Subsurface particle transport shapes the deep critical zone in a granitoid watershed

Understanding the inter-relationships between chemical weathering and physical erosion remains a first order puzzle in Earth surface dynamics. In the Río Icacos watershed in the Luquillo Critical Zone Observatory, Puerto Rico, where some of the world's fastest weathering of granitoid watersheds has been measured, we show that chemical weathering not only releases dissolved solutes, but also weakens the rock around the fractures until particles detach and are mobilised by subsurface flow through fractures. These sand-sized particles are more weathered than corestones, but much less weathered than soils/saprolites. Subsurface removal of these clayenriched, magnetite-depleted particles from the fractures could explain zones with enhanced magnetic susceptibility and decreased terrain conductivity that are observed in geophysical surveys. Subsurface particle transport may thus contribute to geophysical signatures and help sustain high weathering fluxes at Río Icacos and other steep and highly fractured landscapes.