Deep weathering along a granite ridgeline in a subtropical climate

Bulk chemical composition and mineralogy were examined in three soil profiles and a deeper 11-meter profile weathering on a granite ridgeline under subtropical climate conditions in south China. The weathering sequence is delineated by mineralogy and major element variations. Apatite, biotite, hornblende and plagioclase dissolve early during weathering, resulting in nearly 100% Ca and Na loss and significant Mg, Fe and P depletion at depth. The K-feldspar reaction front begins at the depth of depletion of plagioclase, leading to a loss of ~80% of the K at the land surface near the bottom of the ridgeline and almost 100% at the top. Dissolution of quartz and other silicates releases about 60% of Si in the profiles. Kaolinite is the dominant clay mineral and it transforms to gibbsite in the uppermost layer. The soil horizon (upper 100 cm) is the zone dominated by pedogenic processes, including active biological activity, physical erosion and influx of high concentrations of atmospheric components (especially CO₂ and O₂). The pedogenic processes are characterized by low pH (~4.54 to 5.85), high clay content (kaolinite: 6-16 wt.%; gibbsite: 2-8 wt.%) and total organic carbon content (0.13-3.93%) and intensive fracturing and dissolution of quartz and K-feldspar compared with the lower horizons.Accumulation of organic material and resistant minerals downslope is attributed to down-ridge movement of water (termed here, interflow) and weathering products in the uppermost 100 cm down the ridgeline. Using a mass balance model calculation for the catena that assumes steady-state soil thickness at all sites, the bottom profile shows the highest apparent total chemical weathering loss rate (~14 g m^-2 y^-1) whereas the middle position shows the highest physical erosion loss rate (~44 g m^-2 y^-1). SiO₂ accounts for about 84% of the chemical weathering outflux from the soil horizons along the ridgeline hillslope. Chemical weathering rates at the bottom of the hillslope may be accelerated by high concentrations of organic material and by dissolution of kaolinite.This study demonstrates that mineral reaction fronts in granite become separated over depth intervals of meters and that elemental fluxes and release mechanisms vary with position along a ridgeline catena.