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

第一部分：非技术性南极冰盖构成了地球上最大的冰块，仅东南极冰盖就储存了大约53米的海平面当量。因此，东南极冰盖的历史对于了解和预测海平面和地球气候的变化很重要。关于南极东部冰盖在过去两千万年中的长期稳定性，有相互矛盾的证据。为了更好地了解过去的冰盖变化以及横贯北极山脉的历史，需要准确的时间尺度。记录过去冰盖变化的为数不多的适用于南极冰川沉积物的测年方法之一，是测量地表岩石样品中宇宙射线产生的稀有同位素，即宇宙成因核素。当岩石表面暴露/无遮盖时，宇宙射线会在矿物中产生稀有的同位素，如氦-3、铍-10和霓21。该项目将包括测量横贯北极山脉高海拔地区发现的一些最古老的冰川沉积物中的所有三个同位素。由于每种同位素的含量与曝光时间直接相关，因此可以用来计算表面的年龄。这种方法需要知道宇宙辐射产生每种同位素的速率，这取决于矿物成分，目前这是该方法的局限性。该项目的目标是改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，扩大表面测年的可能性。辉石是一种常见于横贯北极山脉的矿物。这一技术进步将使地表暴露测年方法得到更好的应用，这反过来又将有助于重建南极冰盖历史，并提供有关以前冰盖范围的宝贵知识。了解南极洲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：技术描述南极表层岩石样品中原位产生的宇宙成因核素的测量为冰川和地貌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，含辉石和分布广泛的岩性，如横贯北极山脉的铁拉辉绿岩，没有得到充分利用。这项提议旨在改变这一状况，并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。拟议的方法改进将直接适用于重要冰川沉积的侵蚀速度和沉积年龄，如有争议的天狼星群TILS，也适用于较年轻的冰川特征。Bennett地台是本次研究的重点，因为它是横贯北极山脉最南端的天狼星群露头之一，宇宙成因年龄稀少。初步测量表明，天狼星群辉石的3He和21Ne年龄测定结果存在较大差异。一种可能的解释是21Ne的生产率与组成有关。从铁质辉石中分离出来的辉石矿物中的~3He、~(21)Ne和~(10)Be的联合测量将被用来更好地约束产率、主元素和微量元素的相关性以及该方法的假设，并最终推进宇宙成因核素在南极镁铁质岩性中的应用。这项研究的主要目的是改进辉石中10Be的测量方案，并通过测量矿物成分(电子探针)和年轻熔岩流中矿物对中的核素浓度来确定21Ne产率与组成的关系。进一步的目的是通过测量3He/21Ne的产额比，结合对铁铝榴辉岩屏蔽样品的测量，来确认氦的成核贡献和氦扩散损失的影响。在南极洲，辉石中3He、21Ne和10Be的联合测量很少发表单独的样品。这项研究的新的和独特的测量将促进原位产生的宇宙核素在年轻和古老的南极表面的适用性。这项研究将使用现有的样本进行：不要求进行实地工作。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

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.