Miocene evolution of atmospheric carbon dioxide

Changes in pCO₂ or ocean circulation are generally invoked to explain warm early Miocene climates and a rapid East Antarctic ice sheet (EAIS) expansion in the middle Miocene. This study reconstructs late Oligocene to late Miocene pCO₂ from ε(p) values based on carbon isotopic analyses of diunsaturated alkenones and planktonic foraminifera from Deep Sea Drilling Project sites 588 and 608 and Ocean Drilling Program site 730. Our results indicate that highest pCO₂ occurred during the latest Oligocene (~350 ppmv) but decreased rapidly at ~25 Ma. The early and middle Miocene was characterized by low pCO₂ (260-190 ppmv). Lower intervals of pCO₂ correspond to inferred organic carbon burial events and glacial episodes with the lowest concentrations occurring during the middle Miocene. There is no evidence for either high pCO₂ during the late early Miocene climatic optimum or a sharp pCO₂ decrease associated with EAIS growth. Paradoxically, pCO₂ increased following EAIS growth and obtained preindustrial levels by ~10 Ma. Although we emphasize an oceanographic control on Miocene climate, low pCO₂ could have primed the climate system to respond sensitively to changes in heat and vapor transport.