Anisovolumetric weathering is the norm, not the exception, in granitic saprolite
Abstract
Erosion at Earth's surface exposes underlying bedrock to climate-driven chemical and physical weathering, transforming it into a porous, ecosystem-sustaining substrate consisting of weathered bedrock, saprolite, and soil. Weathering in saprolite is typically quantified from bulk geochemistry assuming physical strain is negligible. But recent modeling and measurements suggest that strain in saprolite may be more common than previously appreciated and therefore that anisovolumetric weathering may be widespread. To explore this possibility, we quantified the fraction of porosity produced by physical weathering, F PP , at three sites in granitic bedrock of the Sierra Nevada, California. Elevation and thus climate vary across the sites, with a dry, warm, oak-savanna ecosystem at the low elevation site; a wetter, cooler, oak-conifer forest at the mid elevation site; and a wet, cold, conifer forest at the high elevation site. We found that strain produces more porosity than chemical mass loss at all three sites, indicative of strongly anisovolumetric weathering. This highlights the potential importance of a variety of climate-driven, strain-inducing mechanisms, including seasonal freezing, root wedging, and biotite expansion. To determine if anisovolumetric weathering is also common in landscapes outside the Sierra Nevada, we quantified F PP using volumetric strain and mass loss data from previous work at granitic sites spanning a wider range of climates and erosion rates. Our analysis shows that F PP > 0.12 and thus that weathering is anisovolumetric at all sites in the data compilation. Multiple regression shows that 94% of the variance in F PP can be explained by differences in average precipitation and erosion rate, and that > 98% of Earth's land surface has conditions that promote anisovolumetric weathering in granitic saprolite. This indicates that anisovolumetric weathering is the norm, rather than the exception, and highlights the importance of climate and erosion as drivers of subsurface physical weathering.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMEP0360004C
- Keywords:
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- 1813 Eco-hydrology;
- HYDROLOGY;
- 1824 Geomorphology: general;
- HYDROLOGY;
- 1825 Geomorphology: fluvial;
- HYDROLOGY;
- 1826 Geomorphology: hillslope;
- HYDROLOGY