Pliocene ice preserved in the Allan Hills Blue Ice Area (BIA), East Antarctica

International efforts are currently underway to locate and collect continuous ice core records extending beyond the mid-Pleistocene transition (1200-800 ka) to understand the drivers of this fundamental shift in climate and the pacing of glacial cycles. A complementary approach targets older, stratigraphically disturbed, ice that is retrieved in shallow ice cores at the ice sheet margins. The Allan Hills Blue Ice Area (BIA) represents a particularly attractive study area, as it is characterized by very low ablation rates and horizontal ice velocities resulting in stagnation of very old (> 2 Ma) ice. In 2019 a large diameter (9.25") ice core was drilled at the Allan Hills and dated through precise measurement of the isotopic composition of atmospheric argon (40ArATM) in trapped air bubbles. Preliminary results suggest that the ice at the base of the core is as old as ~4.0 +/- 0.4 Ma. High-precision analyses of Xe/Kr ratios in these same samples provide a record of contemporaneous global mean ocean temperature, that, when considered together with proxies for Antarctic temperature (dDice), provide the first 'snapshots' of ice core climate properties at the dawn of the Pleistocene and beyond. CO2 measurements from the deepest samples in this core are forthcoming, but past results (Yan et al, 2019) suggest that ice in close contact with the bed may not yield reliable CO2 data. However, pristine greenhouse gas records extending to 2.7 Ma are preserved in shallower portions of the 2019 core (see J. Marks Peterson et al, this meeting). Future field seasons at the Allan Hills, as part of the Center for OLDest ice EXploration (COLDEX), will further investigate this unique archive, collecting additional ice cores and carrying out field and laboratory measurements in order to understand how this very old ice is preserved, the extent of stratigraphic disturbance and mixing, and whether even older ice might be preserved in these environments.