Miocene evolution of atmospheric carbon dioxide

Changes in pCO(2) or ocean circulation are generally invoked to explain war m early Miocene climates and a rapid East Antarctic ice sheet (EAIS) expans ion in the middle Miocene. This study reconstructs late Oligocene to late M iocene pCO(2) from epsilon(p) values based on carbon isotopic analyses of d iunsaturated alkenones and planktonic foraminifera from Deep Sea Drilling P roject sites 588 and 608 and Ocean Drilling Program site 730. Our results i ndicate that highest pCO(2) occurred during the latest Oligocene (similar t o 350 ppmv) but decreased rapidly at similar to 25 Ma. The early and middle Miocene was characterized by low pCO(2) (260-190 ppmv), Lower intervals of pCO(2) correspond to inferred organic carbon burial events and glacial epi sodes with the lowest concentrations occurring during the middle Miocene, T here is no evidence for either high pCO(2) during the late early Miocene cl imatic optimum or a sharp pCO(2) decrease associated with EAIS growth. Para doxically, pCO(2) increased following EAIS growth and obtained preindustria l levels by similar to 10 Ma. Although we emphasize an oceanographic contro l on Miocene climate, low pCO(2) could have primed the climate system to re spond sensitively to changes in heat and vapor transport.