Particles can be mobilized in the subsurface by water moving through pore spaces and/or fractures from pedon to catchment scales. This subsurface particle loss, thus, can be an important process in the evolution of the critical zone architecture; however, quantitative investigations on subsurface particle loss in comparison to other weathering processes such as surface erosion and chemical weathering have rarely been carried out. Subsurface particle loss can be particularly significant during shale weathering as compared to other rock types because in other types of crystalline bedrock, small mobile particles must be generated first. In contrast, shale is intrinsically composed of fine particles that can be easily mobile. Particles in the subsurface are primarily mobilized by water: consequently, the mass and chemistry of particles will dynamically change according to the variation of water flow paths in the critical zone. Therefore, to quantify subsurface particle flux, direct observations of both water and particle chemistry during hydrologically active periods are essential. In this study, we documented the chemistry of water and suspended particles in stream and groundwater at short intervals (0.5-48 hours) for six storm events in the Shale Hills Critical Zone Observatory in Pennsylvania. In Shale Hills, suspended particles were enriched with platy-shaped, µm-sized illite and amorphous, submicron-sized Fe-, Al-, and Si-rich oxides. As discharge increased, the mass of suspended particles increased, resulting in greater losses of elements as particles. Combining knowledge of borehole core chemistry and inferred flow path distributions, we concluded that the suspended particles in the stream were derived from both soil and fractured bedrock layers. The particle flux (0.8m Myr-1) determined from our short-term observations was much smaller than solute flux (1.6m Myr-1) and surface erosion rate (15m Myr-1); however, the cumulative effect of subsurface particle loss for a geologic time scale was significantly enough to lower the K and Mg contents of soil and bedrock by 20% and 5%, respectively. Our study demonstrates that physical weathering occurs in the subsurface of shale and can be significant in changing both the rock and soil chemistry.
Kim, H., Gu, X., and Brantley, S.L. (2017): Subsurface particle loss during shale weathering can alter soil and rock chemistry. 2017 American Geophysical Union Fall Meeting, New Orleans, LA, 11-15 Dec .
This Paper/Book acknowledges NSF CZO grant support.