New Cosmogenic 21Ne and 10Be Measurements in the Transantarctic Mountains

横贯南极山脉的新宇宙成因 21Ne 和 10Be 测量

• 批准号: 2048351
• 负责人: Julia Lindow
• 金额: $ 40.14万
• 依托单位: Woods Hole Oceanographic Institution
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2021
• 资助国家: 美国
• 起止时间: 2021-02-01 至 2025-01-31
• 项目状态: 未结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2048351&HistoricalAwards=false
• 关键词: New; Cosmogenic; 21Ne; 10Be; Measurements

Part I: NontechnicalAntarcticas ice sheets constitute the largest ice mass on Earth, with approximately 53 meters of sea level equivalent stored in the East Antarctic Ice Sheet alone. The history of the East Antarctic Ice Sheet is therefore important to understanding and predicting changes in sea level and Earths climate. There is conflicting evidence regarding long-term stability of the East Antarctic Ice Sheet, over the last twenty million years. To better understand past ice sheet changes, together with the history of the Transantarctic Mountains, accurate time scales are needed. One of the few dating methods applicable to the Antarctic glacial deposits, that record past ice sheet changes, is the measurement of rare isotopes produced by cosmic rays in surface rock samples, referred to as cosmogenic nuclides. Whenever a rock surface is exposed/free of cover, cosmic rays produce rare isotopes such as helium-3, beryllium-10, and neon-21within the minerals. This project will involve measurement of all three isotopes in some of the oldest glacial deposits found at high elevation in the Transantarctic Mountains. Because the amount of each isotope is directly linked to the exposure time, this can be used to calculate the age of a surface. This method requires knowledge of the rates that cosmic radiation produces each isotope, which depends upon mineral composition, and is presently a limitation of the method. The goal of this project is to advance and enhance existing measurement methods and expand the range of possibilities in surface dating with new measurements of all three isotopes in pyroxene, a mineral that is commonly found throughout the Transantarctic Mountains. This technological progress will allow a better application of the surface exposure dating method, which in turn will help to reconstruct Antarctic ice sheet history and provide valuable knowledge of former ice-extent. Understanding Antarcticas ice-sheet history is crucial to predict its influence on past and future sea level changes. Part II: Technical DescriptionMeasurements of in-situ produced cosmogenic nuclides in Antarctic surficial rock samples provide unique time scales for glacial and landscape evolution processes. However, due to analytical challenges, pyroxene-bearing and widely distributed lithologies like the Ferrar dolerite of the Transantarctic Mountains, are underutilized. This proposal aims to changes this and to improve the cosmogenic nuclide methodologies for stable isotopes (21Ne and 3He) and radioactive nuclides (10Be) in pyroxenes. Proposed methodological improvements will be directly applicable to erosion rates and deposition ages of important glacial deposits, such as the controversial Sirius Group tills, and also to younger glacial features. Bennett Platform is the focus of this study because it is one of the southern-most Sirius Group outcrops along the Transantarctic Mountains, where cosmogenic ages are sparse.Preliminary measurements demonstrate large discrepancies between 3He and 21Ne age determinations in Sirius Group pyroxenes. One possible explanation is composition dependence of the 21Ne production rates. Coupled measurements of 3He, 21Ne, and 10Be in well-characterized pyroxene mineral separates from Ferrar dolerite will be used to better constrain the production rates, major element and trace element dependencies, the assumptions of the method, and ultimately advance the application of cosmogenic nuclides to mafic Antarctic lithologies. The main goals of this study are to improve measurement protocols for 10Be in pyroxene, and the determination of the composition dependence of 21Ne production rates by measuring mineral compositions (by electron microprobe), and nuclide concentrations in mineral pairs from young lava flows. Further aims are the validation of the nucleogenic contributions and the effects of helium diffusive loss through measurements of 3He/21Ne production ratios, combined with measurements of shielded samples of the Ferrar dolerite. Combined measurements of 3He, 21Ne and 10Be in pyroxenes have rarely been published for individual samples in Antarctica. The new and unique measurements of this study will advance the applicability of in-situ produced cosmogenic nuclides to both young and ancient Antarctic surfaces. The study will be performed using existing samples: no field work is requested.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

第一部分：非技术意义上的南极冰盖构成了地球上最大的冰块，仅南极东部冰盖就储存着相当于海平面约53米的冰川。因此，南极东部冰盖的历史对于理解和预测海平面和地球气候的变化非常重要。在过去的2000万年里，关于南极东部冰盖的长期稳定性，有相互矛盾的证据。为了更好地了解过去的冰盖变化，以及南极横贯山脉的历史，需要精确的时间尺度。适用于记录过去冰盖变化的南极冰川沉积物的为数不多的测年方法之一，是测量由宇宙射线在地表岩石样本中产生的稀有同位素，即宇宙核素。每当岩石表面暴露或没有覆盖物时，宇宙射线就会在矿物中产生稀有的同位素，如氦-3、铍-10和氖-21。该项目将包括在横贯南极山脉高海拔地区发现的一些最古老的冰川沉积物中测量所有三种同位素。由于每种同位素的含量与暴露时间直接相关，因此可以用来计算表面的年龄。这种方法需要了解宇宙辐射产生每种同位素的速率，这取决于矿物组成，这是目前该方法的一个局限性。该项目的目标是推进和加强现有的测量方法，并通过对辉石中所有三种同位素的新测量来扩大表面测年的可能性范围，辉石是一种普遍存在于南极横贯山脉的矿物。这一技术进步将使地表暴露测年方法得到更好的应用，这反过来将有助于重建南极冰盖的历史，并提供有关以前冰层范围的宝贵知识。了解南极冰盖的历史对于预测其对过去和未来海平面变化的影响至关重要。第二部分：技术描述南极地表岩石样品中原位产生的宇宙成因核素的测量为冰川和景观演化过程提供了独特的时间尺度。然而，由于分析方面的挑战，含辉石岩和广泛分布的岩性，如横越南极山脉的费拉尔白云岩，未得到充分利用。本文旨在改变这一现状，改进辉石中稳定同位素（21Ne和3He）和放射性核素（10Be）的宇宙成因核素方法。提出的方法改进将直接适用于重要冰川沉积物的侵蚀速率和沉积年龄，例如有争议的天狼星群丘，也适用于更年轻的冰川特征。班尼特台地是本次研究的重点，因为它是横贯南极山脉最南端的天狼星群露头之一，那里的宇宙形成年龄很少。初步测量表明，天狼星群辉石中3He和21Ne年龄测定存在很大差异。一种可能的解释是21Ne产量的组成依赖性。费拉尔白云岩中辉石矿分离物中3He， 21Ne和10Be的耦合测量将用于更好地约束生产速率，主量元素和微量元素依赖关系，方法的假设，并最终推进宇宙成因核素在南极基性岩性中的应用。本研究的主要目标是改进辉石中10Be的测量方案，并通过测量矿物组成（通过电子探针）和年轻熔岩流矿物对中的核素浓度来确定21Ne产率的组成依赖性。进一步的目的是通过测量3He/21Ne的产生比，结合对屏蔽样品的测量，验证氦的成核贡献和扩散损失的影响。对南极辉石中3He、21Ne和10Be的综合测量很少发表。这项研究的新的和独特的测量将提高原位产生的宇宙形成核素在年轻和古老南极表面的适用性。该研究将使用现有样本进行：不要求实地工作。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。