The respective role of atmospheric carbon dioxide and orbital parameters on ice sheet evolution at the Eocene-Oligocene transition

The continental scale initiation of the Antarctic ice sheet at the Eocene-Oligocene boundary (Eocene-Oligocene transition (EOT), 34 Ma) is associated with a global reorganization of the climate. If data studies have assessed the precise timing and magnitudes of the ice steps, modeling studies have been unable to reproduce a transient ice evolution during the Eocene-Oligocene transition in agreement with the data. Here we simulate this transition using general circulation models coupled to an ice sheet model. Our simulations reveal a threshold for continental scale glaciation of 900 ppm, 100 to 150 ppm higher than previous studies. This result supports the existence of ephemeral ice sheets during the middle Eocene, as similar CO2 levels (900-1000 ppm) have been reached episodically during this period. Transient runs show that the ice growth is accurately timed with EOT-1 and Oi-1, the two delta O-18 excursions occurring during the transition. We show that CO2 and orbital variations are crucial in initiating these steps, with EOT-1 corresponding to the occurrence of low summer insolation, whereas Oi-1 is controlled by a major CO2 drop. The two delta O-18 steps record both ice growth and temperature, representing some 10-30m eustatic sea level fall and 2-4 degrees C cooling at EOT-1 and 70 +/- 20m and 0-2 degrees C for Oi-1. The simulated magnitude of the ice steps (10m for EOT-1 and 63m for Oi-1) and the overall cooling at various locations show a good agreement with the data, which supports our results concerning this critical transition.