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 [1] and high-latitude [2, 3] cooling of ~10^oC, 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 [4] 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 [6]. 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.

[1] Zachos et al. (2001) Science 292. [2] Bijl et al. (2009) Nature 461. [3] Brassell (2014) Paleoceanography 29. [4] Pearson et al. (2009) Nature 461. [5] Sanyal et al. (1996) Paleoceanography 11. [6] Foster (2008) EPSL 271.