PYRITE DISSOLUTION LEADS WATERSHED GEOMORPHOLOGICAL EVOLUTION

Hydrologic flow paths in the subsurface are governed by the development of secondary porosity and permeability that accompanies the conversion of protolith to regolith through chemical and physical weathering. Mineral dissolution plays a key role in the development of secondary porosity and permeability. The depth interval in a weathering profile across which a mineral becomes depleted due to dissolution is termed here a reaction front. The downward propagation of nested reaction fronts into the subsurface can open porosity, enhance permeability and affect water transmittance. To unravel the interplay between water flow and development of nested reaction fronts, we present bulk geochemical analyses from nine boreholes and groundwater geochemistry and age tracers from 25 wells across the Susquehanna Shale Hills Critical Zone Observatory (SSHCZO; PA, USA).

At SSHCZO, borehole material revealed that pyrite oxidative dissolution was the deepest reaction. Chlorite begins to oxidize at the same depth, although over a larger depth interval, and is accompanied by carbonate mineral dissolution. At the base of the 5-8 m deep fractured zone plagioclase begins to dissolve, while illite dissolution becomes important in the uppermost mobile soils. Groundwater levels showed that subsurface water flow reaches the catchment outlet by interflow and regional groundwater flow. Interflow (shallow hillslope flow) is oxygenated and constrained to the upper ~6 m highly fractured zone. At the valley floor, interflow advects to depths of 5–8 m where it mixes with deep groundwater and drives pyrite oxidation. The pyrite oxidation likely weakens the bedrock beneath the valley by generating secondary porosity and sulfuric acid, both of which enhance flow and dissolution.

We hypothesize that pyrite oxidation promotes channel incision, which in turn supports drainage of groundwater from the ridges, slowly lowering the catchment water table. Pyrite oxidation beneath the valley could control both the rate of channel incision and the rate of weathering advance under the uplands. Given this conceptual model, the catchment morphology is controlled by the delivery of interflow and groundwater flow at depth in the valley that drive pyrite reactions over the long-term and culminates in a cascade of clay weathering reactions and soil formation.