THE IMPORTANCE OF GRADUAL CHEMICAL WEATHERING ON NUTRIENT RELEASE IN TWO GLACIATED AREAS IN THE SWISS ALPS

In order to identify processes and rates of alteration of moraine material in the glacier forefield, we studied chemical weathering processes of glacial sediments and rates in the two catchment areas of the Rhône and Oberaar glaciers (Swiss Alps).

We use mineralogy and bulk chemistry (XRD and XRF data) of glacial sediments of different ages from both glaciers; i.e. suspended sediment in meltwater, recent englacial melt-out sediment, terminal moraines dating from little ice age LIA (1300-1850), and older tills. Two of the main questions are: (1) How much does subsequent weathering of sediment after deposition contribute to the overall chemical denudation rate of a catchment? (2) How long is its influence significant? The answers could be used to assess weathering rates during glacial periods.

XRD data show an increase of pedogenic formed vermiculite and a continuous decrease of biotite with increasing age of the sediment. Chlorite seems to be more or less stable in tills up to LIA, in the oldest deposited tills there is a significant decrease. Suspended sediment is fresh, secondary weathering products are missing. Melt-out sediment contains vermiculite suggesting it may be derived from reworked lateral moraines that were previously weathered.

XRF element distributions have been compared to mineralogy in our profiles, which should give an insight in nutrient leaching processes. The distribution of some elements can directly be related to mineral weathering reactions, eg. biotite=> vermiculite: The concentration of Fe2O3, K2O, and MgO decreases as biotite content becomes less. In the pre-LIA tills Fe2O3 is completely leached in the top 30 cm of the profile.

This gradual convergence of biotite to vermiculite over a time scale of 100’s of years emphasises the importance of secondary weathering in glaciated cachments. This pedogenic processes allow the slow release of Fe, probably biolimiting nutrient in proglacial alpine areas.