Landscape Evolution and Ice Sheet Stability: New Chronologies from the Transantarctic Mountains

该奖项由极地方案办公室南极地质和地球物理方案提供，支持研究获得和解释跨南极山脉冰川沉积和地貌的新年代数据。其中一个重点将是确定天狼星群沉积物的年龄，通常认为这些沉积物记录了南极气候的显著变暖，这意味着南极东部冰盖在过去一直不稳定。由于东南极冰盖包含了地球淡水的很大一部分，这对了解过去的气候变化至关重要。南极气候变暖的主要证据是在一些天狼星群的冰碛岩中发现了山毛榉树叶和木材化石。核心问题是这是何时发生的，基于天狼星群化石硅藻的证据表明这是大约300万年前，基于地貌学的证据表明这是至少1000万年前。为了获得这些重要冰川沉积物的最小年龄，将在天狼星群主要露头的单斜辉石矿物分离物中测量宇宙射线产生的氦和氖（3 He和21 Ne）。这种方法通常被称为暴露年龄测定法，它依赖于这样一种假设，即3 He和21 Ne在表面活性剂样品中的积累与宇宙射线暴露有关，因此与时间直接相关。这些研究将包括来自麦克默多湾干谷地区的样本，以及在1995/1996年南方夏季野外季节收集的远在南部的莫德皇后山脉（比尔德莫尔和沙克尔顿冰川地区）的样本。来自班尼特平台（沙克尔顿冰川）的初步年龄证明了该方法的可行性，得出的年龄约为1100万年。新数据将有助于确定各地之间是否存在系统性的年龄差异，这可能有助于调和相互冲突的气候和地质模型。为了确定这些方法的可靠性，将对每个地点的多个样品进行分析，并将使用几种不同的宇宙成因核素来检查一致性。初步数据已经表明，一些地点产生极好的年龄一致性（班尼特平台和桌山），而其他人没有。将利用实地观察来评估该方法的假设，包括对相关基岩样品（应产生较老的年龄）和屏蔽样品（应产生零年龄）的测量。基岩样本的暴露年龄数据将得出跨南极山脉地貌的最小年龄估计值，并可计算最大侵蚀速率，这两项数据都将检验跨南极山脉无冰地貌至少有1 000万年历史且受侵蚀影响最小的假设。

This award, provided by the Antarctic Geology and Geophysics Program of the Office of Polar Programs, supports research to obtain and interpret new chronological data for glacial deposits and landforms in the Transantarctic Mountains. One focus will be to determine the age of the Sirius Group deposits, which are generally believed to record significantly warmer Antarctic climates, implying that the East Antarctic Ice Sheet has been unstable in the past. Because the East Antarctic Ice Sheet contains such a large fraction of the Earth's fresh water this is crucial to understanding past climate changes. The primary evidence for warmer Antarctic climates is the existence of fossil Beech leaves and wood within some Sirius Group tillites. The central question is when this occurred, with evidence based on fossil diatoms in the Sirius Group suggesting that it was roughly 3 million years ago, and evidence based on geomorphology suggesting it was at least 10 million years ago. In order to obtain minimum ages for these important glacial deposits, cosmic ray produced helium and neon (3He and 21Ne) will be measured in clinopyroxene mineral separates from key Sirius Group outcrops. The method, commonly referred to as exposure age dating, relies on the assumption that the accumulation of 3He and 21Ne in surficial samples is related to cosmic ray exposure, and hence is directly related to time. The studies will include samples from the Dry Valleys region of McMurdo Sound as well as those far to the south in the Queen Maude Mountains (Beardmore and Shackleton Glacier area) that were collected during the 1995/1996 Austral summer field season. Preliminary ages from Bennett Platform (Shackleton Glacier) demonstrate the viability of the method, yielding ages of approximately 11 million years. The new data will help determine whether there are systematic age differences between the various localities, which might help reconcile the conflicting climatic and geologic models. In order to establish the reliability of the methods, multiple samples from each locality will be analyzed, and several different cosmogenic nuclides will be used to check for consistency. The preliminary data already demonstrate that some locations yield excellent age consistency (Bennett Platform and Table Mountain) while others do not. Field observations will be used to evaluate the assumptions of the method, including measurements on associated bedrock samples (which should yield older ages) and shielded samples (which should yield zero age). The exposure age data for the bedrock samples will yield minimum age estimates for the landscapes in the Transantarctic Mountains and allow calculation of the maximum erosion rates, both of which will test the hypothesis that ice-free landscapes in the Transantarctic mountains are at least 10 million years old and are modified minimally by erosion.