Refining our estimate of atmospheric CO2 across the Eocene–Oligocene climatic transition

The Eocene–Oligocene transition (EOT) followed by Oligocene isotope event 1 (Oi-1) is a dramatic global switch in climate characterized by deep-sea cooling and the first formation of permanent Antarctic ice. Models and proxy evidence suggest that declining partial pressure of atmospheric carbon dioxide (CO2atm) below a threshold may explain the onset of global cooling and associated ice formation at Oi-1. However, significant uncertainty remains in the estimated values and salient features of reconstructed CO2atm across this interval. In this study, we present novel carbon isotope records from size separated diatom associated organic matter (δ13Cdiatom) preserved in silica frustules. Physical preservation of this material allows concurrent investigation of isotopic and cell size information, providing two input parameters for biogeochemical models and the reconstruction of CO2atm. We estimate CO2atm in two ways; first we use size and reaction–diffusion kinetics of a cell to calculate a CO2atm threshold. Second we use the calibrated relationship between εp(diatom) and carbon dioxide from culture and field studies to create a record of CO2atm prior to and across the transition. Our study, from site 1090 in the Atlantic sector of the Southern Ocean, shows CO2atm values fluctuating between 900 and 1700±100 p.p.m.v. across the EOT followed by a drop to values in the order of 700 to 800±100 p.p.m.v. just prior to the onset of Oi-1. Our values and magnitude of CO2atm change differ from previous estimates, but confirm the overall trends inferred from boron isotopes and alkenones, including a marked rebound following Oi-1. Due to the intricate nature of the climate system and complexities in constraining paleo-proxies, this work emphasizes the importance of a multi-proxy approach to estimating of CO2atm in order to elucidate its role in the emplacement of Antarctic ice-sheets at the EOT. •Diatom-bound organic carbon isotopes used to reconstruct atmospheric CO2 across Eocene/Oligocene transition.•Significant drop in CO2atm to ∼900 p.p.m.v. prior to the onset of Antarctic glaciation (Oi-1).•Rebound to pre-Oi-1 CO2 values (∼1300 p.p.m.v.) following drop.•Supports hypothesis that a CO2atm threshold induces the formation of ice in the Southern hemisphere.•Solidifies values of CO2atm prior to and across major climatic transition.