Abstract

The global cooling from the early Cenozoic ice-free world (~50 Ma) to today’s bipolar icehouse world has long been ascribed to declining levels CO2 levels (Berner et al., 1983, Raymo et al., 1988). This deterioration in climate is recorded by a ~4 ‰ increase in deepwater benthic foraminiferal d18O, reflecting both a ~12 °C cooling, and the formation of Earth’s continental ice sheets (Lear et al., 2000). Roughly half of this signal occurs between the Miocene Climatic Optimum (~16 Ma; MCO ) and today, including a ~1 ‰ step marking the establishment of a permanent East Antarctic ice sheet during the Middle Miocene Climate Transition (MMCT) at ~14 Ma (Zachos et al., 2001). Although most pCO2 reconstructions show an overall decline associated with early Cenozoic cooling, estimates of pCO2 during the Miocene Climatic Optimum are either higher (stomatal index; Kürshner et al. 2008), lower (boron isotopes in foraminifera; Pearson and Palmer, 2000), or around the same (d13C of alkenones; Pagani et al., 1999) as pre-industrial levels, leading to suggestions that either existing proxy records are flawed (Ruddiman, 2010), or climate and pCO2 have been decoupled for some portions of Earth’s history (Kerr, 1999; Pagani et al., 1999). Here I use new boron isotope measurements in planktonic foraminifera to show that the Miocene Climatic Optimum was associated with elevated pCO2 and that drawdown from this maximum coincided with orbitally-paced organic carbon burial during subsequent climatic cooling. Contrary to previous studies (e.g. Pagani et al., 1999) our new boron based pCO2 record therefore reaffirms the link between CO2 , climate and the cyrosphere for this important part of the Cenozoic.