CAREER: Unlocking the Isotopic Signatures of Weathering Recorded in Rivers Through Isotope-Enabled Reactive Transport

水通过固体地球最浅的层的渗透对维持地球上的生命至关重要。提供给土壤的水分灌溉了我们种植的作物，并维持了自然生态系统。深入地下的水重新补给含水层，并为溪流和河流提供基流。这种穿过浅层地球的凌日还引入了水作为一种反应剂，能够风化构成地壳的矿物，允许将岩石转化为土壤的化学变化，并确定了水资源的化学特征。因此，水是这个“临界区”的一个组成部分，也就是我们赖以生存和获取资源的地球表面。这种重要的转变在很大程度上是隐藏在直接研究之外的，因为它们发生在我们脚下的近地表。因此，我们依靠描述流体通过临界区的速度和路径的模型，再加上水、矿物和生命之间发生的化学反应，来预测我们无法直接观察到的情况。然后，将这些模型与更容易获得的样本进行对比，通常使用排泄分水岭的溪流和河流的化学物质。利用新的仪器和模型开发，该项目将首次基于从土壤到河流的过渡过程中收集的流体和固体对这些反应传输模型进行直接验证。这项研究的更广泛影响包括改进水质和临界区功能的化学特征的预测模型。开发和部署这种模型的能力对于推进流域管理和临界区科学至关重要。为了实现这一目标，该项目将创建一个开放获取的在线培训平台，旨在教育下一代地球科学家开发最先进的反应性运输模型。教学模块将作为NSF-RCN奖项的一部分进行部署，并将以符合ADA标准的网页设计为特色，包括自我指导和课堂整合选项。这一资源将促进下一代定量模型的广泛使用和公平利用，并促进全球临界区社区的国际合作。临界区风化剖面中流体输送和化学反应之间的紧密耦合的预测模型仍然在很大程度上根据容易获得的观测数据进行验证，例如涌现为泉水和流向溪流和河流的基流的流体。这些流体流失景观的综合特征提供了测试正演模型的重要手段，目前越来越重视河流稳定同位素比率(例如7Li、30Si、27 mg、44Ca)的使用，因为它们对化学风化的特定成分具有极高的敏感性。将这些有希望的工具集成到反应传输模拟中，可能是建立高保真预测模型的关键，该模型用于化学和流体流动的耦合，排出完整的风化剖面。这一提议将在实现这一目标方面取得三大进展：(1)作为流体旅行时间函数的溶质同位素特征的定量解释；(2)在渗透和排放之间的风化带中验证同位素特征；(3)通过河流排水流域的同位素比率记录的风化过程的预测模型。这将通过利用一系列实验室柱实验来实现，该实验旨在支持模型开发，进而应用于一种新的现场规模的仪器能力，该能力允许在临界区的部分饱和部分直接收集流体样本，在该区域，流体通过风化层排出，然后作为基流涌出到溪流中。这项工作的结果将测试一组假设，这些假设将理论与化学风化产生的溶质中的同位素比率的观测联系起来。这一裁决反映了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。

The percolation of water through the shallowest layers of the solid Earth is vital to sustaining life on this planet. Moisture delivered to soil irrigates the crops we grow and sustains natural ecosystems. Water that reaches further into the subsurface recharges aquifers and supplies the baseflow to streams and rivers. This transit through the shallow Earth also introduces water as a reactive agent capable of weathering the minerals that compose the crust, allowing chemical transformations that convert rock to soil and set the chemical signature of water resources. Thus, water is an integrator of this “critical zone”, i.e., the surface of the Earth where we live and draw resources. Such vital transformations are largely hidden from direct study because they occur below our feet in the near surface. Thus, we rely upon models that describe the rates and pathways of fluid drainage through the critical zone, coupled to the chemical reactions that occur between water, minerals and life, to predict what takes place where we cannot make direct observations. These models are then checked against more accessible samples often using the chemistry of streams and rivers which drain watersheds. Using novel instrumentation and model development, this project will offer the first direct validation of these reactive transport models based on fluids and solids collected within the transition from soil to stream. The broader impacts of this study include the improvement of predictive models for the chemical signatures of water quality and critical zone functioning. The capability to develop and deploy such models is vital to the advancement of watershed management and critical zone science. In pursuit of this goal, the project will create an open access online training platform designed to educate the next generation of Earth scientists in the development of state-of-the-art reactive transport models. The teaching modules will be deployed as part of an NSF-RCN award and will feature ADA-compliant web design with both self-guided and classroom integration options. This resource will foster the expanded use and equitable availability of next generation quantitative models and promote international collaborations across the global critical zone community.Predictive models for the tight coupling between fluid transport and chemical reactivity in critical zone weathering profiles remain largely validated against easily accessible observations such as the fluids that emerge as spring water and baseflow to streams and rivers. These integrated signatures of fluid draining landscapes offer a vital means of testing forward models and an increasing emphasis is now being placed on the use of riverine stable isotope ratios (e.g., 7Li, 30Si, 27Mg, 44Ca) for their exquisite sensitivity to specific components of chemical weathering. Integration of these promising tools into reactive transport simulations could hold the key to high fidelity predictive models for the coupling of chemistry and fluid flow draining intact weathering profiles. This proposal will generate three major advancements towards this goal: (1) quantitative interpretation of solute isotope signatures as a function of fluid travel time; (2) validation of isotope signatures in the zone of weathering between infiltration and discharge; and (3) predictive models for the weathering processes recorded by the isotope ratios of rivers draining watersheds. This will be accomplished through leveraging of a series of laboratory column experiments designed to support model development and in turn application to a novel field-scale instrumentation capability which allows direct collection of fluid samples in the partially saturated section of the critical zone where fluid drains through regolith before emerging as the baseflow to streams. The results of this work will test a set of hypotheses that link theory to observations of isotope ratios in the solutes derived from chemical weathering.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.