The carbon cycle during the Mid Pleistocene Transition

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Köhler, P. , Hönisch, B. and Fischer, H. (2007): The carbon cycle during the Mid Pleistocene Transition , Geophysical Research Abstracts, Vol. 9, 08846, 2007. European Geosciences Union, 4th General Assembly, 15-20 April 2007, Vienna, Austria.. .
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It was recently hypothesised that the glacial variability as seen in sediment cores duringthe last 2 Myr can be explained solely by the obliquity cycle given by a 40-kyrperiodicity (Huybers, 2007). This hypothesis suggests that the glacial cycles were andare continuously governed by obliquity pacing but that late Pleistocene glaciationsrepeatedly skip one or two obliquity cycling, thus resulting in 80 or 120 kyr (on average100-kyr) periodicity. This would solve both the questions what drove the 100-kyrvariability of the late Pleistocene and how the climate system shifted from the 40-kyrtowards the 100-kyr variability during the Pleistocene Transition (MPT). One possibleexplanation for the observed trend towards longer cycles between glacial terminationsis a gradual long-term decrease in greenhouse gases causing global cooling and theability to sustain larger ice-sheets. This impact on the carbon cycle, however, is difficultto assess, because atmospheric CO2 reconstructions from ice cores are until todayrestricted to the last 800 kyr. We extend informations on the global carbon cycle byrunning the global carbon cycle box model BICYCLE (Köhler et al., 2005; Köhlerand Fischer, 2006) over the last 2 Myr and compare its results with benthic d13Crecords and atmospheric pCO2 calculated from pH reconstructions based on boronisotopes (Hönisch and Hemming, 2005). In both model- and data-based approachesatmospheric pCO2 is indeed higher during glacial periods of the early Pleistocene,thus supporting the hypothesis of Huybers. However, the amplitudes in benthic d13Cincreases by a factor of two over the MPT in the simulations, while it stays constantin the sediment cores. This suggests that the gradual changes in the carbon cycle arenot only driven by increasing glacial/interglacial amplitudes in most climate variables(such as temperature, sea level, etc). The main candidate to alternatively explain thislong-term trend in the carbon cycle is an increase of the riverine input of terrestrialweathering as also suggested by Clark et al. (2006).ReferencesClark et al., 2006 Clark, P. U., Archer, D., Pollard, D., Blum, J. D., Rial, J. A.,Brovkin, V., Mix, A. C., Pisias, N. G., and Roy, M.: The Middle PleistoceneTransition: characteristics, mechanisms, and implications for longtermchanges in atmospheric pCO2, Quaternary Science Reviews, doi:10.1016/j.quascirev.2006.07.008, 2006.Hönisch and Hemming, 2005 Hönisch, B. and Hemming, N. G.: Surface ocean pHresponse to variations in pCO2 through two full glacial cycles, Earth and PlanetaryScience Letters, 236, 305314, doi:10.1016/j.epsl.2005.04.027, 2005.Huybers, 2007 Huybers, P.: Glacial variability over the last two million years:an extended depth-derived agemodel, continuous obliquity pacing, andthe Pleistocene progression, Quaternary Science Reviews, 26, 3755; doi:10.1016/j.quascirev.2006.07.013, 2007.Köhler and Fischer, 2006 Köhler, P. and Fischer, H.: Simulating low frequencychanges in atmospheric CO2 during the last 740 000 years, Climate of the Past,2, 5778; SRefID: 18149332/cp/2006257, 2006.Köhler et al., 2005 Köhler, P., Fischer, H., Munhoven, G., and Zeebe, R. E.:Quantitative interpretation of atmospheric carbon records over the lastglacial termination, Global Biogeochemical Cycles, 19, GB4020, doi:10.1029/2004GB002 345, 2005.

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