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Titel A process-based understanding of the late Cenozoic carbon cycle
VerfasserIn P. Köhler, R. S. W. van de Wal, B. de Boer, L. J. Lourens, R. Bintanja, T. Bickert, G. Lohmann
Konferenz EGU General Assembly 2012
Medientyp Artikel
Sprache Englisch
Digitales Dokument PDF
Erschienen In: GRA - Volume 14 (2012)
Datensatznummer 250062083
 
Zusammenfassung
On a million-year time scale the global carbon cycle and atmospheric CO2 are assumed to be largely determined by the so-called solid Earth processes weathering, sedimentation, and volcanic outgassing. However, it is not clear how much of the observed dynamics in the proxy data constraining the carbon cycle over the Cenozoic might be determined by internal processes of the atmosphere-ocean-biosphere subsystem. Here, we apply for the first time a process-based model of the global carbon cycle in transient simulations over the last 20 Myr to identify the contributions of terrestrial carbon storage, solubility pump and ocean gateways on changes in atmospheric CO2 and marine δ13C. We apply the isotopic carbon cycle box model BICYCLE, which consists of atmosphere, terrestrial biosphere and ocean reservoirs, the latter containing the full marine carbonate system. Our simulation results show that the long-term cooling since the Mid Miocene Climatic Optimum (about 15 Myr BP) leads to an intensification of the solubility pump, and a drop in atmospheric CO2 of up to 100 ppmv. This oceanic carbon uptake is largely counterbalanced by carbon loss from the terrestrial biosphere. The reduction in terrestrial C storage over time including the expansion of C4 grasses during the last 8 Myr might explain half of the long-term decline in deep ocean δ13C and would support high CO2 (400 to 450 ppmv) around 15 Myr BP. The closure of the Tethys and the Central America ocean gateways explains the developing gradient in deep ocean δ13C between the Atlantic and Pacific basin. We furthermore calculate the residuals, which are unexplained by our results and are therefore caused by solid Earth processes. From the residuals a rise in both ocean alkalinity and dissolved inorganic carbon over time is detected as reasons for declining atmospheric CO2 which led to Earth’s long-term cooling observed since the Mid Miocene Climate Optimum. Increased continental weathering in combination with changes in volcanic out-gassing of CO2 might explain these changes in marine carbonate chemistry. Around 16 Myr BP we find a prominent regime shift in the carbon cycle-climate system at which the gradient in both deep ocean δ13C and temperature significantly declines. This might be connected with a shrinking seafloor spreading rates which might have caused reduced volcanic activity and thus less CO2 outgassing. The existence of such a regime shift is confirmed if we extend our analysis to deep ocean records of δ18O and δ13C over the whole Cenozoic.