The dynamics of Si cycling during weathering in two small catchments in the Black Forest (Germany) traced by Si isotopes

Silicon stable isotopes have emerged as a powerful proxy to investigate weathering because Si uptake from solution by secondary minerals or by the vegetation causes significant shifts in the isotope composition. In this study, we determined the Si isotope compositions of the principle Si pools within two small catchments located on sandstone and paragneiss, respectively, in the temperate Black Forest (Germany). At both settings, clay formation is dominated by mineral transformation preserving largely the signature of parental minerals with δ30Si values of around − 0.7‰. Bulk soils rich in primary minerals are similar to bulk parental material with δ30Si values close to − 0.4‰. Topsoils are partly different because organic matter degradation has promoted intense weathering leading to δ30Si values as low as − 1.0‰. Water samples expose highly dynamic weathering processes in the soil zone: 1) after spring snowmelt, increased release of DOC and high water fluxes trigger clay mineral dissolution which leads to δ30Si values down to − 0.7‰ and 2) in course of the summer, Si uptake by the vegetation and secondary mineral formation drives dissolved Si to typical positive δ30Si values up to 1.1‰. Groundwater with δ30Si values of around 0.4‰ records steady processes in bedrock reflecting plagioclase weathering together with kaolinite precipitation. An isotope mass balance approach reveals incongruent weathering conditions where denudation of Si is largely driven by physical erosion. Erosion of phytoliths contributes 3 to 21% to the total Si export flux, which is in the same order as the dissolved Si flux. These results elucidate the Si dynamics during weathering on catchments underlain of sedimentary origin, prevailing on the Earth surface and provide therefore valuable information to interpret the isotope signature of large river systems.