H. Fischer, J. Schmitt & P. K¨ ohler

(1)

Use and abuse of Keeling plots in paleoatmospheric research:

What can we learn from δ ¹³ CO ² in polar ice cores?

H. Fischer, J. Schmitt & P. K¨ ohler

Alfred Wegener Institute for Polar and Marine Research Bremerhaven, Germany Contact: hufischer@awi-bremerhaven.de

The alternation of ice and warm ages is connected to glacial/interglacial CO

2

con- centration changes of approximately 80-100 ppmv with significant fine structure during Termination I (Monnin et al. 2001). Changes in the carbon isotopic signature of CO

2

during that time are expected to add to our understanding what processes were responsible for the observed CO

2

changes. First measurements revealed a glacial/interglacial change in δ

¹³

CO

2

of 0.2-0.3 h (Leuenberger et al. 1992) but signi- ficantly higher variations during the termination (Smith et al. 1999). Using the so called Keeling plot approach (δ

¹³

C = a/CO

2

+ b, where b is taken as representative of the isotopic signature of carbon added or extracted from the atmosphere) it was concluded that the terrestrial biosphere was of major importance for CO

2

changes in the glacial and the Holocene (Smith et al. 1999; Fischer et al. 2003). However, this approach known from terrestrial carbon cycle research represents essentially a carbon isotopic mass balance of a two reservoir system and its application on paleoclimatic CO

2

changes is not straightforward. Here we revisit the Keeling plot approach on paleoclimatic time scales using ice core observations, theoretical consi- derations and modelling results. Based on output of transient model runs from our global carbon cycle model BICYCLE during the last transition (K¨ ohler et al. 2005) we constrain the conclusions to be drawn from ice core δ

¹³

CO

2

data and Keeling plot analyses (K¨ ohler et al. 2006). The effective isotopic signatures of various processes calculated by either the Keeling plot approach or theoretically differ widely from the known δ

¹³

C of the source and are very often indistinguishable in the light of the uncertainties. A back calculation from well distinct fluctuations in pCO

2

and δ

¹³

C to identify their origin using the Keeling plot approach seems not possible.

Extending the Keeling plot approach to a three reservoir system

Two reservoir system A=A0+B and AδA=A0δA

0+BδB⇒δA=A0(δA 0−δB)

A +δB

Three reservoir system A+O=A0+O0+B and AδA+OδO=A0δA

0+O0δO 0+BδB A0=600 PgC and O0=38,000 PgCδA

0 =−6.5h,δO

0 = +1.5hδB=−25h εAO≈δA

0−δO

0 ≈δA−δO≈ −8h

Revelle or buffer factorβ=f(temperature, alkalinity, DIC):β:=

¡

d[CO2]/[CO2]

dDIC/DIC

¢

. βin recent surface waters varies between 8 and 16 (Sabine et al. 2004). Preindustrial:β(surface ocean boxes): 11.5, with 9 in equatorial waters and 12 in the high latitudes.

Finally:δ∆A=

A0+O0+B−O A0+O0+B (A0δA

0+O0δO

0+BδB+εAOO)−A0δA 0

O0+B−O .

0 200 400 600 800 1000 B [PgC]

-10.0 -9.8 -9.6 -9.4 -9.2 -9.0

A

[

o

/

oo

]

AO= -8^o/_oo, = 11.5, A = 600 PgC

A

-12 -10 -8 -6 -4 -2 0

AO

[

^o

/

oo

] 1

3 5 7 9 11 13 15 17 19

[- ]

A[^o/_oo] with B = 10 PgC, A = 600 PgC

-30

-30 -25-25

-20

-20 -15-15

-14 -14

-13 -13 -12

-12

-11 -11 -10

-10

-9 -9 -8 -8

-9.75 -9.75 -9.5

-9.5

-9.25 -9.25 -9

-9

-8.75 -8.75 -8.5

-8.5

-8.25 -8.25 -8

-8

-7.75 -7.75

B

Results of the three reservoirs approach. Effective isotopic signature of the atmosphereδ∆Aas function of (A) the size of the terrestrial release and (B) the Revelle Factorβand the fractionation during gas exchangeεAO. The cross in B marks the preindustrial state (β=11.5,εAO =−8.0h).

Box model of the Isotopic Carbon cYCLE BICYCLE

100 m

1000 m

DEEP SURFACE

MEDIATEINTER−

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

16

22 5

10 1 4 16

9 6

3 30

9 16

20 1

18 19

15

6

9 9

16

12 5 2 1

3

200 250 300 350

C O

2

[p p m v ]

13C CO₂

A

TD

30 1

Time [kyr BP]

LD 1000 2000

Time [yr AD]

2000 PB 1980 2000

Time [yr AD]

-8.5 -8.0 -7.5 -7.0 -6.5 -6.0

13

C [

o

/

oo

]

0.003 0.004 0.005 1/CO

₂

[1/ppmv]

-8.5 -8.0 -7.5 -7.0 -6.5

-6.0 300 250 200

CO

2

[ppmv]

...

. . . ... . . ..

. . ...

. ..

... ..

. . . ..

.... . .. . .

. .. . ... .

. ..

.. . ...

. ..

. ...

. . .... . ... . . ..

... . ..

. ..

... ..

. . ..

... . . . . ..

...

. ..

... ..

. ..

... . ...

..

. ..

...

. . . .. . ...

. . . . ...

. ..

...

. ..

.. . ...

. ..

... ..

. . . . .

...

. . . ... . . ..

. . ...

. ..

... ..

. . . ..

... . .. . .

. .. . ... . ..

.. . ...

. ..

... ..

... . . . . .... . ... . . ..

.. . ... . . ..

... ..

. ..

. . ...

. . . ..

...

. ..

... ..

. ..

... . ..

. ..

...

. . . .. . ...

. . . . ... . . ..

. . ...

. ..

.. . ...

. ..

... . .. . . . . .

. .

Point Barrow, detrended

.

Point Barrow Law Dome Taylor Dome LGM Taylor Dome GIG Taylor Dome HOL

TD HOL: y0= -9.5^o/oo, r²= 40%

TD GIG: y0= -5.8^o/oo, r²= 34%

TD LGM: y0= -9.5^o/oo, r²= 48%

LD ANT: y0= -13.1^o/oo, r²= 96%

PB ORG: y0= -16.7^o/oo, r²= 68%

PB DET: y0= -25.3^o/oo, r²= 96%

B

A: Data sets of measured CO2 andδ13C. B: Keeling plot. Point Barrow monthly resolved (1982−2002) (Keeling & Whorf 2005; Keeling et al. 2005); original data (PB ORG); detrended (PB DET). Law Dome (1 kyr) (Francey et al. 1999; Trudinger et al. 1999) (LD ANT). Taylor Dome (30 kyr) (Smith et al. 1999); Holocene (TD HOL), glacial/interglacial transition (TD GIG), LGM (TD LGM).

Summary of y-axis intercept y0 of the steady state Keeling plot analysis for processes changing over Termination I.

Process y0 (h) Comment

Linear rise in terrestrial carbon storage −8.6 increase non-linear, steepest slope−25h Decrease in marine export production −8.6 steeper slope during first 50 yr (y0=−9.7h) Rise in NADW formation −7.8 varies with time; mixture with changes in marine export

production during Heinrich 1 event; during Younger Dryas and resumption in the Holocene y0=−7.15±

0.05h, steep slope during first 50 yr (y0=−9.5h) Rise in Southern Ocean vertical mixing −8.2 steep slope during first 50 yr (y0=−11.0h) Decline in sea ice cover −0.7 regression over whole data set:−3.8h; different in

North (−4.8h) and South (−77.2h)

Rise in sea level −6.4

Rise in temperature −3.6

Sediment/ocean interaction −5.8

266 268 270 272 274

p C O

2

[ a tm ]

13C pCO₂

A

-2 0 2 4 6 8

Time [kyr]

-7.0 -6.9 -6.8 -6.7 -6.6 -6.5

13

C [

o

/

oo

]

0.0037 0.00375 1/pCO

₂

[1/ atm]

-7.0 -6.9 -6.8 -6.7 -6.6

-6.5 270 260

pCO

2

[ atm]

05 PgC, -23ô/_oo 10 PgC, -33ô/oo 10 PgC, -13ô/_oo 10 PgC, -23ô/_oo y₀= -23.8ô/_oo y₀= -8.4ô/_oo y₀= -18.7ô/_oo; r²=68%

year 1

year 0

B

250 300 350 400 450 500

p C O

2

[ a tm ]

13C pCO₂

A

-2 0 2 4 6 8

Time [kyr]

-7.6 -7.4 -7.2 -7.0 -6.8 -6.6

13

C (a tm ) [

o

/

oo

]

0.002 0.003 0.004

1/pCO

₂

[1/ atm]

-7.6 -7.4 -7.2 -7.0 -6.8 -6.6

500 400 300

pCO

₂

[ atm]

y0= -8.8^o/oo y0= -10.2^o/oo

y0= -8.0^o/oo r²= 93%

year 0

B

year 100 year 8000

20 18 16 14 12 10 Time [kyr BP]

160 180 200 220 240 260

p C O

2

[ a tm ]

A

20 18 16 14 12 10 Time [kyr BP]

-7.0 -6.8 -6.6 -6.4 -6.2

13C pCO₂

0.004 0.005 0.006 1/pCO

2

[1/ atm]

-7.0 -6.8 -6.6 -6.4 -6.2

13

C [

o

/

oo

]

270 230 190 170 pCO

2

[ atm]

1: y0=-07.3^o/oo, r²= 65%

2: y0=-10.0^o/oo, r²=100%

3: y0=-05.2^o/oo, r²= 97%

4: y0=-11.4^o/oo, r²= 83%

5: y0=-13.3^o/oo, r²= 90%

6: y0=-13.0^o/oo, r²= 76%

7: y0=-06.9^o/oo, r²= 06%

B

Examples for Keeling plots out of simulation results: Top: Fast terrestrial carbon release. Middle: Switching from abiotic to biotic ocean. Different regression models in Top and Middle: first year only in green; prior/after (steady states) in black; equilibration time in magenta. Bottom: Identifying events with differentδ13 C signal during Termination I.

References:

Francey, et al. 1999. Tellus, 51B:170–193.Fischer, et al. 2003. Memoirs National Institute Polar Research, 57:121–

138.Keeling, et al. 2005. Oak Ridge National Laboratory, U.S. Department of Energy,, Oak Ridge, Tenn., USA.

Keeling, & Whorf 2005. Oak Ridge National Laboratory, U.S. Department of Energy,, Oak Ridge, Tenn., USA.

K¨ohler et al. 2005. GBC, 19:GB4020, doi: 10.1029/2004GB002345.K¨ohler et al. 2006. Biogeosciences Discussions, in press.Leuenberger et al. 1992. Nature, 357:488–490.Monnin et al. 2001. Science, 291:112–114.Sabine et al.

2004. Science, 305:367–371.Smith et al. 1999. Nature, 400:248–250.Trudinger et al. 1999. Tellus, 51B:233–248.

H. Fischer, J. Schmitt & P. K¨ ohler

Use and abuse of Keeling plots in paleoatmospheric research:

What can we learn from δ 13 CO 2 in polar ice cores?