Christophe Dumas

Modeling the evolution of Antarctic ice sheet over the last 420,000 years: Implications for altitude changes in the Vostok region

A new thermomechanical three-dimensional model designed to simulate the evolution of the Antarctic ice sheet over long time periods is presented. This model incorporates the various types of ice flow found in Antarctica: relatively slow inland ice flow that is essentially due to ice deformation, fast ice flow in the regions with ice streams, and ice shelf flow. By coupling these three types of flow, it is possible to predict grounding line migration. Simulations covering four glacial-interglacial cycles have been conducted by forcing this model with a temperature record from Vostok and a sea level record from marine cores. Our findings indicate that grounding line migration induced by sea level changes is the primary factor governing the evolution of the Antarctic ice volume. On the other hand, the altitude of the ice sheet surface at Vostok is driven by accumulation rate variations. The amplitude of the altitude change does not exceed 150 m and is very similar for all the sites located on the Antarctic Plateau.

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 CO 2 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 δ 18 O excursions occurring during the transition. We show that CO 2 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 CO 2 drop. The two δ 18 O steps record both ice growth and temperature, representing some 10-30 m eustatic sea level fall and 2-4°C cooling at EOT-1 and 70 ± 20 m and 0-2°C for Oi-1. The simulated magnitude of the ice steps (10 m for EOT-1 and 63 m 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.