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U-Series Comminution Age Constraints on Taylor Valley Erosion

U 系列粉碎年龄对泰勒谷侵蚀的限制

基本信息

批准号：
1644171
负责人：
Terrence Blackburn
金额：
$ 35.92万
依托单位：
University of California-Santa Cruz
依托单位国家：
美国
项目类别：
Standard Grant
财政年份：
2017
资助国家：
美国
起止时间：
2017-06-15 至 2021-05-31
项目状态：
已结题

来源：
https://www.nsf.gov/awardsearch/showAward?AWD_ID=1644171&HistoricalAwards=false
关键词：
Series Comminution Age Constraints Taylor

项目摘要

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.

该项目的主要科学目标是测试南极洲泰勒谷在过去200万年（Ma）内是否受到冰川的严重侵蚀。泰勒谷是横贯南极山脉的干旱山谷之一，其特点是年平均气温低，降水量少，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被发现在山谷的墙壁和地板上，在海拔较低的地方保存着越来越年轻的沉积物。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。以前在干谷地区的研究表明，像泰勒冰川这样的极冷冰川，一个进入山谷的主要出口冰川，在过去的几百万年里没有侵蚀。这项研究将测试一个新的假设，即泰勒冰川下的冰川侵蚀和沉积物生产在过去几百万年中一直很活跃。这一假设将使用一种新的同位素测年方法进行测试，这种方法被称为“粉碎测年”，可以确定被称为粉砂的细粒沉积物颗粒的形成时间。如果沉积物的年龄是年轻的，那么结果将表明冰川过程比以前认为的更有活力。总的来说，这项研究将增加我们对南极洲过去冰川的性质和范围的了解。由于侵蚀沉积物产生的淤泥是当地生态系统的营养物质，因此研究结果也将揭示营养物质向高极地地区土壤、河流和沿海地区的输送。该项目将由一名早期职业科学家领导，并包括一名博士的培训。学生. 关于东南极冰盖的稳定性存在着长期的科学争议，许多证据集中在南维多利亚地的干谷地区。地貌学家的普遍观点是，景观一直非常稳定，冰川作用的影响在过去的15 Ma已经很小。该项目将区分泰勒冰川的冰川侵蚀和沉积的两种端元情况，泰勒冰川是南极洲东部冰盖的一个出口冰川，终止于南极洲干旱河谷地区的泰勒谷。在第一种情况下，所有的山谷救济之前产生的15马时，非极地气候使温暖的冰川切割和扩大古代河道。在这种情况下，较年轻的冰川沉积物记录了冰量减少和有限的冰川侵蚀的冷基冰川的进展，沉积物生成导致冰川沉积物主要由较老的再循环沉积物组成。在第二种情况下，谷底的选择性侵蚀继续加深泰勒谷，但在过去的2Ma没有影响到相邻的山峰。在这种情况下，随着时间的推移，位于海拔逐渐降低的第四纪冰川沉积物的“浴缸环”可能是由于冰下侵蚀造成的谷底下降，以及新沉积物的产生，这些沉积物现在已被纳入这些沉积物中。虽然这两种情况都将导致现今的地形，但它们在区域冰川体积随时间的演变以及山谷地貌生产和细粒颗粒生成的时间方面有所不同。这两个方案将通过使用U系列粉碎测年法对细颗粒物的产生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由于α反冲而导致的234 U的损失。只有在具有足够高的表面积与体积比的细粒颗粒中才能检测到234 U的不足，这可能会导致可观的234 U损失。粉碎时间和粒度控制234U损失的大小。虽然这一地质年代学工具尚处于起步阶段，但这一建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显示，这是一个很容易解决的二元问题。