Signal Preservation in the Deepest Part of the EPICA Dome C Ice Core and Application to Palaeoclimate Reconstruction from 600,000 to 800,000 years ago

In the framework of the new ice core project Beyond EPICA an ice core is currently being drilled to provide a climate record extending over the past 1.5 million years. This ice core will cover the Mid-Pleistocene Transition (~1.2 − 0.8 million years before present) where the glacial-interglacial cycles shifted from following the obliquity to the eccentricity periodicity, as well as the Marine Isotope Stage 19 interglaciation which is considerably the best analogue for a natural Holocene climate regarding the orbital configuration. However, dating the old ice and the interpretation of the climate signal is hampered by extensive annual layer thinning at that depth, high basal temperatures close to melting point, and long residence time favouring diffusive exchanges leading to muted signals even if high-resolution data can be obtained. We are particularly concerned by the possible diffusion of the δ(O2/N2) and δ18O of O2 signals in the deepest part since these two parameters, measured in the air bubbles, are essential tools to provide dating of the deep part of the ice core. To investigate how these signals will be preserved in the bottom part of the ice core after diffusion, we present new high-resolution records of the elemental and isotopic composition of O2 and N2 from the deepest 200 m of the EPICA Dome C ice core spanning over the period from 600,000 to 800,000 years before present. We address the effect of diffusion by comparing the amplitude of the orbital scale variability of the δ(O2/N2) and δ18O of O2 signals in the deepest part of the EPICA Dome C ice core to the expected amplitude of these signals without diffusion and propose some perspectives for the analysis of the Beyond EPICA ice core.