Predicting sedimentary bedrock subsurface weathering fronts and weathering rates

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 com...

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Bibliographic Details
Published in:Scientific reports 2019-11, Vol.9 (1), p.17198-10, Article 17198
Main Authors: Wan, Jiamin, Tokunaga, Tetsu K, Williams, Kenneth H, Dong, Wenming, Brown, Wendy, Henderson, Amanda N, Newman, Alexander W, Hubbard, Susan S
Format: Article
Language:English
Subjects:
Climate change
element cycles
GEOSCIENCES
hydrology
Minerals
Organic matter
Oscillations
Oxidation
Pyrite
Shale
Water flow
Water quality
Water table
Weathering
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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, we applied a multiphase approach along a mountainous watershed hillslope transect underlain by marine shale. Here we report three findings. First, 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. Secondly, carbonate minerals and potentially rock organic matter share the same weathering front depth with pyrite, contrary to models where weathering fronts are stratified. Thirdly, the measurements-based weathering rates from subsurface shale are high, amounting to base cation exports of about 70 kmol ha y , yet consistent with weathering of marine shale. Finally, by integrating geochemical and hydrological data we present a new conceptual model that can be applied in other settings to predict weathering and water quality responses to climate change.
ISSN:2045-2322
2045-2322
DOI:10.1038/s41598-019-53205-2