Landscape controls on pore water chemistry and chemical weathering rates in the Critical Zone Observatory: Shale Hills Catchment (Central Pennsylvania, USA)

We investigate controls, mechanisms and rates of shale weathering and soil formation at the Susquehanna/Shale Hills Critical Zone Observatory of central Pennsylvania. The Shale Hills is forested and V-shaped catchment, with slopes around 16-18%. The parent shales, of Silurian Rose Hill Formation, are comprised of primarily illite, quartz, and chlorite. The dominant chemical reactions in the soil profiles are dissolution of chlorite and illite to form more stable kaolinite, through intermediate phases vermiculite and hydroxy interlayered vermiculite. Previous hydrologic studies have included monitoring the soil moisture contents and modeling the water flow dynamics in the unsaturated zones. Depth to the bedrock in the catchment depends mainly on the landscape positions, with thinner soils observed at the ridge tops, and much thicker soils at the valley floors and topographically depressional areas. Study sites were selected to investigate the propagation rates of the weathering front (the interface between intact bedrock and weathered material) with increasing complexity: fluid flow above the bedrock interface is largely vertical at ridgetop (1D site), downslope along a planar transect (2D site), and convergent downslope along a swale transect (3D site). Weekly soil waters were collected at these sites and the soil water chemistry and mineral dissolution kinetics are integrated at these characteristic landscapes, to investigate weathering scenario for the whole catchment. Soil at the 1D site is only 20 cm thick, with soils slightly depleted relative to parent composition. Here, porewater chemistry is controlled by chemical weathering reactions with some contribution from rainfall. At the 2D planar transect, soil thickness increases from the ridge top to valley floor, and soil waters become more concentrated downslope as mineral dissolution progresses. The depth variation of water chemistry is distinctively different among the sites, which is closely related to soil-texture controlled water flowpaths. Due to its topography, the 3D swale transect was greatly influenced by sediment transport during peri-glacial periods and has experienced different stages of weathering and soil development. As a result, both soil and pore water chemistry depends on local geomorphologic features and landscape positions. The stream reflects mixing among soil waters of different chemistry (shallow versus deep), and also mixing of soil waters and groundwaters, controlled by catchment hydrology. 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- and low-flow seasons. Mineral weathering rates are calculated from water chemistry on both catchment scale and pedon scale. These data and those laboratory-derived dissolution rates differ, reflecting different controlling mechanisms under field and experimental conditions.