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：我们提出了一项初步研究，将对开发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。我们将在从南非稀树大草原采集的精心挑选的、特征良好的土壤剖面上测量U系列同位素。这些土壤构成了地形连带序列的一个特例，土壤通过长期向下重新分布溶质和风化产物而发生了分化。直到现在，这些层序才被现代地球化学和地貌学方法量化。这标志着风化剖面的地球化学研究向前迈进了一大步，传统上，风化剖面研究的重点是山脊或地貌稳定的表面，它们代表了地球上S表面的一小部分。铀衰变系列同位素在过去十年中作为一种强大的示踪剂出现在时间尺度上不到一百万年的地球过程中。该工具在研究表面过程方面一直未得到充分利用，但最近的研究显示出了巨大的前景。这些研究大多从河流角度探讨风化作用，试图从悬浮沉积物和河水中的U系列不平衡来量化综合地貌的风化和侵蚀速率。这些研究本质上依赖于对通过在土壤中分馏U系列元素(U、Th和Ra)而产生不平衡的过程的理解，但这种理解是缺乏的。对土壤中铀系列同位素所做的少数研究往往缺乏对铀系列数据进行可靠解释所需的地貌、成壤作用和地球化学特征。他们将测量来自9个土坑的43个样品的痕量元素(包括U、Th和Nb)的浓度，这些土坑包括一个描述良好的链状序列的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约1 Ma、500ka和10ka的时间尺度上对痕量元素的再分配十分敏感。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)数据，将被用来开发和测试链状植物分化的盒子模型。这一方法与之前将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学10Be数据的限制。该环境下的10Be~220ka土壤年龄与U系列数据的适用时间尺度非常吻合。更广泛的影响：该项目将资助皮？S在伍兹霍尔海洋研究所休假期间启动的工作的实验室费用，在那里他将开始与世界卫生组织科学家Bernhard Peucker-Ehrenbrink和Ken Sims的新合作。重要的是，这里收集的数据和形成的想法意在适用于南非单一的一组土壤，并应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系列同位素的研究提供急需的风化分馏限制。根据这一试点提案中收集的数据，PI打算提交一份更大的提案，以支持波士顿大学未来的博士生在气候变化环境中进一步开发U系列应用。