Warm fjords and vegetated landscapes in early Pliocene East Antarctica

The response of the Antarctic ice sheets to future warming is uncertain. The IPCC are predicting minimal melt from Antarctica while others suggest increased meltwater contributions are possible. The Pliocene period (5.333 Ma to 2.58 Ma) may provide insights into future ice sheet response, because atmospheric CO2 concentrations were similar to today (350-450 ppmv) and the earth surface was between 2 °C and 4 °C warmer than the preindustrial conditions. Geological records indicate that Antarctica's ice sheets were smaller and more dynamic at this time and many sea-level estimates require meltwater input from the Greenland, West (WAIS) and East Antarctic Ice sheets (EAIS). However, only a few records exist proximal to the Antarctic ice sheet which allow for reconstruction of the Pliocene climate state. We present a multiproxy climate reconstruction from a sedimentary succession that was deposited in an ancient fjord within the Transantarctic Mountains, covering discrete intervals between the early Pliocene and the late Pleistocene. In contrast to modern frigid conditions, our records indicate sea surface temperatures of about 5.6 °C at c. 4.1 Ma, the presence of a plant community at the fjord margins and evidence of soil formation. Simulations of potential vegetation cover in the Pliocene indicate our reconstruction is most compatible with a complete collapse of the WAIS and a large scale retreat of the EAIS from the subglacial basins with atmospheric CO2 levels of less than 450 ppmv. Our study indicates that under present day atmospheric CO2 conditions, in the early Pliocene, the Antarctic ice sheets retreated significantly. Understanding the mechanisms driving this large-scale ice sheet retreat would enable us to assess whether current atmospheric CO2 concentrations will lead to the same ice sheet configuration once the Earth system has come to a new equilibrium state.