Age, thinning and spatial origin of the Beyond EPICA ice from a 2.5D ice flow model

Abstract. The European Beyond EPICA – Oldest Ice consortium is currently conducting an ice core drilling project at Little Dome C (LDC) in Antarctica with the aim of retrieving a continuous ice core up to 1.5 Ma. In order to determine the age of the ice at a given depth, 1D numerical models are often employed. However, they do not take into account any effects due to horizontal flow. We present a 2.5D inverse model that determines the age–depth profile along a flow line from Dome C (DC) to LDC that is assumed to be stable in time. The model is constrained by dated radar internal reflecting horizons. Surface velocity measurements are used to determine the flow line and ascertain the flow tube width, which also allows the model to consider lateral divergence. This new model therefore improves on the results produced by 1D models previously applied to the DC area. By inferring a mechanical ice thickness, the model predicts either the thickness of a basal layer of stagnant ice or a basal melt rate.

Results show that the deepest ice at Beyond EPICA Little Dome C (BELDC) originates from around 15 km upstream. The oldest ice with useful age resolution, i.e. with an age density of 20 kyr m^-1, is predicted to be 1.12 Ma at BELDC. Over the LDC area, the 2.5D model predicts a basal layer 200–250 m thick at the base of the ice sheet. Modelled ice particle trajectories suggest that this layer could be composed of stagnant ice, accreted ice or even disturbed ice containing debris. We explore the possibilities, though this is an open question that may only be answered by analysis the Beyond EPICA ice core once it has been drilled. Finally, we discuss in detail a thinning in the basal layer which is less than predicted by the model, as observed in other ice cores. This could mean that modelled ages are significantly over-estimated in the deepest part of the ice column. Given that the age estimate from the 2.5D model is younger than previous estimates, we suggest that horizontal flow is an important factor in this region. However, our model assumes that the flow line features such as flow direction and dome location have not change over the time period considered, which might not be the case.