Climate of the Past Discussions www.clim-past-discuss.net 1814-9340 1814-9359 4 4 2008 10.5194/cpd-4-809-2008 http://www.clim-past-discuss.net/4/809/2008/ http://www.clim-past-discuss.net/4/809/2008/cpd-4-809-2008.html http://www.clim-past-discuss.net/4/809/2008/cpd-4-809-2008.pdf 809 858 2008-07-09 The carbon cycle during the Mid Pleistocene Transition: the Southern Ocean Decoupling Hypothesis P. Köhler R. Bintanja Alfred Wegener Institute for Polar and Marine Research, PO Box 120161, 27515 Bremerhaven, Germany KNMI (Royal Netherlands Meteorological Institute), Wilhelminalaan 10, 3732 GK De Bilt, Netherlands Various hypotheses were proposed within recent years for the interpretation of the Mid Pleistocene Transition (MPT), which occurred during past 2 000 000 years (2 Myr). We here add to already existing theories on the MPT some data and model-based aspects focusing on the dynamics of the carbon cycle. We find that the average glacial/interglacial (G/IG) amplitudes in benthic δ¹³C derived from sediment cores in the deep Pacific ocean increased across the MPT by ~40%, while similar amplitudes in the global benthic δ¹⁸O stack LR04 increased by a factor of two over the same time interval. The global carbon cycle box model BICYCLE is used for the interpretation of these observed changes in the carbon cycle. Our simulation approach is based on regression analyses of various paleo-climatic proxies with the LR04 benthic δ¹⁸O stack over the last 740 kyr, which are then used to extrapolate changing climatic boundary conditions over the whole 2 Myr time window. The observed dynamics in benthic δ¹³C cannot be explained if similar relations between LR04 and the individual climate variables are assumed prior and after the MPT. According to our analysis a model-based reconstruction of G/IG amplitudes in deep Pacific δ¹³C before the MPT is possible if we assume a different response to the applied forcings in the Southern Ocean prior and after the MPT. This behaviour is what we call the "Southern Ocean Decoupling Hypothesis". This decoupling might potentially be caused by a different cryosphere/ocean interaction and thus changes in the deep and bottom water formation rates in the Southern Ocean before the MPT, however an understanding from first principles remains elusive. Our hypothesis is also proposing dynamics in atmospheric <i>p</i>CO₂ over the past 2 Myr. Simulated <i>p</i>CO₂ is varying between 180 and 260 <i>μ</i>atm before the MPT. 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