TY  - JOUR
A1  - Steinhöfel, Grit
A1  - Breuer, Jörn
A1  - von Blanckenburg, Friedhelm
A1  - Horn, Ingo
A1  - Sommer, Michael
T1  - The dynamics of Si cycling during weathering in two small catchments in the Black Forest (Germany) traced by Si isotopes
JF  - Chemical geology : official journal of the European Association for Geochemistry
N2  - 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 delta Si-30 values of around -0.7%. Bulk soils rich in primary minerals are similar to bulk parental material with delta Si-30 values close to -0.4%. Topsoils are partly different because organic matter degradation has promoted intense weathering leading to delta Si-30 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 delta Si-30 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 delta Si-30 values up to 1.1%. Groundwater with delta Si-30 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.
KW  - Weathering
KW  - Sedimentary rocks
KW  - Biogeochemical Si cycle
KW  - Silicon isotopes
KW  - UV femtosecond laser ablation
Y1  - 2017
U6  - https://doi.org/10.1016/j.chemgeo.2017.06.026
SN  - 0009-2541
SN  - 1878-5999
VL  - 466
SP  - 389
EP  - 402
PB  - Elsevier
CY  - Amsterdam
ER  -