.The carbon cycle during the Mid Pleistocene Transition: the Southern Ocean Decoupling Hypothesis

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Köhler, P. , Fischer, H. , Hönisch, B. and Bintanja, R. (2008): .The carbon cycle during the Mid Pleistocene Transition: the Southern Ocean Decoupling Hypothesis , Climate change: from the geologic past to the uncertain future: A tribute to André Berger - 26 to 29 May 2008 - Louvain-la-Neuve University - Belgium. .
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We here use the global carbon cycle box model BICYCLE to investigate changes in thecarbon cycle during the Mid Pleistocene Transition (MPT) covering the past 2,000,000years (2 Myr). While there exist so far no ice core record of atmospheric CO2 beyond800,000 years our simulated atmospheric carbon dioxide partial pressure across theMPT can only be compared with pCO2 calculated from new pH reconstructions based onboron isotopes measured in planktic foraminifer shells. We validate our results furtherby a comparison of simulated d13C with paleo reconstructions of benthic d13C in thedeep Pacific Ocean. Our approach is based on regression analyses of variouspaleoclimatic proxies with the LR04 benthic d18O stack, which are then used toextrapolate changing climatic boundary conditions over the whole 2 Myr time window.The focus of our investigation is on the changes in the glacial/interglacial (G/IG)amplitudes in climate (represented by LR04) and the carbon cycle (represented bybenthic d13C) across the MPT. We find that the G/IG amplitudes in LR04 increased by afactor of two across the MPT, those of benthic d13C measured in cores in the deep Pacificonly by 40%. According to our model this difference in the dynamic of the climate systemand the carbon cycle can be explained if we assume a different response to the appliedforcings in the Southern Ocean prior and after the MPT. This behaviour is what we callthe "Southern Ocean Decoupling Hypothesis" of the climate and carbon dynamics. Wediscuss how our ?ndings are related to the various hypotheses on the causes of the MPTpublished within the last years. This study is a temporal extension of the "EPICAchallenge", which tried to estimate variations in atmospheric CO2 from published paleoreconstructions before new ice core measurements of CO2 were made public. We ?nallyhypothesise that as consequence of our analysis the close relationship between Antarctictemperature and atmospheric CO2 found in ice cores breaks down prior to the MPT.

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