On the application and interpretation of Keeling plots in paleo climate research ? deciphering ?¹³C of atmospheric CO₂ measured in ice cores

P. Köhler (1) , J. Schmitt (1) , H. Fischer (1)

1 Department of Bentho-pelagic processes

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 (pCO₂) and of ?¹³C of CO₂ 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. Our analysis shows that features seen in pCO₂ and ?¹³C during the industrial period can be interpreted with respect to the Keeling plot. However, only a maximum of approximately half of the signal can be explained by this method. 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. For less abrupt changes as occurring during glacial cycles carbon uptake by the ocean cannot longer be neglected. We introduce an equation for the calculation of the effective isotopic signature of long-term changes in the carbon cycle, in which the ocean is introduced as third reservoir. This is a paleo extention of the two reservoir mass balance equations of the Keeling plot approach. Steady state analyses of changes in the terrestrial and marine biosphere lead to similar effective isotopic signatures (?8.6 per mil) of the carbon fluxes perturbing the atmosphere. These signatures are more positive than the ?¹³C signals of the sources, e.g. the terrestrial carbon pools themselves (~?25 per mil). In all other cases the effective isotopic signatures are larger (?8.2 per mil to ?0.7 per mil), and very often indistinguishable in the light of the uncertainties. Therefore, a back calculation from well distinct fluctuations in pCO₂ and ?¹³C to identify their origin using the Keeling plot approach seems not possible.

Domaines: Cosmologie et astrophysique extra-galactique [astro-ph.CO]; Astrophysique [astro-ph]; Interfaces continentales, environnement; Océan, Atmosphère; Sciences de la Terre

P. Köhler, J. Schmitt, H. Fischer. On the application and interpretation of Keeling plots in paleo climate research ? deciphering ?¹³C of atmospheric CO₂ measured in ice cores. Biogeosciences Discussions, 2006, 3 (3), pp.513-573. ⟨hal-00297805⟩