Particle fluxes in groundwater change subsurface shale rock chemistry over geologic time

Abstract

Most models of landscape evolution posit that particles leave the land surface by physical weathering (i.e., erosion) and solutes leave the subsurface by chemical weathering. They assume that erosion does not affect the rock and soil chemistry. However, in this study of a shale catchment we discovered that particles are mobilized out of soil and weathering rock and are transported through the subsurface, resulting in changes in the rock and soil chemistry. We studied the solute and particle fluxes during six storms in the Susquehanna Shale Hills Critical Zone Observatory in Pennsylvania, USA and compared those to the record in regolith chemistry. The stream's suspended particles primarily consisted of platy-shaped, μm-sized illite, commonly coated with patchy, amorphous, submicron-sized Al-, Fe- and Si-rich oxides. The chemistry of the stream particles always differed from that of surface soils except during intense dry-season rainstorms. Stream particles were chemically similar to the laboratory-extracted soil colloids at high discharge but to groundwater particles at low discharge, implying a central role of flow path variations in controlling subsurface particle transport. Zr was effectively immobile in Shale Hills. Regolith chemistry revealed that the cumulative effects of particle loss to depths of 5–8 m in the fractured bedrock zone were estimated to account for 58% of K and 24% of Mg losses. In shale landscapes, we propose that subsurface particle transport must be considered in landscape evolution models as an important contributor to changes in rock and soil chemistry over geologic time periods.