High but not Super High Atmospheric CO2 During the Early Cenozoic
Abstract:The early Cenozoic (~53-33Ma) marks the most recent climatic shift in Earth’s history from a greenhouse to an icehouse world. This interval is characterized by a gradual deep-sea  and high-latitude [2, 3] cooling of ~10oC, and only moderate cooling of the tropics [e.g. 2] leading to the Eocene/Oligocene transition (EOT) marked by widespread continental Antarctic glaciation.
The cause of long-term Eocene cooling is currently poorly known but a gradual decline in the concentration of atmospheric CO2 is most frequently invoked. However, the majority of available early Eocene CO2 records are uncertain and only weakly correlated with climate variability. The exception to that is the final transition into the icehouse  where a decline in the CO2 content of the atmosphere has been suggested as the trigger. Therefore we generated new records of boron isotopes (δ11B) in planktonic foraminifera, a proven proxy of seawater pH [e.g. 5], using multicollector ICPMS . We utilised depth profiles of very well preserved multi-species planktonic foraminifera recovered by the Tanzanian Drilling Project for five time slices spanning 53-37 Ma. Additionlly, we generated approximately 0.8My resolution planktonic foraminifera δ11B records from the Ocean Drilling Program (ODP) Sites 865 and 1258/1260. Our new records show consistent results of elevated atmospheric CO2 in the early Eocene that decreases through to the late Eocene. We will discuss our new reconstructions of seawater pH and derived atmospheric CO2 concentrations, not only in view of diagenesis, but also of estimates of seawater δ11B composition and alkalinity and their significance for Eocene Antarctic glaciation, in light of potential mechanisms for modulating climate.
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