TY - JOUR
T1 - Chemical reactions, porosity, and microfracturing in shale during weathering
T2 - The effect of erosion rate
AU - Gu, Xin
AU - Rempe, Daniella M.
AU - Dietrich, William E.
AU - West, A. Joshua
AU - Lin, Teng Chiu
AU - Jin, Lixin
AU - Brantley, Susan L.
N1 - Funding Information:
Access to the small-angle neutron scattering instruments was provided by the Center for High Resolution Neutron Scattering, a partnership between the National Institute of Standards and Technology and the National Science Foundation under Agreement No. DMR-1508249 . XG acknowledges help and advice on neutron scattering from David F.R. Mildner, David Cole, and Gernot Rother. XG and SLB acknowledge funding from DOE OBES DE-FG02-OSER15675 and NSF Critical Zone Observatory grants NSF , United States 12-39285 and 13-31726 for support for working on the Susquehanna Shale Hills Critical Zone Observatory in The Pennsylvania State University, United States Stone Valley Forest, which is funded by the Penn State College of Agriculture Sciences and the Department of Ecosystem Science and Management, and is managed by the staff of the Forestlands Management Office. WED and DMR acknowledge support from the Keck Foundation, National Science Foundation Grant NSF , United States 1331930 for the Eel River Critical Zone Observatory, the Angelo Coast Range Reserve and the University of California Reserve System, and the National Center for Airborne Laser Mapping. Ben Houlton, Randy Dahlgren, and Scott Mitchell ( University of California, Davis ) are thanked for sharing NSF-funded elemental analysis data for the Eel River CZO. The Taiwan Forestry Research Institute (TFIR) is thanked for help organizing sampling and allowing access to the Fushan site. SLB acknowledges M. Lebedeva for insights and three reviewers are thanked for their comments. Appendix A
Funding Information:
Access to the small-angle neutron scattering instruments was provided by the Center for High Resolution Neutron Scattering, a partnership between the National Institute of Standards and Technology and the National Science Foundation under Agreement No. DMR-1508249. XG acknowledges help and advice on neutron scattering from David F.R. Mildner, David Cole, and Gernot Rother. XG and SLB acknowledge funding from DOE OBES DE-FG02-OSER15675 and NSF Critical Zone Observatory grants NSF, United States 12-39285 and 13-31726 for support for working on the Susquehanna Shale Hills Critical Zone Observatory in The Pennsylvania State University, United States Stone Valley Forest, which is funded by the Penn State College of Agriculture Sciences and the Department of Ecosystem Science and Management, and is managed by the staff of the Forestlands Management Office. WED and DMR acknowledge support from the Keck Foundation, National Science Foundation Grant NSF, United States 1331930 for the Eel River Critical Zone Observatory, the Angelo Coast Range Reserve and the University of California Reserve System, and the National Center for Airborne Laser Mapping. Ben Houlton, Randy Dahlgren, and Scott Mitchell (University of California, Davis) are thanked for sharing NSF-funded elemental analysis data for the Eel River CZO. The Taiwan Forestry Research Institute (TFIR) is thanked for help organizing sampling and allowing access to the Fushan site. SLB acknowledges M. Lebedeva for insights and three reviewers are thanked for their comments.
Publisher Copyright:
© 2019 Elsevier Ltd
PY - 2020/1/15
Y1 - 2020/1/15
N2 - 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 examining 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, ERCZO); 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 yr−1 in Fushan ≫ 0.2–0.4 mm yr−1 in ERCZO ≫ 0.01–0.025 mm yr−1 in SSHCZO). The natural experiment did not totally isolate erosion as a variable: mean annual precipitation varied along the erosion gradient (4.2 m yr−1 in Fushan > 1.9 m yr−1 in ERCZO > 1.1 m yr−1 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 conditions. Thus, microfracturing is a process that can couple physical erosion and chemical weathering in shales.
AB - 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 examining 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, ERCZO); 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 yr−1 in Fushan ≫ 0.2–0.4 mm yr−1 in ERCZO ≫ 0.01–0.025 mm yr−1 in SSHCZO). The natural experiment did not totally isolate erosion as a variable: mean annual precipitation varied along the erosion gradient (4.2 m yr−1 in Fushan > 1.9 m yr−1 in ERCZO > 1.1 m yr−1 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 conditions. Thus, microfracturing is a process that can couple physical erosion and chemical weathering in shales.
KW - Chemical weathering
KW - Erosion
KW - Microfracture
KW - Porosity
KW - Shale
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U2 - 10.1016/j.gca.2019.09.044
DO - 10.1016/j.gca.2019.09.044
M3 - Article
AN - SCOPUS:85074661915
SN - 0016-7037
VL - 269
SP - 63
EP - 100
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
ER -