H52C-06
Using WITCH to determine the factors that govern shale weathering and solute fluxes in the Critical Zone

Friday, 18 December 2015: 11:35
3016 (Moscone West)
Pamela L Sullivan, University of Kansas, Lawrence, KS, United States, Yves Godderis, GET CNRS, Toulouse, France, Yuning Shi, Pennsylvania State University Main Campus, University Park, PA, United States, Xin Gu, Penn state university, State College, PA, United States, Jacques Schott, CNRS, Paris Cedex 16, France, Christopher Duffy, The Pennsylvania State University, Department of Civil and Environmental Engineering, University Park, PA, United States and Susan L Brantley, Earth and Environmental Systems Institute, Penn State, Univ. Pk, PA, United States
Abstract:
Quantifying the effects of climate and biota on silicate and carbonate mineral weathering rates is crucial for predicting regolith formation and global weathering fluxes. Recent efforts demonstrate that geochemical, vegetation and climate models can be linked to understand the controls on solute fluxes in the critical zone. These efforts also elucidate field weathering rates and mineralogical evolution. To investigate the controls on shale weathering and solute flux, we developed a baseline shale weathering and solute flux model utilizing field observations from the Susquehanna Shale Hills Critical Zone Observatory (SSHCZO). Our modeling approach linked the physically-based land surface hydrologic model, Flux-PIHM (Penn State Integrated Hydrologic Model), to the numerical chemical weathering model WITCH.

We are progressively testing the importance of mineral reactive surface area, aspect, clay thermodynamic constants, and vegetation cycling (uptake and decomposition) on soil water solute fluxes. Under baseline conditions, WITCH simulated the range in soil water Mg2+ concentrations observed on the sun-facing hillslope but slightly underestimated the concentrations observed on the shaded slope. The baseline WITCH modeling suggests Mg2+ solute concentrations are primarily controlled by clay weathering but that Ca2+, K+, Na+ and Si are driven by other critical zone processes.

The inclusion of aspect resulted in lower recharge rates on the sun-facing side because ET was higher on that side. This in turn also resulted in lower soil water solute concentrations on the shaded hillslope. Cooler temperatures on that side also reduced the simulated chlorite and illite dissolution rates. When a vegetation cycling module was incorporated in WITCH, the simulation reproduced the range in Ca2+, K+, and Si concentrations in soil waters as observed in the field for both hillslopes. Modeling results suggest the inclusion of aspect and vegetation cycling are key in simulating soils water solute concentrations.

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