Biogeosciences
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2006
10.5194/bg-3-539-2006
http://www.biogeosciences.net/3/539/2006/
http://www.biogeosciences.net/3/539/2006/bg-3-539-2006.html
http://www.biogeosciences.net/3/539/2006/bg-3-539-2006.pdf
539
556
2006-11-15
On the application and interpretation of Keeling plots in paleo climate research – deciphering δ<sup>13</sup>C of atmospheric CO<sub>2</sub> measured in ice cores
P. Köhler
pkoehler@awi-bremerhaven.de
H. Fischer
J. Schmitt
G. Munhoven
Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research, P.O. Box 12 01 61, 27515 Bremerhaven, Germany
Laboratoire de Physique Atmosphérique et Planétaire, Institut d'Astrophysique et de Géophysique, Université de Liège, 17 avenue du Six-Ao{\^u}t, B–4000 Liège, Belgium
The Keeling plot analysis is an interpretation method widely used in
terrestrial carbon cycle research to quantify exchange processes of carbon
between terrestrial reservoirs and the atmosphere. Here, we analyse measured
data sets and artificial time series of the partial pressure of atmospheric
carbon dioxide (<i>p</i>CO<sub>2</sub>) and of δ<sup>13</sup>C of CO<sub>2</sub> over industrial and
glacial/interglacial time scales and investigate to what extent the Keeling
plot methodology can be applied to longer time scales. The artificial time
series are simulation results of the global carbon cycle box model BICYCLE.
The signals recorded in ice cores caused by abrupt terrestrial carbon uptake
or release loose information due to air mixing in the firn before bubble
enclosure and limited sampling frequency. Carbon uptake by the ocean cannot
longer be neglected for less abrupt changes as occurring during glacial cycles. We
introduce an equation for the calculation of long-term changes in the
isotopic signature of atmospheric CO<sub>2</sub> caused by an injection of terrestrial carbon to the atmosphere,
in which the
ocean is introduced as third reservoir. This is a paleo extension of the two
reservoir mass balance equations of the Keeling plot approach.
It gives an explanation for the bias between the isotopic signature of the terrestrial
release and the signature deduced with the Keeling plot approach for long-term processes, in which the oceanic reservoir cannot be neglected.
These deduced isotopic signatures are similar (−8.6‰) for steady state
analyses of long-term changes in the terrestrial and marine biosphere which both perturb the atmospheric carbon reservoir.
They are more positive than the δ<sup>13</sup>C signals of
the sources, e.g. the terrestrial carbon pools themselves (−25‰).
A distinction of specific processes acting on the global carbon cycle from the Keeling
plot approach is not straightforward. In general, processes related to biogenic fixation
or release of carbon have lower y-intercepts in the Keeling plot than changes in
physical processes, however in many case they are indistinguishable (e.g. ocean circulation from biogenic carbon fixation).
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