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

供给源头溪流的山坡中硅酸盐岩石的风化作用决定了排水集水区的化学特征、从大陆到海洋的物质运输以及大气CO2和陆地表面之间的关键反馈。然而，定量模型之间的基本关系的水从景观排放率（Q）和溶质浓度（C）在水中仍然是一个重大的挑战。这部分是由于基岩的溶解作用和新的次生矿物的形成之间的密切耦合，我们称之为硅酸盐风化。这种不确定性的核心是一个实际问题：硅酸盐风化的速度很慢。这意味着在实验室中建立的典型的流通柱甚至不能捕获高地流域硅酸盐风化的简化表示。相比之下，天然山坡是复杂的，难以约束。在这项工作中，研究人员将克服这种差异，使用独特的中尺度景观演变观测台（LEO），它提供了三个复制收敛山坡上建造的世界上最大的称重蒸渗仪。LEO设施位于生物圈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实例和演示，每年有超过10万名公众参观。这包括10，000名使用生物圈2号作为STEM课程的学生。该项目将培训两名博士生，从而形成一个跨三个机构的合作研究小组，并制作“展示”项目，作为生物圈2号教育之旅的一部分，包括有关工作目的和状态的信息。最后，本项目开发和校准的反应性输运模拟将作为当前NSF研究协调网络的一个例子：地球科学数值模拟的社区教育基础设施。即使在完全均匀的系统中，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，这些反应通过经受不稳定渗透的不均匀流动路径发生。因此，一个关键的需要，以推进基于过程的C-Q关系的理解是提供额外的约束，嵌入在同一模型框架内，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沉淀之间的关系。具体来说，他们将对硅同位素（δ 30 Si）和锗硅比（Ge/Si）进行配对，它们各自对风化系统中次生矿物形成的速率和性质非常敏感，通过扩展到C-R-Q（浓度-同位素/元素比-放电）框架来揭示有助于C-Q观测的次生沉淀反应的平衡。目前，描述三角洲30 Si和Ge/Si在二次矿物生长过程中的分配的参数的实验室表征研究正在扩大，以及这些比率与放电在现场规模的数据集。然而，在将该信息与受约束的流体通过时间分布跨流通系统配对以验证所观察到的行为的适当模型表示方面存在关键差距。这一差距取决于业务限制。硅酸盐水-岩石相互作用的缓慢风化速率阻碍了在合理尺度下使用标准流通柱设计，而自然系统的复杂性限制了开发反应性和流体旅行时间之间的约束关系的能力。在这里，他们将使用LEO并采用一种新的通量加权时间方法来约束整个系统的瞬态流体旅行时间分布。通过这种独特的实验设施，新颖的瞬态旅行时间约束，反应传输模型和（伪）同位素示踪剂的组合，他们相信在过程级的表示和预测的C-R-Q关系的变革性进步是可以实现的。这个奖项反映了NSF的法定使命，并已被认为是值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估的支持。

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.