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

摘要

Beacon Valley, Antarctica, contains unique remnants of glacier ice underneath a till layer covering the valley floor. To constrain the age and evolution of this important indicator of Antarctic paleoclimate, we analyzed two dolerite erratics from the till surface and one from within the ice for cosmogenic helium and neon. A conservative minimum exposure age of the older surface sample is 2.3 Ma, 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 surface sample, although its current shielding would imply only a three times lower production rate. This indicates that the ice level has slowly been lowered by sublimation at the rate of a few m/Ma. The high exposure age of the surface sample as well as the very low sublimation rate of the relict ice both support the conclusion that the remnant ice in Beacon Valley was deposited many million years ago [Sugden et al., Nature 376 (1995) 412-414] and has never been thinner than at present. In addition, we found that cosmogenic helium and neon are released quantitatively from pyroxene at temperatures of <900 ℃ and >1000 ℃, respectively.