Collaborative Research: EAR-Climate: Physical Controls on CO2 Release from Shale Weathering

Board Of Regents, Nshe, Obo University Of Nevada, Reno, Reno NV

Investigators

Abstract

Shales, commonly found sedimentary rocks, contain a large amount of organic carbon and have been mined for oil, natural gas, and other fossil fuels. Analogous to how the burning of fossil fuels releases carbon dioxide (CO2) to the atmosphere, the natural weathering of shale also releases CO2. While this CO2 release occurs slowly, it has potential to change Earth’s climate over million-year timescales. The scientific community currently lacks understanding about how shale weathering (and associated CO2 release) occurs, and this limits understanding of both past changes in Earth’s climate and predictions of future changes. This project examines how changes in climate and erosion influence the rate of shale weathering via performing a detailed field study in shale rock exposed throughout California. Field work results will be incorporated into a mathematical model that will allow estimates of the rate of CO2 release from shale weathering across the globe. This project will thus both advance understanding of an important natural control on Earth’s climate, and provide a framework to improve predictions of climate change in the future. In addition to these benefits, the project will also provide training for graduate and undergraduate students and project members will engage in K-8 and community outreach to provide geoscience education. Despite previous work on the chemical and biological processes that drive shale weathering, there does not exist a mechanistic understanding of how physical processes modulate CO2 release from shale weathering. This project addresses this knowledge gap by quantifying how variations in physical erosion rate, precipitation rate, and local topography influence shale weathering. The project tests the hypothesis that feedbacks between the supply of carbon during conversion of rock to regolith, chemical kinetics, and topographic controls on weathering zone thickness cause CO2 release from shale weathering to be maximized for areas with modest erosion rates, modest precipitation rates, and high topographic curvature (i.e., ridges). To accomplish this, the researchers will measure the loss of organic carbon in depth-profiles of shale up to 10 m deep, focusing on shales of the Monterey, Rincon, and Cozy Dell formations in the Santa Ynez Mountains, California, where a >6-fold erosion gradient allows assessment of how variation in erosion influences shale weathering. The Santa Ynez Mountain samples will be supplemented with depth profiles of Monterey Shale from Point Reyes National Seashore and Carrizo Plain, California, allowing exploration of shale weathering over a 5-fold gradient in precipitation within the same lithologic formation. The field data will be used to calibrate and modify a reactive-transport model based on physical forcing, thereby providing new opportunities to link geomorphic transport laws with biogeochemical models, and predict CO2 release from shale weathering across a wide range of spatial and temporal scales. Documenting links between physical processes and silicate weathering has led to major advances in the understanding of feedbacks between climate, tectonics, and topography, and documenting such tradeoffs for shale weathering in this project is a logical, yet critical next step to advance understanding of the geologic carbon cycle. 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.