The clay fraction in the subsoil illuvial Bh–Bs horizons is isotopically lighter (i.e., enriched in light 28Si) than “unweathered” BC clay fraction (δ30Si from −0.60 ± 0.06‰ to −0.84 ± 0.08‰ ‰; Fig. 4B, C). The magnitude of light Si depletion/enrichment in the clay fraction increases with soil age, with 30ΔSiE–BC varying from +0.20‰ (at t = 175 years) to +0.42‰ (at t = 335 years); and 30ΔSiB–BC varying from −0.17‰ (at t = 175 years) to −0.32‰ (at t = 335 years).