Chemical Weathering of Shales in the Shale Hills Catchment (Central Pennsylvania, USA)

Shales have been documented to be important in determining global fluxes of C, P and Pt-group elements worldwide as large amounts of shales are exposed at the Earth's surfaces. However, few studies have been performed to understand the chemical weathering rates of shales and their contribution to the overall silicate weathering fluxes on global scales. The Shale Hills catchment is situated in central Pennsylvania, on Rose Shale formation of Silurian age, and it is one of several sites where shale weathering rates will be studied and compared along a latitude and thus a climate gradient. The Shale Hills is a 7.9 ha, V-shaped catchment, with slopes around 16-18%. Previous hydrologic experimentations have included monitoring the soil moisture contents and modeling the water flow dynamics in the soil zones. In this study, we characterized soils and soil waters in three locations along an elevation transect in the Shale Hills catchment (ridge top, middle slope and valley floor), as well as the first-order streams. The parent materials are comprised of primarily illite, quartz and K-feldspar. The dominant chemical reactions in the soil profiles are dissolution of K-feldspar to form illite and further dissolution of illite to more stable kaolinite. The depths to bedrock are controlled by elevation, where the soils are the shallowest at the ridge top (only 30 cm), and the soils in the valley floor are deepest, over 60 cm deep. The extent of elemental loss due to shale weathering also shows dependence on topography, from high in valley floor to low at the ridge top. In all three sites, Ca is leached more quickly than other cations and more than 50% has been weathered during soil development. The soil water composition is very consistent with soil chemistry. Soil waters from the ridge top are dilute, while those from the valley floor are much higher in ionic strength. In all soil waters, Ca is the dominant cation, followed by Mg, Na and K. The variation of cation concentrations with depth is distinctively different among three sites, which is related to soil texture and thus the water flowpaths. The stream reflects mixing among soil waters of different chemistry (shallow versus deep), as ground waters contribute less than 5% of riverine discharge. Temporal variations of stream chemistry are related not only to the temperature-dependent mineral weathering rates, but also the different proportions of these sources during high-flow and low-flow seasons. This work focuses on a hillslope transect, and well-characterized hydrology of the catchment makes it possible to investigate chemical weathering processes in multi-dimensions.