Formation rates of smectites derived from two Holocene chronosequences in the Swiss Alps

The formation rate of smectitic components in alpine soils is derived from two chronosequences in the Swiss Alps covering a time span of 11 500 years. The soils developed on granitic parent material. The lithologies between the two chronosequences (investigation areas), however, showed some distinct differences with respect to the main mineralogy and, to a lower extent, to the chemistry. The chronofunctions of smectitic phase formation (g/kgclay/year) were fitted to data using an exponential formation model. Extensive mineral weathering resulted in significant losses of chlorite and mica and the corresponding formation of smectite and regularly and irregularly interstratified mica–smectite. This formation is especially pronounced in the early stages of soil formation. Our investigations reveal that the greatest changes in soil clay mineralogy of alpine soils on granitic or gneiss host material occur within the first 1000–3000 years of soil development. At both investigation sites, the annual formation rates are in a similar order of magnitude with 0.1 g/kg/year or even higher at the start of soil formation and with values <0.01 g/kg/year after about 7000 years. The pedogenetic smectites from the E or Bs horizons included several populations with various interlayer charges. This heterogeneity seems to be related to the nature of their precursors.