/all but one processes)
Process I II III IV V VI VII VIII
Physical processes
SST 36/27 37/31 24/22 30/20 35/26 11/2 34/24 29/13
Sea level –16/–9 –15/–11 –7/–4 –12/–7 –7/–5 –5/–1 –12/–8 7/5
Sea ice –11/–5 –9/–5 –4/–2 –7/–2 –11/–3 –2/–1 –10/–6 –16/–12
Ocean circulation
THC 13/27 13/22 6/21 10/11 13/49 0/3 0/10 0/2
Heinrich events 7/10 6/7 4/8 11/1 11/32 0/0 7/9 0/0
SO vertical mixing 30/37 28/38 23/41 30/38 14/26 14/11 23/29 15/19
Biology and biogeochemistry
Fluxes ocean sediment 4/31 3/34 1/23 3/31 3/31 1/15 2/25 –2/7
Fe fertilisation 19/16 19/22 4/5 19/16 8/5 19/22 19/25 5/6
Terrestrial biosphere –5(–20)/–7 –8(–22)/–10 –5(–17)/–6 –7(–24)/–7 –4/–5 –2/–4 –4/–8 –3/–2 (in brackets forced with Vostok pCO2)
Sum 90/127 87/128 52/108 87/101 75/156 36/41 59/100 35/38
Simulated (scenario S) 104 102 70 94 100 48 77 46
Vostok 102 97 84 112 – – – –
0 20 40 60 80 100
IRD(%)
A
5 10 15
SST(oC)
B
5 4 3 2
18O(o/oo)
C
0 -1 -2
18O(o/oo)
D
2 0 -2
18O(o/oo)
E
400 200 0 -200 -400
depth(m)
F
-450 -420 -390 -360
D(o/oo)
G
1 5 7
9 11 13 15 17
0 500 1000 1500
dust(ppbv)
H
700 600 500 400 300 200 100 0
Time (kyr BP)
160 200 240 280
pCO2(ppmv)
I
I II
III IV V VI VII VIII
The EPICA challenge(K¨ohler and Fischer, submitted to Nature): Records used to force the BICYCLE model (A-H), measured and simulated pCO2 (I). SST reconstructions (A), IRD (B) and benthicδ18 O from core ODP980 (N Atlantic). D: Planktonicδ18 O of ODP677. E: Stacked benthicδ18O of SPECMAP. F: Changes in the depth of the Pacific lysocline. DeuteriumδD (G) and atmospheric dust contents (H) as measured in the EDC ice core. I: Measured Vostok pCO2 (circles) plotted on the orbitally tuned age scale and simulated pCO2 with (S, red) and without (S-H, black) a shut-down of the THC during Heinrich events.
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Acknowledgements:The EPICA challenge team for the inspiring scientific quest.
Box model of the Isotopic Carbon cYCLE BICYCLE
100 m
1000 m
DEEP SURFACE
MEDIATE INTER−
Rock
carbon
water C3
FS SS
NW W D C4 Atmosphere
Atlantic Indo−Pacific
Sediment
40°N
50°N 40°S 40°S
SO
Biosphere
-7.0 -6.8 -6.6 -6.4 -6.2
13 C[o /oo]
TD 13C Interval I II III IV H1 BA YD
200 220 240 260 280
pCO2[ppmv]
EDC pCO2
-450 -440 -430 -420 -410 -400 -390 -380
D[o /oo]
EDC D
0 10 20 30 40 50
2+ nss-Ca[ppb] 60
EDC nss-Ca2+
400 500 600 700
CH4[ppbv]
GISP2 EDC CH4
-42-41 -40-39 -38-37 -36-35 -34
18 O[o /oo]
GISP2 18O
-120 -100 -80 -60 -40 -20 0
sealevel[m]
sea level
20 18 16 14 12 10 GISP2 Age [kyr BP]
0 5 10 15 20
Flux[106 m3 /s]
SO mixing NADW
-7.0 -6.8 -6.6 -6.4 -6.2
13 C[o /oo]
Interval I II III IV H1 BA YD
-7.0 -6.8 -6.6 -6.4 -6.2
13 C[o /oo]
Interval I II III IV H1 BA YD
180 200 220 240 260 280
pCO2[ppmv]
A-TB0YD A-TB2 A-TB1 A-TB0
20 18 16 14 12 10 GISP2 Age [kyr BP]
0 100 200 300 400 500
14 C[o /oo]
Termination I(K¨ohler et al., submitted to GBC): Top: Forcings of BICYL- CE. Bottom: Simulated and measured atmospheric CO2,δ13 C,∆14 C.
