U-Series Comminution Age Constraints on Taylor Valley Erosion

对该项目的非技术描述该项目的主要科学目标是测试南极洲的泰勒山谷在过去的200万年(Ma)是否受到冰川的显著侵蚀。泰勒谷是横跨北极山脉的干燥山谷之一，其特点是年平均气温低，降雨量少，侵蚀有限。这些条件使脆弱的冰川地貌得以保存长达15 Ma。被冰川侵蚀和沉积的沉积物在山谷的墙壁和底部被发现，在较低的海拔处保存着逐渐年轻的沉积物。科学家可以测定冰川沉积物的年代，以了解过去冰川侵蚀的过程和时间。之前在干旱山谷地区的研究表明，像泰勒冰川这样的极端寒冷的冰川在过去的几百万年里并没有受到侵蚀。泰勒冰川是进入山谷的主要出口冰川。这项研究将检验一个新的假设，即泰勒冰川下的冰川侵蚀和沉积物生产在过去的几百万年里一直活跃。这一假说将通过一种名为“粉碎测年”的新的同位素测年方法来验证，该方法可以确定细粒沉积物颗粒(称为粉砂)的形成时间。如果沉积年龄较早，那么结果将表明冰川过程比之前认为的更具活力。总体而言，这项研究将增加我们对南极过去冰川的性质和范围的了解。由于侵蚀沉积物产生的泥沙是当地生态系统的养分，研究结果也将有助于将养分输送到高极地区的土壤、溪流和海岸地带。该项目将由一名早期职业科学家领导，并包括一名博士生的培训。关于该项目的技术描述，关于南极东部冰盖的稳定性存在长期的科学争议，许多证据集中在南维多利亚陆地的干燥山谷地区。地貌学家的一种普遍观点是，在过去的15 Ma左右，地貌非常稳定，冰川作用的影响微乎其微。该项目将区分两种终端成员的冰川侵蚀和泰勒冰川沉积的情景，泰勒冰川是东南极冰盖的出口冰川，终止于南极洲干燥山谷地区的泰勒山谷。在第一种情况下，所有的山谷起伏都是在15 Ma之前产生的，当时非极地气候使温暖的冰川切割和加宽了古老的河流水道。在这种情况下，较年轻的冰川沉积记录了冰量减少和冰川侵蚀有限的冷基冰川的进展，沉积物的产生导致了主要由较老的循环沉积物组成的冰川沉积。在第二种情况下，谷底的选择性侵蚀继续加深了泰勒山谷，但在过去2 Ma并没有影响到相邻的山峰。在这种情况下，随着时间推移，第四纪冰川沉积物的“浴盆环”位于海拔逐渐降低的位置，这可能是由于冰川下侵蚀导致谷底下降，并随之而来的是产生新的沉积物，这些沉积物现在被并入这些沉积物中。虽然这两种情况都会导致今天的地形，但它们在区域冰川体积随时间的隐含演变以及山谷地貌产生和细粒颗粒产生的时间上存在差异。这两种情况将通过使用U系列粉碎测年对细颗粒生产施加时间限制来进行测试。这种新的地质年代学工具利用了阿尔法反冲造成的234U的损失。只有在比表面积与体积比足够高的细粒颗粒中才能检测到234U的缺陷，这可能会导致明显的234U损失。粉碎时间和粒度控制着234U损失的大小。虽然这一地质年代学工具还处于初级阶段，但这一提议的科学目标可以通过解决古代和最近粉碎的细颗粒之间的问题来实现，初步建模和测量数据表明，这是一个很容易解决的二元问题。

A nontechnical description of the project The primary scientific goal of the project is to test whether Taylor Valley, Antarctica has been eroded significantly by glaciers in the last ~2 million years (Ma). Taylor Valley is one of the Dry Valleys of the Transantarctic Mountains, which are characterized by low mean annual temperatures, low precipitation, and limited erosion. These conditions have allowed fragile glacial landforms to be preserved for up to 15 Ma. Sediment eroded and deposited by glaciers is found on the valley walls and floors, with progressively younger deposits preserved at lower elevations. Scientists can date glacial deposits to understand the process and timing of past glacial erosion. Previous work in the Dry Valleys region suggested that extremely cold glaciers like Taylor Glacier, a major outlet glacier entering the valleys, were not erosive during the last several million years. This research will test a new hypothesis that glacial erosion and sediment production beneath Taylor Glacier have been active in the last few million years. This hypothesis will be tested using a new isotopic dating method called "comminution dating' which determines when fine-grained sediment particles called silt were formed. If the sediment age is young, then the results will suggest that glacial processes have been more dynamic than previously thought. Overall, this study will increase our understanding of the nature and extent of past glaciations in Antarctica. Because the silt produced by erosion sediment is a nutrient for local ecosystems, the results will also shed light on delivery of nutrients to soils, streams, and coastal zones in high polar regions. This project will be led by an early career scientist and includes training of a Ph.D. student. A technical description of the project There is a long-standing scientific controversy about the stability of the East Antarctic Ice Sheet with much evidence centered in the Dry Valleys region of South Victoria Land. A prevailing view of geomorphologists is that the landscape has been very stable and that the effects of glaciation have been minimal for the past ~15 Ma. This project will distinguish between two end-member scenarios of glacial erosion and deposition by Taylor Glacier, an outlet glacier of the East Antarctic Ice Sheet that terminates in Taylor Valley in the Dry Valleys region of Antarctica. In the first scenario, all valley relief is generated prior to 15 Ma when non-polar climates enabled warm-based glaciers to incise and widen ancient river channels. In this case, younger glacial deposits record advances of cold-based glaciers of decreasing ice volume and limited glacial erosion, and sediment generation resulted in glacial deposits composed primarily of older recycled sediments. In the second scenario, selective erosion of the valley floor has continued to deepen Taylor Valley but has not affected the adjacent peaks over the last 2 Ma. In this scenario, the "bathtub rings" of Quaternary glacial deposits situated at progressively lower elevations through time could be due to the lowering of the valley floor by subglacial erosion and with it, production of new sediment which is now incorporated into these deposits. While either scenario would result in the present-day topography, they differ in the implied evolution of regional glacial ice volume over time and the timing of both valley relief production and generation of fine-grained particles. The two scenarios will be tested by placing time constraints on fine particle production using U-series comminution dating. This new geochronologic tool exploits the loss of 234U due to alpha-recoil. The deficiency in 234U only becomes detectable in fine-grained particles with a sufficiently high surface-area-to-volume ratio which can incur appreciable 234U loss. The timing of comminution and particle size controls the magnitude of 234U loss. While this geochronologic tool is in its infancy, the scientific goal of this proposal can be achieved by resolving between ancient and recently comminuted fine particles, a binary question that the preliminary modeling and measured data show is readily resolved.