Abstract The behavior of the Greenland Ice Sheet during the Pleistocene remains uncertain due to the paucity of evidence predating the Last Glacial Maximum. Here, we employ a novel approach, cosmogenic nuclide analysis of individual subglacially-derived cobbles, which allows us to make inferences about ice sheet processes and subglacial erosion. From three locations in western Greenland, we collected 86 cobbles from the current ice sheet margin and nine cobbles exposed on the modern proglacial land surface. We measured the concentration of in situ 10Be in all cobbles (n = 95) and 26Al and 14C in a subset (n = 14). Cobbles deposited during Holocene retreat have 10Be exposure ages generally consistent with the timing of ice retreat determined by other methods. Conversely, most of the 86 subglacial cobbles contain very low concentrations of 10Be (median 1.0×103 atoms g−1), although several have ∼104 and one has ∼105 atoms g−1. The low concentrations of 10Be in most subglacial cobbles imply that their source areas under the Greenland Ice Sheet are deeply eroded, preserving minimal evidence of surface or near-surface exposure. The presence of measurable 14C in ten of the cobbles requires that they experienced cosmogenic nuclide production within the past ∼30 ka; however, 14C/10Be ratios of ∼6 suggest that nuclide production occurred during shielding by overlying material. Only two of the 86 subglacial cobbles definitively have cosmogenic nuclide concentrations consistent with prior surface exposure. Overall, isotopic analysis of subglacial cobbles indicates that much of western Greenland's subglacial landscape is characterized by deep erosion and minimal subaerial exposure.

中文翻译：

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测量冰川鹅卵石中的多种宇宙成因核素揭示了格陵兰冰盖过程

摘要 由于在末次盛冰期之前缺乏证据，格陵兰冰盖在更新世期间的行为仍然不确定。在这里，我们采用了一种新方法，即对单个冰下衍生的鹅卵石进行宇宙成因核素分析，这使我们能够对冰盖过程和冰下侵蚀进行推断。从格陵兰西部的三个地点，我们从当前冰盖边缘收集了 86 块鹅卵石，并在现代前冰期地表上收集了 9 块鹅卵石。我们测量了所有鹅卵石 (n = 95) 中的原位 10Be 浓度以及子集 (n = 14) 中的 26Al 和 14C。全新世退缩过程中沉积的鹅卵石的 10Be 暴露年龄与其他方法确定的冰退缩时间基本一致。反过来，86 个冰下鹅卵石中的大多数含有非常低浓度的 10Be（中值 1.0×103 个原子 g-1），尽管有几个含有~104 个原子，一个含有~105 个原子 g-1。大多数冰下鹅卵石中 10Be 的低浓度意味着它们在格陵兰冰盖下的源区被深度侵蚀，保留了极少的地表或近地表暴露的证据。10 个鹅卵石中可测量的 14C 的存在要求它们在过去 30 ka 内经历了宇宙成因核素的产生；然而，约 6 的 14C/10Be 比率表明在覆盖材料的屏蔽过程中会产生核素。86 个冰下鹅卵石中只有两个明确具有与先前表面暴露一致的宇宙成因核素浓度。总体而言，对冰下鹅卵石的同位素分析表明，格陵兰西部的大部分地区