Miocene to Pleistocene glacial history of West Antarctica inferred from Nunatak geomorphology and cosmogenic-nuclide measurements on bedrock surfaces
Miocene to Pleistocene glacial history of West Antarctica inferred from Nunatak geomorphology and cosmogenic-nuclide measurements on bedrock surfaces
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DOI：
10.2475/10.2020.01
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发表时间：
2020-10
影响因子：
2.9
通讯作者：
P. Spector;J. Stone;G. Balco;Trevor R. Hillebrand;M. Thompson;T. Black
中科院分区：
文献类型：
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作者：
P. Spector;J. Stone;G. Balco;Trevor R. Hillebrand;M. Thompson;T. Black
我们报告地貌观测和宇宙成因核素测量基岩表面从三个孤立的冰原岛峰组在南极洲西部：皮里特山和纳什山，位于威德尔海部门，和惠特莫尔山脉，位于罗斯威德尔分水岭。本文的目标是（一）建立一个年表景观发展在这些网站和（二）量化的长期历史的冰厚度变化在南极洲西部。这些冰原岛峰展示了其上叠加着风化基岩表面的遗留的高山景观。在皮里特丘陵，一条侵蚀的纵倾线被蚀刻在高山山脊上，并将下面的光滑山脊与上面的锯齿状山脊分开。在纵倾线以下，地貌证据表明有重复的冻基冰盖，而在纵倾线以上，冰盖证据完全不存在。也没有地貌证据表明惠特莫尔山脉的冰比现在厚。来自惠特莫尔山脉和皮里特丘陵纵倾线上方的最古老基岩表面中的宇宙成因核素浓度表明，在极低的侵蚀速率下，不间断地暴露了1.12亿年。这使得南极洲西部高山景观形成的时间限制较低，我们假设这发生在中新世中期冷却之前相对温暖的气候期间。在惠特莫尔山脉没有更厚的冰的证据是一致的假设，即在更新世冰期，由于积累减少，分水岭通常比现在更薄。纵倾线下的基岩表面有低得多的核素浓度，最容易解释的情况下，反复冻结的冰盖和偶尔冰下采摘，这是一致的地貌观测。现代冰平面附近的基岩表面似乎已经被覆盖了一半以上的时间，而较高的海拔表面表明逐渐减少覆盖。纵倾线和相关的地貌特征与埃尔斯沃思山脉的一条突出的纵倾线非常相似，我们得出结论，这些实际上是同一特征的一部分。Ellsworth trimmline被假设为在中新世中期冷却和从高山到大陆冰川的过渡期间形成的，我们的研究结果与这一假设是一致的。
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.