We report geomorphic observations and cosmogenic-nuclide measurements on bedrock surfaces from three isolated nunatak groups in West Antarctica: the Pirrit Hills and Nash Hills, located in the Weddell Sea sector, and the Whitmore Mountains, located on the Ross-Weddell divide. The objectives of this paper are to (i) establish a chronology for landscape development at these sites and (ii) quantify the long-term history of ice-thickness variations in West Antarctica. These nunataks display relic alpine landscapes on which weathered bedrock surfaces are superimposed. In the Pirrit Hills, an erosional trimline is etched into alpine ridges and separates smooth-crested ridges below from serrated ridges above. Below the trimline, geomorphic evidence indicates repeated frozen-based ice cover, while above the trimline evidence for ice cover is entirely absent. There is also no geomorphic evidence for thicker-than-present ice at the Whitmore Mountains. Cosmogenic nuclide concentrations in the oldest bedrock surfaces from the Whitmore Mountains and from above the Pirrit Hills trimline indicate uninterrupted exposure for ∼12 Myr at extraordinarily low erosion rates. This places a lower limit on the timing of the formation of alpine landscapes in West Antarctica, and we hypothesize that this occurred during the relatively warm climates prior to the mid-Miocene cooling. The absence of evidence for thicker ice at the Whitmore Mountains is consistent with the hypothesis that the divide was typically thinner than present during Pleistocene glacial periods due to reduced accumulation. Bedrock surfaces below the trimline have much lower nuclide concentrations and are most easily explained by a scenario of repeated frozen-based ice cover and occasional subglacial plucking, which is consistent with geomorphic observations. Bedrock surfaces near the modern ice level appear to have been covered more than half of the time, while higher elevation surfaces indicate progressively less cover. The trimline and associated geomorphic features are very similar to a prominent trimline in the Ellsworth Mountains, and we conclude that these are, in fact, part of the same feature. The Ellsworth trimline is hypothesized to have formed during the mid-Miocene cooling and the transition from alpine to continental glaciation, and our results are consistent with this hypothesis.

中文翻译：

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从 Nunatak 地貌和基岩表面的宇宙成因核素测量推断出的西南极洲中新世至更新世冰川历史

我们报告了南极洲西部三个孤立的 nunatak 群的基岩表面的地貌观测和宇宙成因核素测量：位于威德尔海区的皮里特山和纳什山，以及位于罗斯-威德尔分水岭上的惠特莫尔山。本文的目的是 (i) 为这些地点的景观发展建立一个年表，以及 (ii) 量化南极洲西部冰厚变化的长期历史。这些 nunatak 展示了古老的高山景观，风化的基岩表面叠加在上面。在皮里特山，侵蚀修剪线蚀刻在高山山脊上，将下方的平滑脊与上方的锯齿状山脊分开。在修剪线下方，地貌证据表明重复冻结的冰盖，而在修剪线上方冰盖的证据完全不存在。也没有地貌证据表明惠特莫尔山脉的冰比现在更厚。来自惠特莫尔山脉和皮里特山修剪线上方的最古老基岩表面的宇宙成因核素浓度表明，以极低的侵蚀率连续暴露约 12 Myr。这对南极洲西部高山景观形成的时间设置了下限，我们假设这发生在中中新世变冷之前相对温暖的气候中。没有证据表明惠特莫尔山脉的冰更厚，这与以下假设一致，即由于积聚减少，分水岭通常比更新世冰川时期更薄。修剪线以下的基岩表面的核素浓度要低得多，并且最容易通过重复冻结的冰盖和偶尔的冰下采伐的情况来解释，这与地貌观察一致。现代冰面附近的基岩表面似乎有一半以上的时间被覆盖，而海拔越高的表面表明覆盖越来越少。修剪线和相关的地貌特征与埃尔斯沃思山脉的一条突出的修剪线非常相似，我们得出的结论是，这些实际上是同一特征的一部分。埃尔斯沃思修剪线被假设是在中中新世冷却和从高山冰川到大陆冰川的过渡期间形成的，我们的结果与这一假设一致。