Towards a chemical weathering rate chronometer: A U-series investigation of catena development in Kruger Park, South Africa

迈向化学风化速率计时仪：对南非克鲁格公园链式开发的 U 系列调查

• 批准号: 0819870
• 负责人: Andrew Kurtz
• 金额: $ 2.77万
• 依托单位: Trustees of Boston University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2008
• 资助国家: 美国
• 起止时间: 2008-09-01 至 2010-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0819870&HistoricalAwards=false
• 关键词: Towards; chemical; weathering; rate; chronometer

Scientific Merit: We propose a pilot study that will make a significant contribution towards the development of a new chemical weathering rate chronometer and address fundamental issues involving chemical weathering and soil development on hillslopes. We will measure U-series isotopes on carefully selected, well-characterized set of soil profiles sampled from the South African savanna. The soils comprise an exceptional example of a topographic catena sequence, where soils have differentiated through long-term downslope redistribution of solutes and weathering products. These sequences are only now being quantified by modern geochemical and geomorphic approaches. This represents a major step forward for geochemical studies of weathering profiles, which have traditionally focused on ridgetops or geomorphically stable surfaces, which represent a small fraction of Earth?s surface. Uranium-decay-series isotopes have emerged in the last decade as a powerful tracer of Earth processes on timescales shorter than one million years. The tool has been underexploited for studying surface processes, but recent studies have shown great promise. Most of these studies have approached weathering from the riverine perspective, seeking to quantify landscape-integrated weathering and erosion rates from U-series disequilibria in suspended sediments and river water. These studies inherently depend on an understanding of the processes that produce disequilibria by fractionating U-series elements (U, Th, and Ra) in soils, but this understanding is lacking. The few studies that have been done on U-series isotopes in soils often lack the geomorphic, pedogenic, and geochemical characterization required to develop robust interpretations of U-series data. They will measure trace element (including U, Th, and Nb) concentrations on 43 samples from 9 soil pits comprising a 1km transect of a well-characterized catena sequence. These data will be used to select 20 samples for full U-series disequilibria (234U/238U, 230Th/234U, and 226Ra/230Th) analyses. These isotope ratios are sensitive to trace element redistribution on timescales of roughly 1 ma, 500 ka, and 10 ka, respectively. These time-sensitive data, plus additional soil physical (textural, hydrologic) and chemical (mass balance) data will be used to develop and test box models of catena differentiation. Theit approach is in contrast to prior applications of U-series data to weathering profiles that have sought to determine soil ages, an approach that is overly sensitive to model assumptions. Instead, soil age here is constrained by existing cosmogenic 10Be data. The 10Be-derived ~220ka soil age in this environment is well-suited to the applicable time scale of U-series data. Broader Impact: This project will support lab costs for work initiated during the PI?s sabbatical at the Woods Hole Oceanographic Institution, where he will begin a new collaboration with WHOI scientists Bernhard Peucker-Ehrenbrink and Ken Sims. Importantly, the data collected and ideas developed here are intended to be applicable beyond a single set of soils in South Africa, and should improve our understanding of hillslope-scale weathering processes in general, and provide much-needed constraints on weathering fractionations for future studies of U-series isotopes in river studies. Based on data collected in this pilot proposal, the PI intends to submit a larger proposal that will support a future PhD student at BU to further develop U-series applications in the weathering environment.

科学价值：我们提出了一项试点研究，这将对开发一种新的化学风化速率计时器做出重大贡献，并解决涉及化学风化和山坡土壤发育的基本问题。我们将在精心挑选的、具有良好特征的南非稀树大草原土壤剖面样本上测量u系列同位素。土壤是地形连环序列的一个特殊例子，土壤通过长期的溶质和风化产物的下坡再分配而分化。这些序列直到现在才被现代地球化学和地貌学方法量化。这代表了风化剖面的地球化学研究向前迈出的重要一步，传统上，地球化学研究的重点是山脊顶或地貌稳定的表面，这些表面只占地球的一小部分。年代的表面。铀衰变系列同位素在过去十年中出现，作为地球过程的有力示踪剂，其时间尺度短于100万年。该工具尚未被充分利用来研究表面过程，但最近的研究显示出很大的希望。这些研究大多是从河流的角度来研究风化，试图通过悬浮沉积物和河水中的u系列不平衡来量化景观综合风化和侵蚀速率。这些研究本质上依赖于对土壤中U系列元素（U， Th和Ra）分选产生不平衡过程的理解，但这种理解是缺乏的。对土壤中铀系同位素进行的少数研究往往缺乏对铀系数据进行可靠解释所需的地貌、成土和地球化学特征。他们将测量来自9个土壤坑的43个样品的微量元素（包括U、Th和Nb）浓度，这些土壤坑包括一个特征良好的连环序列的1公里样带。这些数据将用于选择20个样品进行全u系列不平衡（234U/238U， 230Th/234U和226Ra/230Th）分析。这些同位素比值分别在大约1 ma、500 ka和10 ka的时间尺度上对微量元素的再分布敏感。这些对时间敏感的数据，加上额外的土壤物理（质地、水文）和化学（质量平衡）数据，将用于开发和测试连环分化的盒子模型。他们的方法与之前将u系列数据应用于风化剖面的方法形成对比，后者试图确定土壤年龄，这种方法对模型假设过于敏感。相反，这里的土壤年龄受到现有宇宙成因10Be数据的限制。该环境下10be导出的~220ka土壤年龄与u系列资料的适用时间尺度非常吻合。更广泛的影响：该项目将支持在项目计划期间启动的工作的实验室成本。他将在伍兹霍尔海洋研究所休假，在那里他将与WHOI的科学家Bernhard Peucker-Ehrenbrink和Ken Sims开始一项新的合作。重要的是，这里收集的数据和发展的想法旨在适用于南非的单一土壤，并且应该提高我们对山坡尺度风化过程的总体理解，并为未来河流研究中u系列同位素的风化分馏提供急需的约束。基于该试点提案中收集的数据，PI打算提交一份更大的提案，以支持未来BU的博士研究生进一步开发u系列在风化环境中的应用。