The oldest ice on Earth in Beacon Valley, Antarctica: new evidence from surface exposure dating

Beacon Valley, Antarctica, contains unique remnants of glacier ice undernea th a till layer covering the valley floor. To constrain the age and evoluti on of this important indicator of Antarctic paleoclimate, we analyzed two d olerite erratics from the till surface and one from within the ice for cosm ogenic helium and neon. A conservative minimum exposure age of the older su rface sample is 2.3 Mal but taking into account erosion, the true exposure age of this boulder is likely to be considerably higher. The buried sample contains more than 20 times less cosmogenic noble gases than the old surfac e sample, although its current shielding would imply only a three times low er production rate. This indicates that the ice level has slowly been lower ed by sublimation at the rate of a few m/Ma. The high exposure age of the s urface sample as well as the very low sublimation rate of the relict ice bo th support the conclusion that the remnant ice in Beacon Valley was deposit ed many million years ago [Sugden et al., Nature 376 (1995) 412-414] and ha s never been thinner than at present. In addition, we found that cosmogenic helium and neon are released quantitatively from pyroxene at temperatures of < 900 degrees C and > 1000 degrees C, respectively. (C) 2000 Elsevier Sc ience B.V. All rights reserved.