Measuring multiple cosmogenic nuclides in glacial cobbles sheds light on Greenland Ice Sheet processes
Measuring multiple cosmogenic nuclides in glacial cobbles sheds light on Greenland Ice Sheet processes
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测量冰川卵石中的多种宇宙核素为格陵兰冰盖过程提供了线索
DOI：
10.1016/j.epsl.2020.116673
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发表时间：
2021
影响因子：
5.3
通讯作者：
Caffee, Marc W.
中科院分区：
文献类型：
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作者：
Corbett, Lee B.;Bierman, Paul R.;Neumann, Thomas A.;Graly, Joseph A.;Shakun, Jeremy D.;Goehring, Brent M.;Hidy, Alan J.;Caffee, Marc W.
由于缺乏末次冰盛期之前的证据，格陵兰冰盖在更新世期间的行为仍然不确定。在这里，我们使用了一种新的方法，即对单个次冰川来源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做出推断。我们从格陵兰岛西部的三个地点收集了86块现今冰盖边缘的鹅卵石和9块暴露在现代冰川陆地表面的鹅卵石。我们测量了所有鹅卵石中10Be的浓度(n=95)和子集中的26Al和14C的浓度(n=14)。在全新世消退期间沉积的鹅卵石的暴露年龄为10Be，与其他方法确定的冰消退的时间大体一致。相反，在86个冰下鹅卵石中，大多数含有非常低浓度的10Be(中位数为1.0×103个原子g−1)，尽管有几个含有∼104，一个含有∼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的格陵兰冰盖下的源区被深度侵蚀，保存了最少的地表或近地表暴露的证据。在10个鹅卵石中存在可测量的14C，这要求它们在过去的∼30ka内经历了宇宙源核素的产生；然而，∼6的14C/10Be比率表明，核素的产生是在被上覆物质屏蔽的过程中发生的。在86个冰下鹅卵石中，只有两个确切地具有与之前表面暴露的宇宙成因核素浓度一致的浓度。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冰下景观具有深度侵蚀和极少的地面暴露的特点。
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 situ10Be in all cobbles (n = 95) and26Al and14C in a subset (n = 14). Cobbles deposited during Holocene retreat have10Be 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 of10Be (median 1.0×103atoms g−1), although several have ∼104and one has ∼105atoms g−1. The low concentrations of10Be 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 measurable14C 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.