Collaborative Research: Concentration - Ratio - Discharge (C-R-Q) relationships of transient water-age distributions

供给水源的山坡中硅酸盐岩石的风化决定了排水集水区的化学特征，质量从大陆到海洋的运输，以及大气二氧化碳和陆地表面之间的关键反馈。然而，建立定量模型来描述某一景观的排水率(Q)和该水域内的溶质浓度(C)之间的基本关系，仍然是一个重大挑战。这在一定程度上是由于基岩的溶解作用和新次生矿物的形成之间的紧密耦合，我们称之为硅酸盐风化作用。这种不确定性的核心是一个实际问题：硅酸盐的风化速度很慢。这意味着，实验室中建立的典型流动柱甚至不能捕捉到高地流域硅酸盐风化的简化表示。相比之下，天然山坡比较复杂，很难约束。在这项工作中，研究人员将使用独特的中尺度景观演变天文台(LEO)来克服这种差异，该天文台提供了三个重复的会聚山坡，这些山坡建在世界上最大的称重蒸渗仪上。LEO设施设在生物圈2号中心内，该中心将地球系统科学研究转化为易于处理的例子和演示，每年有超过10万名公众参观者。这包括将生物圈2作为其STEM课程的一部分的10,000名学生。该项目将培训两名博士生，从而组成一个跨三个机构的合作研究小组，并制作“展出”项目，作为生物圈2号教育之旅的一部分，其中包括关于工作目的和现状的信息。最后，该项目开发和校准的反应性运输模拟将作为当前NSF研究协调网络的一个例子：用于地球科学数值模拟的社区教育基础设施。即使在完全均匀的体系中，这些串联溶解和沉淀速率的无限组合也可能导致相同的溶质浓度。此外，这些反应通过受非稳定渗透影响的非均匀流动路径发生。因此，推进对C-Q关系的基于过程的理解的一个关键需要是提供嵌入在同一模型框架内的附加约束，并减少自由参数的数量。在这里，研究人员将利用稳定同位素和微量元素比值的特征变化来诊断原生硅酸盐风化和次生矿物降水之间的关系。具体地说，它们将配对硅同位素(德尔塔30Si)和锗硅比(Ge/Si)，每一种都对风化系统中次生矿物形成的速度和性质唯一敏感，通过扩展到C-R-Q(浓度-同位素/元素比-放电)框架，揭示有助于C-Q观测的二次沉淀反应的平衡。目前，描述次生矿物生长过程中三角洲30Si和Ge/Si分配的参数的实验室表征研究正在扩大，以及这些比率与野外规模的流量之间的数据集。然而，在将这些信息与受限的流体通过时间分布配对以验证观察到的行为的适当模型表示方面存在关键差距。这一差距取决于操作限制。硅酸盐水-岩石相互作用的缓慢风化速度阻碍了合理规模的标准过流柱设计的使用，而自然系统的复杂性限制了在反应性和流体旅行时间之间建立受限关系的能力。在这里，他们将使用LEO，并使用一种新的流量加权时间方法来限制整个系统中的瞬时流体旅行时间分布。通过这种独特的实验设备、新的瞬时旅行时间约束、反应传输建模和(伪)同位素示踪剂的组合，他们认为在C-R-Q关系的过程级别表示和预测方面取得了革命性的进步。该奖项反映了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 weathering of silicate rock in the hillslopes that feed headwater streams sets the chemical characteristics of water draining catchments, the transport of mass from continents to oceans and critical feedbacks between atmospheric CO2 and the land surface. Yet quantitative models for the basic relationship between the rate of water discharge from a landscape (Q) and the concentration of solutes (C) within that water remains a significant challenge. This is in part due to close coupling between the solubilization of bedrock and the formation of new secondary minerals, which we term silicate weathering. At the core of this uncertainty is a practical issue: the rates of silicate weathering are slow. This means that typical flow-through columns built in laboratories cannot capture even a simplified representation of silicate weathering in upland watersheds. In contrast, natural hillslopes are complicated and difficult to constrain. In this work, investigators will overcome this disparity using the unique mesoscale Landscape Evolution Observatory (LEO), which affords three replicate convergent hillslopes constructed on the world's largest weighing lysimeters. The LEO facility is housed within the Biosphere 2 center, which translates Earth system science research into tractable examples and demonstrations for over 100,000 public visitors per year. This includes 10,000 students who use the Biosphere 2 as part of their STEM curriculum. The project will train two PhD students, thus forming a collaborative research group across three institutions, and produce 'on-display' projects as part of the Biosphere 2 educational tour including information about the purpose and status of the work. Finally, the reactive transport simulations developed and calibrated by this project will be leveraged as an example for a current NSF Research Coordination Network: Community-based educational infrastructure for numerical simulation in the Earth Sciences. Even in a perfectly homogeneous system, an infinite combination of these tandem dissolution and precipitation rates could lead to the same solute concentration. Further, these reactions occur through non-uniform flow paths subject to unsteady infiltration. Thus, a critical need to advance process-based understanding of the C-Q relationship is the provision of additional constraints which embed within the same model framework and reduce the number of free parameters. Here, the researchers will use the characteristic shifts in stable isotope and trace element ratios to diagnose the relationship between primary silicate weathering and secondary mineral precipitation. Specifically, they will pair silicon isotopes (delta 30Si) and germanium-silicon ratios (Ge/Si), which are each uniquely sensitive to the rate and nature of secondary mineral formation in weathering systems, to unmask the balance of secondary precipitation reactions contributing to C-Q observations through expansion to a C-R-Q (concentration – isotope/element ratio – discharge) framework. At present, laboratory characterization studies of the parameters which describe partitioning of delta 30Si and Ge/Si during secondary mineral growth are expanding, as well as datasets of these ratios versus discharge at the field scale. Yet a critical gap existing in pairing this information across a flow-through system with constrained fluid transit time distributions to verify appropriate model representation of observed behavior. This gap is contingent upon operational limitations. The slow weathering rates of silicate water-rock interactions impede the use of standard flow-through column designs at reasonable scales, while the complexity of natural systems limits the capacity to develop constrained relationships between reactivity and fluid travel time. Here, they will use LEO and employ a novel flux-weighted time approach to constrain transient fluid travel time distributions across the system. Through this combination of unique experimental facility, novel transient travel time constraint, reactive transport modeling, and (pseudo)isotopic tracers, they believe that a transformative advancement in process-level representation and prediction of C-R-Q relationships is achievable.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.