New insights from the basal section of the Beyond EPICA ice core (Little Dome C)

The basal section of the 2,800-m-long Beyond EPICA ice core (the lowermost 316 m) is of particular interest, not only because it potentially represents the oldest part of the core, but also because it contains important clues about past and present ice-sheet dynamics. We present here field observations and ongoing analyses aimed at characterizing the different ice units above the basal interface. Below the bottom of the “stratified ice” at 2,506 m, electrical conductivity, water-isotope composition, and crystal size allow the identification of a thinned and distorted zone extending down to 2,583 m, with no clear evidence of preserved climatic cycles. Below 2,583 m, the conductivity signal shows a sharp increase that persists over more than 200 m, although with some variability, and is associated with a marked increase in ice crystal size, reaching more than 10 cm.

The first occurrence of basal rock debris is observed at 2,795 m, where sand-sized and finer particles are embedded within the ice matrix, together with a few angular centimettric pebbles. This debris-rich section exhibits a heterogeneous composition, with decameter-scale alternations of banded, dispersed facies and clear-ice intervals. Density estimates of these basal units indicate that debris generally accounts for less than 1% of the mass, although it may locally reach a few percent. Two angular pebbles with volumes of several cubic centimeters are currently being processed for luminescence dating. Mineralogical observations suggest multiple source lithologies, supporting the hypothesis that the debris result from lateral transport and that the core did not reach the underlying bedrock. Scanning electron microscopy (SEM) observations of grain textures, together with geochemical analyses (Sr and Nd isotopes) and gas content will add additional constraints and help refine this interpretation and look for possible evidences of past deglaciation events. In addition, clay minerals and large (by XRD) will be analyzed to identify partially melted basal conditions in the past or weathering during ice free conditions.