A model-based interpretation of low frequency changes in the carbon cycle during the last 120,000 years and its implications for the reconstruction of atmospheric ∆ 14 C

A model-based interpretation of low frequency changes in the carbon cycle during the last 120,000 years and its implications for the reconstruction of atmospheric ∆ ¹⁴ C

— AGU2006 Poster ID: PP33A-1781

Peter K¨ ohler

¹

, Raimund Muscheler

²

& Hubertus Fischer

¹

1: Alfred Wegener Institute for Polar & Marine Research Bremerhaven, Germany (Peter.Koehler@awi.de)

2: NASA/Goddard Space Flight Center, Climate and Radiation Branch, Greenbelt, MD, USA (raimund@climate.gsfc.nasa.gov)

A main caveat in the interpretation of observed changes in atmospheric∆¹⁴C du- ring the last 50,000 years is the unknown variability of the carbon cycle, which together with changes in the¹⁴C production rates determines the¹⁴C dynamics.

A plausible scenario explaining glacial/interglacial dynamics seen in atmospheric CO2andδ¹³C was proposed recently (K¨ohler et al. 2005). A similar approach and expanding its interpretation to the¹⁴C cycle is an important step towards a deeper understanding of∆¹⁴C variability (K¨ohler et al. 2006). This approach is based on an ocean/atmosphere/biosphere box model of the global carbon cycle (BICYCLE) to reproduce low frequency changes in atmospheric CO2as seen in Antarctic ice cores. The model is forced forward in time by various paleo-climatic records deri- ved from ice and sediment cores. The simulation results of our proposed scenario match a compiled CO2record from various ice cores during the last 120,000 years with high accuracy (r²= 0.89). We analyze scenarios with different¹⁴C produc- tion rates, which are either constant, based on¹⁰Be measured in Greenland ice cores, or the recent high-resolution geomagnetic field reconstruction GLOPIS-75 and compare them with the available∆¹⁴C data covering the last 50,000 years.

Our results suggest that during the last glacial cycle in general less than 110h of the increased atmospheric∆¹⁴C are based on variations in the carbon cycle, while the largest part (5/6) of the variations has to be explained by other factors.

Glacial atmospheric∆¹⁴C larger than 700hcannot not be explained within our framework, neither through carbon cycle-based changes nor through variable¹⁴C production. Superimposed on these general trends might lie positive anomalies in atmospheric∆¹⁴C of∼50hcaused by millennial-scale variability of the northern deep water production during Heinrich events and Dansgaard/Oeschger climate fluctuations. According to our model the dominant processes that increase glacial

∆¹⁴C are a reduced glacial ocean circulation (+∼40h), a restricted glacial gas exchange between the atmosphere and the surface ocean through sea ice coverage (+∼20h), and the enrichment of DIC with¹⁴C in the surface waters through isotopic fractionation during higher glacial marine export production caused by iron fertilization (+∼10h).

Keywords:carbon cycle,¹⁴C cycle,¹⁴C production rates, glacial/interglacial, mo- deling, box model, radionuclides

⇐Forcing ⇓Simulation Results

0.0 0.5 1.0 1.5 2.0 2.5

18O(o/oo)

A

-120 -100 -80 -60 -40 -20 0

sealevel(m))

B

-400 -200 0 200 400

depth(m)

C

-15 -10 -5 0

T(K)

D

-45 -40 -35

18O(o/oo)

E

-450 -420 -390 -360

D(o/oo)

F

-1500 -1000 -500 0

dust(ppbv)

G

120 100 80 60 40 20 0

Time (kyr BP) 160

200 240 280

CO2(ppmv)

H

120 100 80 60 40 20 0

Time (kyr BP) 160

200 240 280

CO2(ppmv)

10pt running mean simulation

-60 -40 -20 0 20 40

(CO2)[ppmv]

overall CaCO3chemistry Terrestrial biosphere Fe fertilisation SO mixing NADW Sea ice Sea level Temperature

A

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CO2[ppmv]

B

-40 -20 0 20 40

(14C)[o/oo] C

120 100 80 60 40 20 0

Time [kyr BP]

-20 0 20 40 60

14C[o/oo]

D

Left:Different data sets forcing the model (A: equatorial SST proxy, B: sea level; C: lysocline; D: northern hemisphere temperature; E: North Atlantic SST proxy; F: Southern Ocean SST proxy; G: dust input in Southern Ocean; H: Data and simulation results for pCO2.Right:Simulation results. A: Process contribution to∆(pCO2); B: pCO2; C:

Process contribution to∆(∆14C); D:∆14C with constant 14C production rate.

Model —∆¹⁴C data — Results

000000 000000 000000 111111 111111 111111

00000000 00000000 00000000 11111111 11111111 11111111 000000 000000 111111 111111

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Box model of the Isotopic Carbon cYCLE BICYCLE

1000 m

Rock

carbon

water C3

FS SS

NW W D C4

Atmosphere

Atlantic Indo−Pacific

Sediment SO

Biosphere

16

22 5

101 4 16

9 6

3 30

9 16

20 1

18 19

15

6

9 9

16

12 5 2 1

3 40°N 40°S

40°S 50°N

100 m

-200 0 200 400 600 800 1000 1200 1400 1600

0 10000 20000

30000 40000

50000 14C [‰]

Age [years BP]

Voelker et al., 1998 Bard et al., 1998 Schramm et al., 2000 Kitagawa & van der Pflicht, 2000 Goslar et al., 2000 Hughen et al, 2000, 2004 Beck et al., 2001 Reimer et al., 2004 Fairbanks et al., 2005

IntCal04

0 1 2 3 4

Time [kyr]

-20 0 20 40 60 80 100 120

(14C)[o/oo]

0 1 2 3 4

Time [kyr]

IG THC, 2 modern

30 29 28 27 26 25 Time [kyr BP]

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30 29 28 27 26 25 Time [kyr BP]

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(CO2)[ppmv]

transient, 2 modern -20

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(14C)[o/oo]

IG THC, modern

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(CO2)[ppmv]

transient, modern (CO2)

(¹⁴C) Top:Data based reconstruction of∆14C.

References:

Bard et al. 1998. Radiocarbon, 40:1085–1092.Beck et al. 2001. Science, 292:2453–2458. Fairbanks et al. 2005. Quaternary Science Reviews, 24:1781–1796.

Goslar et al. 2000. Nature, 403:877–880.Hughen et al. 2000. Science, 290:1951–1954.Hughen et al. 2004.

Science, 303:202–207.Kitagawa & van der Pflicht 2000.

Radiocarbon, 42:369–380.K¨ohler et al. 2005. GBC, 19:GB4020, doi: 10.1029/2004GB002345.K¨ohler et al.

2006. GGG, 7, Q11N06, doi: 10.1029/2005GC001228.

Laj et al. 2005. Geophs. Monog. Series, 145:255–265.

Muscheler et al. 2005. QSR, 24:1849–1860.Reimer et al. 2004. Radiocarbon, 46:1029–1058. Schramm et al. 2000. EPSL, 175:27–40.Voelker et al. 1998.

Radiocarbon, 40:517–534.

Top:Shutdown of the NADW from interglacial (IG) THC or transient experiments (30–25 kyr BP) with constant14C production rates (mo- dern or 2×modern level), length of shutdown:

500, 1000, 1500, 2000 years.

Variable¹⁴C production rates

0.0 0.5 1.0 1.5 2.0 2.5

70 60 50 40 30 20 10 0 Time [kyr BP]

0.0 0.5 1.0 1.5 2.0 2.5

relative14 Cproductionrate[-]

S4: f(GLOPIS-75) S3: f(¹⁰Be)

S2: constant at 2 modern level S1: constant at modern level

10Be from Muscheler et al. (2005) and GLOPIS from Laj et al. (2005).

yellow/blue: 1/3000 yr−1cutoff frequency.

Simulated∆¹⁴C based on variable¹⁴C production rates

-100 0 100 200 300 400 500 600 700 800 900

50 40 30 20 10 0

Time [kyr BP]

-100 0 100 200 300 400 500 600 700 800 900

14 C[o /oo]

S4x: f(GLOPIS-75) S3x: f(¹⁰Be) S1: constant (modern)

INTCAL04 Reimer et al (2004)

The content of this poster is published as K¨ohler et al. (2006) in GGG.

(Bard et al. 1998; Voelker et al. 1998; Goslar et al. 2000; Hughen et al. 2000; Hughen et al. 2004; Kitagawa & van der Pflicht 2000; Schramm et al. 2000; Beck et al. 2001;

Reimer et al. 2004; Fairbanks et al. 2005; K¨ohler et al. 2005; K¨ohler et al. 2006; Muscheler et al. 2005; Laj et al. 2005)

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