The distribution of weathering fronts under valley and ridge

Abstract:

The spatial distribution of weathered rock across landscapes strongly influences how water and solutes are routed throughout the landscape, but we know little about this distribution. To understand the controls on the evolution of weathering under hilly and mountainous regions, we investigated the spatial distribution of the oxidation front for pyrite at the Shale Hills catchment in central Pennsylvania using geochemical measurements on materials recovered from boreholes across the catchment as well as geophysical measurements. As a subcatchment of the Susquehanna Shale Hills Critical Zone Observatory, intensive field monitoring and measurements of surface water and groundwater hydrology and aqueous chemistry in the past decade allow us to test how the reaction front recorded in the illite/chlorite/quartz-dominated shale is related to flow paths and solute fluxes.

The V-shaped Shale Hills catchment contains an ephemeral, westward-flowing stream. The bedrock of the catchment is mainly composed of Fe-rich, organic-poor Rose Hill shale (Silurian-aged) with increasing abundances of interbedded carbonate near the outlet of the catchment. Under the ridge, the oxidation of pyrite occurs sharply across a front (~1 m in thickness) above the water table, while under the valley, the oxidation initiates at ~ 21 m below the water table and completes at ~ 6 m below the water table. This results in a narrow reaction front under the ridge that widens to a ~15 m wide front under the outlet. Under both ridge and valley, chlorite dissolution initiates at the same depth as pyrite oxidation completes whereas illite dissolution only occurs in mobile soil. The dissolution fronts of chlorite and illite can be traced by seismic tomography. The concentrations of pyrite-derived sulfate and carbonate-derived divalent cations (Ca²⁺ and Mg²⁺) increase downstream, which indicates the OWP and carbonate dissolution is highly coupled. All the observations lead to a proposed flow scheme for Shale Hills: shallow flow of O₂-rich water in the upper fractured zone (5-8 m below land surface) mixes with deeper O₂-poor groundwater along and beneath the stream channel. The coupling of weathering and hillslope hydrology determines deep critical zone structure and may exert a long-term control on morphology evolution under valley and ridge.