Mineral weathering and elemental transport during hillslope evolution at the Susquehanna/Shale Hills Critical Zone Observatory

Located in the uplands of the Valley and Ridge physiographic province of Pennsylvania, the Susquehanna/Shale Hills Critical Zone Observatory (SSHO) is a tectonically quiescent, first-order catchment developed on shales of the Silurian Rose Hill Formation. We used soil cores augered at the highest point of the watershed and along a subsurface water flowline on a planar hillslope to investigate mineral transformations and physical/chemical weathering fluxes. About 25 m of bedrock was also drilled to estimate parent composition. Depletion of carbonate at tens of meters of depth in bedrock may delineate a deep carbonate-weathering front. Overlying this, extending from ∼6 m below the bedrock-soil interface up into the soil, is the feldspar dissolution front. In the soils, depletion profiles for K, Mg, Si, Fe, and Al relative to the bedrock define the illite and chlorite reaction fronts. When combined with a cosmogenic nuclide-derived erosion rate on watershed sediments, these depletion profiles are consistent with dissolution rates that are several orders of magnitudes slower for chlorite (1-5 × 10⁻¹⁷ mol m⁻² s⁻¹) and illite (2-9 × 10⁻¹⁷ mol m⁻² s⁻¹) than observed in the laboratory. Mineral reactions result in formation of vermiculite, hydroxy-interlayered vermiculite, and minor kaolinite. During weathering, exchangeable divalent cations are replaced by Al as soil pH decreases.The losses of Mg and K in the soils occur largely as solute fluxes; in contrast, losses of Al and Fe are mostly as downslope transport of fine particles. Physical erosion of bulk soils also occurs: results from a steady-state model demonstrate that physical erosion accounts for about half of the total denudation at the ridgetop and midslope positions. Chemical weathering losses of Mg, Na, and K are higher in the upslope positions likely because of the higher degree of chemical undersaturation in porewaters. Chemical weathering slows down in the valley floor and Al and Si even show net accumulation. The simplest model for the hillslope that is consistent with all observations is a steady-state, clay weathering-limited system where soil production rates decrease with increasing soil thickness.