Climate of the Past Discussions www.clim-past-discuss.net 1814-9340 1814-9359 4 4 2008 10.5194/cpd-4-859-2008 http://www.clim-past-discuss.net/4/859/2008/ http://www.clim-past-discuss.net/4/859/2008/cpd-4-859-2008.html http://www.clim-past-discuss.net/4/859/2008/cpd-4-859-2008.pdf 859 895 2008-08-12 Constraining atmospheric CO₂ content during the Middle Miocene Antarctic glaciation using an ice sheet-climate model P. M. Langebroek petra@palmod.uni-bremen.de A. Paul M. Schulz Faculty of Geosciences, University of Bremen, Bremen, Germany MARUM – Center for Marine Environmental Sciences, Univ. of Bremen, Bremen, Germany Foraminiferal oxygen isotopes from deep-sea sediment cores suggest that a rapid expansion of the Antarctic ice sheet took place in the Middle Miocene around 13.9 million years ago (Ma). The origin for this transition is still not understood satisfactorily. Among the proposed causes are a drop in the partial pressure of atmospheric carbon dioxide (<i>p</i>CO₂) in combination with orbital forcing. An additional complication is the large uncertainty in the magnitude and age of the reconstructed <i>p</i>CO₂ values and the low temporal resolution of the available record in the Middle Miocene. We used an ice sheet-climate model with an energy and mass balance module to assess variations in ice-sheet volume induced by <i>p</i>CO₂ and insolation forcing and to better constrain atmospheric CO₂ in the Middle Miocene. The ice-sheet sensitivity to atmospheric CO₂ was tested in several scenarios using constant <i>p</i>CO₂ forcing or a regular decrease in <i>p</i>CO₂. Small, ephemeral ice sheets existed under relatively high atmospheric CO₂ conditions (between 400–450 ppm), whereas more stable, large ice sheets occurred when <i>p</i>CO₂ is less than 400 ppm. 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