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

科学价值：我们提出了一个试点研究，将作出重大贡献的发展，一个新的化学风化速率计时器，并解决基本问题，涉及化学风化和山坡上的土壤发育。我们将在南非大草原上精心挑选的、特征良好的土壤剖面上测量铀系列同位素。土壤包括一个特殊的例子，地形链序列，土壤通过长期下坡溶质和风化产物的再分布分化。这些序列现在才被量化的现代地球化学和地貌的方法。这代表了风化剖面的地球化学研究向前迈出了重要一步，传统上风化剖面的研究集中在山脊顶或地貌稳定的表面上，而这些表面只占地球的一小部分。s表面。铀衰变系列同位素是在过去十年中出现的，作为一种强有力的地球过程示踪剂，其时间尺度短于100万年。该工具在研究表面过程方面尚未得到充分利用，但最近的研究显示出很大的希望。这些研究中的大多数已经接近从河流的角度来看，试图量化的U系列不平衡在悬浮沉积物和河水中的风化和侵蚀速率的累积。这些研究本质上依赖于对土壤中U系元素（U、Th和Ra）分馏产生不平衡的过程的理解，但缺乏这种理解。对土壤中U系列同位素的少数研究往往缺乏对U系列数据进行可靠解释所需的地貌、成壤和地球化学特征。他们将测量来自9个土坑的43个样品的微量元素（包括U，Th和Nb）浓度，这些土坑包括一个具有良好特征的链状序列的1公里横断面。这些数据将用于选择20个样品进行完整的U系列不平衡（234 U/238 U，230 Th/234 U和226 Ra/230 Th）分析。这些同位素比值是敏感的微量元素再分配的时间尺度上大约1万年，500万年，和10万年，分别。这些时间敏感的数据，加上额外的土壤物理（质地，水文）和化学（质量平衡）的数据将被用来开发和测试连锁分化箱模型。Theit的方法是在对比以前的应用程序的U系列数据风化剖面，试图确定土壤年龄，一种方法，是过于敏感的模型假设。相反，这里的土壤年龄受到现有宇宙成因10 Be数据的限制。该环境下的~ 220 ka土壤年龄与U系列数据的适用时间尺度相吻合。更广泛的影响：该项目将支持PI期间启动的工作的实验室成本？他在伍兹霍尔海洋研究所休假，在那里他将开始与世界卫生组织科学家伯恩哈德·皮克-埃克林克和肯·西姆斯进行新的合作。重要的是，这里收集的数据和开发的想法是为了适用于南非的一组土壤之外，并应提高我们对山坡尺度风化过程的理解一般，并提供急需的约束风化分馏为未来的研究在河流研究中的U系列同位素。根据在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BU未来的博士生进一步开发U系列在风化环境中的应用。

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.