Trimlines, blockfields, mountain-top erratics and the vertical dimensions of the last British-Irish Ice Sheet in NW Scotland
Trimlines, blockfields, mountain-top erratics and the vertical dimensions of the last British-Irish Ice Sheet in NW Scotland
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苏格兰西北部最后一个英爱冰盖的修剪线、区块、山顶的起伏和垂直尺寸
DOI：
10.1016/j.quascirev.2012.09.002
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发表时间：
2012
影响因子：
4
通讯作者：
Fabel D
中科院分区：
文献类型：
--
作者：
Fabel D
将英国和爱尔兰山脉冰川侵蚀的低坡与块地覆盖的高原分开的冰缘纵倾线被解释为（1）代表末次冰盛期（LGM）最后一个冰盖的最大高度，或（2）将处于压力熔点的湿基冰与山顶高原上的冷基冰分开的热边界。我们通过对苏格兰西北部五座山脉纵倾线以上的高水平不稳定巨石进行10 Be暴露测年来测试这些相互竞争的假设。14个测年异常中有9个测得的末次大冰期后暴露年龄范围为14.0 ± 0.7 ka ~ 16.5 ± 0.9 ka或14.9 ± 0.9 ka ~ 17.6 ± 1.1 ka，取决于暴露年龄计算中所用的10 Be产生率。这些年龄反驳了假设（1），因为它们意味着最后的冰盖覆盖了山脉。山顶上保存着明显完整的块体，这意味着冷基冰盖，支持假说（2）。由于高度一致的高水平的纵倾线延伸从我们的采样点横跨苏格兰西北部和赫布里底群岛的大部分地区，我们的结论适用于所有的纵倾线在这个更广泛的地区，并可能在不列颠群岛的其他地方的所有高水平的纵倾线。冷基冰下块体的保存与整个第四纪大部分或全部时期高原上块体的演变是一致的。平均暴露年龄为15-16 ka的高原顶部erratics意味着冰原岛峰的出现，从下消冰盖之前的区域再进的冰缘（西罗斯Readvance）和快速变暖之前，在14.7 ka的开始，在晚冰期间冰期，但在冰盖变薄的时间追溯最近的代理气候驱动的热-机械耦合模型。我们的研究结果提供了一个额外的限制，这种模式的未来发展，这意味着高水平的纵倾线代表了前过渡区之间的冰在压力熔点和冰低于压力熔点的高度。
Periglacial trimlines separating glacially eroded lower slopes from blockfield-covered plateaus on British and Irish mountains have been interpreted either (1) in terms of representing the maximum altitude of the last ice sheet during the Last Glacial Maximum (LGM), or (2) as a thermal boundary separating wet-based ice at pressure melting point from cold-based ice on summit plateaus. We test these competing hypotheses through10Be exposure dating of high-level erratic boulders above trimlines on five mountains in NW Scotland. Nine out of 14 erratics yielded post-LGM exposure ages ranging from 14.0 ± 0.7 ka to 16.5 ± 0.9 ka or from 14.9 ± 0.9 ka to 17.6 ± 1.1 ka, depending on the10Be production rate employed in exposure age calculation. These ages refute hypothesis (1) as they imply that the last ice sheet overtopped the mountains. Preservation of apparently intact blockfields on the summits implies cold-based ice cover, supporting hypothesis (2). As altitudinally consistent high-level trimlines extend from our sampled sites across much of NW Scotland and the Hebrides, our conclusions apply to all trimlines in this broader area, and probably to all high-level trimlines elsewhere in the British Isles. Preservation of blockfields under cold-based ice is consistent with blockfield evolution on plateaus throughout much or all of the Quaternary. Averaged exposure ages of ∼15–16 ka for plateau-top erratics implies nunatak emergence from the downwasting ice sheet prior to a regional readvance of the ice margin (the Wester Ross Readvance) and before rapid warming at ∼14.7 ka at the onset of the Lateglacial Interstade, but after the timing of ice-sheet thinning as retrodicted by recent proxy climate-driven thermo-mechanical coupled models. Our findings provide an additional constraint on the future development of such models by implying that high-level trimlines represent the altitude of a former transition zone between ice at pressure-melting point and ice below pressure melting point.