On the interpretation of the stable carbon isotope ratio, δ 13 C, during the last 2,000,000 years:
From millennial-scale variability in atmospheric δ 13 CO 2
to the Mid Pleistocene Transition in deep Pacific δ 13 C
Peter K¨ ohler 1 , Richard Bintanja 2 , Jochen Schmitt 1,3 and Hubertus Fischer 1,3
1
: Alfred Wegener Institute for Polar and Marine Research P.O. Box 12 01 61, D-27515 Bremerhaven, Germany
2
: KNMI Royal Netherlands Meteorological Institute, Wilhelminalaan 10, 3732 GK De Bilt, Netherlands
3
: Climate and Environmental Physics, Physics Institute and Oeschger Centre for Climate Change Research
1
: University of Bern, Bern, Switzerland
1
: email: peter.koehler@awi.de, bintanja@knmi.nl, hubertus.fischer@climate.unibe.ch, schmitt@climate.unibe.ch
Abstract
The ratio of the stable carbon isotopes, δ
13C, contains valuable information on the processes which are opera- ting on the global carbon cycle-climate system. It can help to pinpoint, which exchange processes among the different reservoirs of the global carbon cycle significantly alter atmospheric CO
2as δ
13C is recorded in ice cores and benthic organisms buried in the sediments, respec- tively. Here we show with the help of the carbon cycle box model BICYCLE [K¨ ohler et al., 2005; K¨ ohler and Fischer, 2006] how much additional information on carbon cycle and climate dynamics might be extracted from δ
13C and where we find significant limitations. Our time frame of in- terest is spanning from the variability during fast climate fluctuations of the Dansgaard/Oeschger (D/O) events to the rise in the glacial/interglacial amplitudes and the shift in the frequency spectra from 40 kyr to 100 kyr during the Mid Pleistocene Transition (MPT) [K¨ ohler and Bintanja, 2008].
The Model
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Box model of the Isotopic Carbon cYCLE
BICYCLE
Rock C3
FS SS
NW W D C4 Atmosphere
Atlantic Indo−Pacific
Sediment SO
Biosphere mediate
inter−
surface
deep
water carbon
Southern Ocean vertical exchange
(red arrow)is related to SST after MPT, but decoupled from SST before, which we call
Southern Ocean Decoupling Hypothesis.Conclusions
(1) Based on our model convolution of various independently dated climate records there is no 100-kyr cycle in atmosphe- ric δ
13CO
2.
(2) Millennial-scale climate variability leads to fast changes in the terrestrial C cycle. The corresponding δ
13CO
2signal is dilluted quickly through gas exchange with the ocean.
(3) The δ
13CO
2amplitude which is recorded in ice cores de- pends on the gas age distribution in the firn, which dampens the recorded signal (60% at LGM in EPICA Dome C).
(4) We suggest a decoupling of SST in the Southern Ocean from the vertical mixing rates before the Mid Pleistocene Transition (before 1,000,000 years) to find glacial/interglacial amplitudes in δ
13C in the deep Pacific which are in line with reconstruction.
(5) The 400 kyr cycle found in all deep ocean δ
13C recon- structions and its complete lack in δ
18O (and in our simula- tion results) still holds some surprises in the understanding of the carbon cycle-climate interactions.
Millennial-scale variability in atmospheric δ 13 CO 2
-7.0 -6.8 -6.6 -6.4 -6.2 -6.0
13
CO
2(
o/
oo)
CTRL Taylor Dome ice core
A
-0.6 -0.4 -0.2 0.0 0.2 0.4
(
13CO
2) (
o/
oo)
TB Fe fert.
SO mixing NADW Sea ice Sea level
Temp.
B
700 600 500 400 300 200 100 0
Time (kyr BP) -0.6
-0.4 -0.2 0.0 0.2 0.4
(
13CO
2) (
o/
oo)
D/O minus TB+SST TB+SST minus TB+
TB+ minus CTRL
C
Simulated atmospheric
δ13CO2 record over the last 740 kyr (A) does not contain any significant power in the 100 kyr periods (see power spectra below) due to opposing effects of the terrestrial biosphere and the different marine carbon pumps (B) Also: Taylor Dome ice cores data [Smith et al., 1999]. C: No millennial scale variability in CTRL: TB+: Fast changes in terrestrial carbon storage. TB+SST: Scenario TB+ and fast changes in North Atlantic SST. D/O: Scenario TB+SST and fast changes in Atlantic meridional overturning.
101 2 5102 2 5103 Period (kyr) 10-1
2 5 100 2 5 101 2 5 102 2
Spectralpower(-) 99% conf.
CTRL 34 19 15
(100)
0 500 1000 1500 2000 Gas age (yr) 0
1 2 3 4 5 6
Probability(o/oo) E=213yr
E=590yr LGM firn model LGM lognormal PRE firn model PRE lognormal
0 50 100 150
(TBC)(PgC)
A
220 230 240 250 260 270 280 290
pCO2(atm) 5000 yr 1000 yr 0500 yr 0100 yr
B
0 1 2 3 4
Time after start of event (kyr) -7.4
-7.2 -7.0 -6.8 -6.6 -6.4 -6.2 -6.0 -5.8 -5.6
13CO2(o/oo) 590 yr lognormal filter 213 yr lognormal filter original
C
Left top:
Maximum entropy spectral analysis (MESA) of
δ13CO 2 in CTRL .
Left bottom:Gas age distribution as function of climate state, here preindustrial (PRE) and LGM conditions. Calculation by Joos & Spahni [2008], approximated by lognormal functions.
Right:Simulation of terrestrial carbon uptake of 150 PgC (
δ13C = –22h) in 100 yr, followed after 1 kyr by the release of 150 PgC within 100, 500, 1000, 5000 years (A) and effects on atmospheric pCO2 (B) and
δ13CO 2 (C). Thick lines: Original results. Thin and thinnest lines: After filtering with a lognormal function with mean gas age distribution of 213 yr (PRE) and 590 yr (LGM) to mimic amplitude attenuation during gas enclose in the ice.
Mid Pleistocene Transition in deep Pacific δ 13 C
5 4 3
18
O (
o/
oo)
A
180 200 220 240 260 280 300
pCO
2( atm) 180 200 220 240 260 280 300
B
-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2
2.0 1.5 1.0 0.5 0
Time (Myr BP) -1.0
-0.8 -0.6 -0.4 -0.2 0.0 0.2
13
C (
o/
oo)
LR04 SODH
C
100-kyr MPT
40-kyr
(A) LR04 deep ocean benthic
δ18 O stack [Lisiecki and Raymo, 2005], and (B) simulated and measured atmospheric pCO2 and (C) deep Pacific
δ13C over the last 2,000,000 years. Grey: data from ice cores (B) and sediments (C), black:
reconstructed pCO2 based on
δ11 B from planktic foraminifer [H¨onisch et al., 2009]. Scenario LR04: Climate is similarly related to the LR04 benthic
δ18O prior and after the MPT. Scenario SODH: The Southern Ocean Decoupling Hypothesis.
40K MPT 100K 0.0 0.2 0.4 0.6 0.8 1.0 1.2
f
ratio(-)
SODH LR04 data
Left:
Glacial/interglacial amplitudes in deep Pacific
δ13C normalised to the 100k-world (fratio = 1) in data and both simulation scenarios (LR04, SODH).
Below:
As consequence of the Southern Ocean Decoupling Hy- pothesis the relation between Southern Ocean temperature and CO2 breaks up (left). This is also seen in the latest CO2 data set from EPICA Dome C between Antarctic temperature and CO2 (right) [L¨uthi et al., 2008].
-3 -2 -1 0 1 2 3 4 5 SO SST [
oC]
160 180 200 220 240 260 280 300
pCO
2[ atm]
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-3 -2 -1 0 1 2 3 4 5 SO SST [
oC]
160 180 200 220 240 260 280 300
pCO
2[ atm]
40k world
y=164+24x, r2=92%
100k world
y=189+17x, r2=95%
A
-10 -8 -6 -4 -2 0 2 4 6 T [K]
180 200 220 240 260 280 300
CO
2[ppmv]
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-10 -8 -6 -4 -2 0 2 4 6 T [K]
180 200 220 240 260 280 300
CO
2[ppmv]
y = 263+6.77x 050-270 kyr BP
y = 257+8.01x 650-800 kyr BP
References:
