CAREER: A Hydrologic Thermostat for the Global Carbon Cycle?

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The chemical weathering of minerals moderates the concentration of carbon dioxide in the atmosphere, the supply of key nutrients to terrestrial and aquatic ecosystems and the release of naturally occurring contaminants. As such, chemical weathering is a critical component of the Earth system. The movement of water through soils and hillslopes is an important control on overall chemical weathering rates, and the resulting connection between the movement of water and chemical reactions in the subsurface is thus critical for understanding not only Earth's history, but also how Earth's systems will respond to future climatic and anthropogenic changes. However, current models cannot predict the observed export of dissolved solutes from landscapes. To address this problem, this project will use a combination of field studies and reactive transport modeling approaches to determine (1) how the competition between the rates of chemical reactions and solute transport dictates concentration-discharge relationships in rivers and the resulting chemical fluxes, (2) the consequences of weathering reactions and fluid mixing on the stable and radiogenic isotopic composition of waters, and (3) the operation of a 'hydrologic thermostat' that governs global scale chemical weathering rates, and by extension atmospheric carbon dioxide levels, over geologic timescales. Evaluation of the latter is an overarching goal of this project that has the potential to provide new information about the mechanism underlying one of the most profound features of Earth's history: its long-term temperature stability. Students from diverse backgrounds and across different levels will benefit from the exposure to field measurement, data synthesis and numerical modeling during the course of the project. The reactive transport approach is powerful tool for understanding the chemical evolution of Earth's environments. Nevertheless, there are very few opportunities for professional training in this area at or beyond the graduate school level, although reactive transport approaches are widely used in both industry and research. A major educational goal of this project is to build a solid conceptual understanding of reactive transport approaches and models by offering a yearly short-course on reactive transport at Stanford University for graduate students, postdoctoral fellows and faculty from a broad spectrum of U.S. institutions. The course will serve as an introduction to key reactive transport concepts and their application to biogeochemical systems, and will thus provide an entry point into the field and access to a growing community of people that use and develop these models. Each year the course will include several instructors and provide an opportunity for participants to discuss their research and receive feedback and recommendations. The research proposed here will also be featured as examples, and the graduate students from my research group will play key roles in continuing to develop and improve the course, gaining experience as instructors that will carry over into their future careers.