November 20, 2019

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Predicting Sedimentary Bedrock Subsurface Weathering Fronts and Weathering Rates

The new conceptual model coupling subsurface water table positions with weathering fronts of multiple minerals and components in sedimentary bedrocks. The water table remains approximately parallel to the soil surface along the hillslope. The deepest seasonal water table determines the pyrite weathering front. Carbonate minerals and fossil-organic matter share the same weathering front with pyrite.

[Reprinted under a Creative Commons Attribution 4.0 International License (CC BY 4.0) from Wan, J., et al. "Predicting Sedimentary Bedrock Subsurface Weathering Fronts and Weathering Rates." Scientific Reports 9 17198 (2019). DOI:10.1038/s41598-019-53205-2.]

The Science

For the first time, researcher directly determined subsurface bedrock weathering rates from in-situ measurements. The weathering front coincides with the depth of deepest seasonal water table for sedimentary bedrocks. Carbonates and rock organic matter share the same weathering front depth with pyrite, contrary to models that stratify their weathering fronts.

The Impact

This new conceptual model linked to subsurface hydrology makes predictions of bedrock weathering fronts and rates more feasible, and connects to water quality and climate change impacts. The approach can be applied to other settings of a watershed.

Summary

Although bedrock weathering strongly influences water quality and global carbon and nitrogen budgets, the weathering depths and rates within subsurface are not well understood nor predictable. Determination of both porewater chemistry and subsurface water flow are needed in order to develop more complete understanding and obtain weathering rates. In a long-term field study, researchers applied a multiphase approach along a mountainous watershed hillslope transect underlain by marine shale. Researchers found that the deepest extent of the water table determines the weathering front, and the range of annually water table oscillations determines the thickness of the weathering zone. Below the lowest water table, permanently water-saturated bedrock remains reducing, preventing deeper pyrite oxidation. Researchers also found that carbonate minerals and potentially rock organic matter share the same weathering front depth with pyrite, contrary to models where weathering fronts are stratified. Additionally, the measurements-based weathering rates from subsurface shale are high, amounting to base cation exports of about 70 kmolc ha−1 y−1, which is consistent with weathering of marine shale. By integrating geochemical and hydrological data, researchers presented a new conceptual model that can be applied in other settings to predict weathering and water quality responses to climate change.

Principal Investigator

Susan Hubbard
Lawrence Berkeley National Laboratory
[email protected]

Program Manager

Jennifer Arrigo
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
[email protected]

Funding

This work was conducted as part of the Watershed Function Scientific Focus Area at Lawrence Berkeley National Laboratory and was supported by the U.S. Department of Energy (DOE) Environmental System Science Program, DOE Office of Science, Biological and Environmental Research (BER) Program, under Contract Number DE-AC02-05CH11231.

References

Wan, J., et al. "Predicting Sedimentary Bedrock Subsurface Weathering Fronts and Weathering Rates." Scientific Reports 9 17198  (2019). https://doi.org/10.1038/s41598-019-53205-2.