Identification of 320 000-year-old blue ice at the surface of the Elephant Moraine region, East Antarctica

Abstract. To address important paleoclimatic questions, such as the cause of the Mid-Pleistocene Transition (MPT), the search for 1-million-year-old ice is of great interest. Antarctic blue-ice areas (BIAs), where ancient ice outcrops on the surface of ice sheets, offer promising sites for identifying ice spanning the MPT period. To date, only two sites, the Allan Hills BIA and the Mullins Glacier in East Antarctica, have been identified as areas that contain ancient ice older than 1 million years. We investigated icefields in the Elephant Moraine and Reckling Moraine regions of East Antarctica to contribute to the search for ancient ice spanning the MPT. Ice-penetrating radar surveys revealed that ice thickness ranged from 200 to 800 m across the icefields. 81Kr dating of the surface ice (< 10 m) showed ages of 83–119 kyr BP (before present) and 93–124 kyr BP for blue ice in the Meteorite City Icefield and 320–385 kyr BP in the Elephant Moraine Main Icefield. We also analyzed several gas compositions (δ15N–N2, δ18O–O2, δO2 / N2, δAr / N2, CO2, CH4, and N2O) and revealed that gas records at very shallow depths are altered. A comparison of stable water isotopes (δ18Oice and δ2Hice) indicated that the original deposition site of the Elephant Moraine Main Icefield experienced colder conditions than those of the nearby icefields. Given these findings, ice spanning the MPT period could be retrieved from the Elephant Moraine Main Icefield with only a few hundred meters of drilling.