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The rate of chemical weathering has been observed to increase with the rate of physical erosion in published comparisons of many catchments, but the mechanisms that couple these processes are not well understood. We investigated this question by exam- ining the chemical weathering and porosity profiles from catchments developed on marine shale located in Pennsylvania, USA (Susquehanna Shale Hills Critical Zone Observatory, SSHCZO); California, USA (Eel River Critical Zone Observatory, ERC- ZO); and Taiwan (Fushan Experimental Forest). The protolith compositions, protolith porosities, and the depths of regolith at these sites are roughly similar while the catchments are characterized by large differences in erosion rate (1–3 mm yr1 in Fushan 0.2–0.4 mm yr1 in ERCZO 0.01–0.025 mm yr1 in SSHCZO). The natural experiment did not totally isolate erosion as a variable: mean annual precipitation varied along the erosion gradient (4.2 m yr1 in Fushan > 1.9 m yr1 in ERCZO > 1.1 m yr1 in SSHCZO), so the fastest eroding site experiences nearly twice the mean annual temperature of the other two. Even though erosion rates varied by about 100, the depth of pyrite and carbonate depletion (defined here as regolith thickness) is roughly the same, consistent with chemical weathering of those minerals keeping up with erosion at the three sites. These minerals were always observed to be the deepest to react, and they reacted until 100% depletion. In two of three of the catchments where borehole observations were available for ridges, these minerals weathered across narrow reaction fronts. On the other hand, for the rock-forming clay mineral chlorite, the depth interval of weathering was wide and the extent of depletion observed at the land surface decreased with increasing erosion/precipitation. Thus, chemical weathering of the clay did not keep pace with erosion rate. But perhaps the biggest difference among the shales is that in the fast-eroding sites, microfractures account for 30–60% of the total porosity while in the slow-eroding shale, dissolution could be directly related to secondary porosity. We argue that the microfractures increase the influx of oxygen at depth and decrease the size of diffusion-limited internal domains of matrix, accelerating weathering of pyrite and carbonate under high erosion-rate condi- tions. Thus, microfracturing is a process that can couple physical erosion and chemical weathering in shales.
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Deep abiotic weathering of pyrite
Pyrite is a ubiquitous iron sulfide mineral that is oxidized by trace oxygen. The mineral has been largely absent from global sediments since the rise in oxygen concentration in Earth’s early atmosphere. We analyzed weathering in shale, the most common rock exposed at Earth’s surface, with chemical and microscopic analysis. By looking across scales from 10 −9 to 10 2 meters, we determined the factors that control pyrite oxidation. Under the atmosphere today, pyrite oxidation is rate-limited by diffusion of oxygen to the grain surface and regulated by large-scale erosion and clast-scale fracturing. We determined that neither iron- nor sulfur-oxidizing microorganisms control global pyrite weathering fluxes despite their ability to catalyze the reaction. This multiscale picture emphasizes that fracturing and erosion are as important as atmospheric oxygen in limiting pyrite reactivity over Earth’s history.
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- PAR ID:
- 10275436
- Date Published:
- Journal Name:
- Science
- Volume:
- 370
- Issue:
- 6515
- ISSN:
- 0036-8075
- Page Range / eLocation ID:
- eabb8092
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Chemical weathering influences many aspects of the Earth system, including biogeochemical cycling, climate, and ecosystem function. Physical erosion influences chemical weathering rates by setting the supply of fresh minerals to the critical zone. Vegetation also influences chemical weathering rates, both by physical processes that expose mineral surfaces and via production of acids that contribute to mineral dissolution. However, the role of vegetation in setting surface process rates in different landscapes is unclear. Here we use 10Be and geochemical mass balance to quantify soil production, physical erosion, and chemical weathering rates in a landscape where a migrating drainage divide separates catchments with an order-of magnitude contrast in erosion rates and where vegetation spans temperate rainforest, tussock grassland, and unvegetated alpine ecosystems in the western Southern Alps of New Zealand. Soil production, physical erosion, and chemical weathering rates are significantly higher on the rapidly eroding versus the slowly eroding side of the drainage divide. However, chemical weathering intensity does not vary significantly across the divide or as a function of vegetation type. Soil production rates are correlated with ridgetop curvature, and ridgetops are more convex on the rapidly eroding side of the divide, where soil mineral residence times are lowest. Hence our findings suggest fluvially-driven erosion rates control soil production and soil chemical weathering rates by influencing the relationship between hillslope topography and mineral residence times. In the western Southern Alps, soil production and chemical weathering rates are more strongly mediated by physical rock breakdown driven by landscape response to tectonics, than by vegetation.more » « less
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1126/science.abb8092
