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.

科学优点：我们提出了一项试点研究，该研究将为开发新的化学风化速率计时仪做出重大贡献，并解决涉及山坡上化学风化和土壤发展的基本问题。我们将在精心挑选的，良好的特征化土壤概况中测量U系列同位素，从南非稀树草原采样。土壤构成了地形CATENA序列的一个特殊例子，其中土壤通过长期下坡重新分布的溶质和风化产品而分化。这些序列仅通过现代地球化学和地貌方法才能量化。这是对风化曲线的地球化学研究的重要一步，这些研究传统上专注于Ridgetops或地球形态稳定的表面，这些表面代表了地球表面的一小部分。在过去的十年中，铀 - 赛车系同位素同位素已成为时代尺度的强大示踪剂，超过一百万年。该工具已被毫无用处地研究表面过程，但是最近的研究表现出了很大的希望。这些研究中的大多数从河流的角度开始接近风化，旨在量化U系列中景观集成的风化和侵蚀速率，使悬浮的沉积物和河水中的水平不平衡。这些研究固有地取决于对土壤中U系列元素（U，TH和RA）分离的过程的理解，但是缺乏这种理解。关于土壤中U系的同位素进行的少数研究通常缺乏开发对U系列数据的强大解释所需的地貌，儿子和地球化学表征。他们将在93个土壤坑中的43个样品上测量痕量元件（包括U，Th和NB），其中包含一个良好的CATENA序列的1公里样品。这些数据将用于选择20个样本，以使完整的U系列不平衡（234U/238U，230th/234U和226RA/230th）分析。这些同位素比分别对大约1 mA，500 ka和10 ka的时间尺度上的跟踪元件重新分布敏感。这些时间敏感的数据以及其他土壤物理（纹理，水文）和化学（质量平衡）数据将用于开发和测试CATENA分化的模型。与试图确定土壤年龄的u系列数据的先前应用相反，这是对模型假设过度敏感的方法。取而代之的是，这里的土壤年龄受到现有的宇宙基因10BE数据的约束。在这种环境中，衍生的〜220KA土壤年龄非常适合适用的U系列数据的时间尺度。更广泛的影响：该项目将支持在伍兹霍尔海洋学院休假期间发起的工作的实验室成本，在那里他将与WHOI科学家Bernhard Peucker-Ehrenbrink和Ken Sims开始新的合作。重要的是，此处开发的数据和想法旨在超越南非的一组田，并应提高我们对山坡尺度的风化过程的理解，并在河流研究中对U-Series同进的未来研究提供急需的限制。根据该试点建议中收集的数据，PI打算提交一个更大的建议，该提案将支持BU的未来博士生，以进一步在风化环境中开发U系列应用程序。