Altitude and configuration of the last British-Irish ice sheet: testing the nunatak hypothesis using cosmogenic Be-10 exposure dating

在26,000-21,000年前的最后一次冰川盛期(LGM)期间，冰川冰占据了地球现在陆地面积的约30%。重建当时存在的冰盖的尺寸和行为对于确定气候变化、海洋、大气和冰盖之间的联系以及了解过去和未来的海平面变化至关重要。在末次冰盖发生时，全球海平面比现在低约120米，最后一次英国-爱尔兰冰盖向西延伸，横跨大西洋大陆架，在北海地区与来自斯堪的纳维亚半岛的冰盖相遇。然而，事实证明，很难确定横跨不列颠群岛的前冰面的形状和高度。模拟冰层厚度的尝试产生了广泛矛盾的结果，因此迫切需要开发出某种方法，根据现场证据来限制以前的冰层高度和形状。在整个不列颠群岛的山脉上，下部冰川侵蚀的基岩地带和上层冰冻侵蚀的碎屑地带之间存在明显的上坡对比，没有证据表明最后一次冰盖的侵蚀。这样的分界线已经在苏格兰西北部以及斯诺多尼亚、英格兰湖区和爱尔兰的山区绘制了地图。这张地图的准确性已经被一种名为暴露测年的技术所证实，该技术确定了岩石表面暴露在大气中的时间长度。切线以上的基岩表面产生的暴露年龄总是早于大冰川(即26,000年)，而切线以下的基岩表面提供的年龄记录了从变薄的冰盖下暴露的时间(21,000-14,000年)。关于这条边界线是如何形成的，有两种相互矛盾的解释。第一种是“nunatak假说”，即切线代表最后一次冰盖的上限。这就是最初对切线的解释，使以前的冰面能够在山区进行3D重建。另一种解释是，剪切线代表了延伸到山顶的厚厚冰盖内熔点的高度。根据这一观点，剪切线以下的冰处于融点，能够滑动并侵蚀基岩。相反，剪切线以上的冰被设想为低于熔点，冻结到下层基质，因此不能进行重大侵蚀。如果这种解释是正确的，它意味着边界线标志着冰盖的最低高度，它允许我们计算出冰的热状况--这是在数值模式中限制冰盖厚度的重要输入。基岩表面的暴露年代没有区分这两种假设，因为在这两种情况下，暴露年龄都是26,000年。我们将通过获取出现在苏格兰西北部和爱尔兰西北部一些山顶上的冰川运输巨石的暴露年龄来解决这个问题。如果这些暴露年龄大于LGM的年龄(&gt；26,000岁)，那么假设1被证实。如果它们产生了26,000年的暴露年龄，就意味着最后一次冰盖沉积了不稳定的部分，这表明冰盖越过了山顶，切线代表了冰盖内熔点的高度。由于相邻山脉上的剪切线高度可以连接起来形成一致的‘剪切线表面’，所以从几个采样点获得的结果可以外推到给定区域的所有剪切线。曝光日期将从苏格兰西北部四次峰会的16个样本和爱尔兰西北部峰会的4个样本获得。结果将决定性地证明，不列颠群岛的最后一块冰盖是一个低调的地貌，有许多山峰突出在冰上，还是冰覆盖了最高的山峰，但在大片高地上冻结在冰床上。

During the last glacial maximum (LGM) 26,000-21,000 years ago, glacier ice occupied ~30% of the Earth's present land area. Reconstructing the dimensions and behaviour of the ice sheets that existed at that time is vitally important for establishing the linkages between climate change, the oceans, atmosphere and ice sheets, as well as for understanding past and future sea-level change. At the time of the LGM, global sea level was about 120 m lower than now, and the last British-Irish ice sheet extended westwards across the Atlantic continental shelf and met an ice sheet from Scandinavia in the North Sea area. It has proved difficult, however, to determine the configuration and altitude of the former ice surface across the British Isles. Attempts to model ice thickness have produced widely conflicting results, so there is an urgent need to develop some way of constraining former ice altitude and configuration from field evidence. On mountains throughout the British Isles there is a marked upslope contrast between a lower zone of glacially-eroded bedrock and an upper zone of frost-weathered debris that exhibits no evidence for erosion by the last ice sheet. Such trimlines have been mapped across NW Scotland, as well as in Snowdonia, the English Lake District and the mountains of Ireland. The accuracy of this mapping has been confirmed by a technique known as exposure dating, which determines how long rock surfaces have been exposed to the atmosphere. Bedrock surfaces above trimlines have invariably produced exposure ages older than the LGM (i.e > 26,000 years), whereas those below trimlines give ages recording the timing of exposure from under the thinning ice sheet (21,000-14,000 years). There are two competing explanations of how this trimline formed. The first, the 'nunatak hypothesis', is that the trimline represents the upper limit of the last ice sheet. This is how the trimline was initially interpreted, enabling 3D reconstruction of the former ice surface across mountain areas. The alternative interpretation is that trimlines represents the altitude of the melting point within a thick ice sheet that extended over mountain summits. According to this view, the ice below trimlines was at melting point, and capable of sliding over and eroding bedrock. Conversely, the ice above trimlines is envisaged as having been below melting point, frozen to the underlying substrate and thus incapable of significant erosion. If this interpretation is correct, it implies that the trimline marks the minimum altitude of the ice sheet, and it allows us to work out the thermal regime of the ice - a vital input for constraining ice-sheet thickness in numerical models. Exposure dating of bedrock surfaces does not distinguish between these two hypotheses, as in both cases exposure ages >26,000 years are to be expected. We will overcome this problem by obtaining exposure ages for glacially-transported boulders (erratics) that occur above trimlines on some summits in NW Scotland and NW Ireland. If these give exposure ages greater than the age of the LGM (> 26,000 years), then hypothesis 1 is validated. If they yield exposure ages < 26,000 years, deposition of the erratics by the last ice sheet is implied, demonstrating that the ice sheet over-rode mountain summits and that trimlines represent the altitude of the melting point within the ice sheet. As trimline altitudes on adjacent mountains can be linked to form a consistent 'trimline surface', the results obtained for a few sampling sites can be extrapolated to all trimlines in a given region. Exposure dates will be obtained for 16 samples from four summits in NW Scotland, and 4 from a summit in NW Ireland. The results will demonstrate decisively whether the last ice sheet in the British Isles was a low-profile feature with numerous summits protruding above the ice, or whether the ice covered the highest summits but was frozen to its bed across extensive areas of high ground.