A SEM STUDY OF THE CHEMICAL WEATHERING RATES OF APATITES IN TWO GLACIATED CATCHMENTS IN THE SWISS ALPS

Quantifying the chemical weathering rate of apatite is critical for our understanding of the rate of biological sequestration of carbon. This is because apatite is a major source of phosphorus, a biolimiting nutrient. Using a mass balance method (inputs of phosphate in the form of rain and snow subtracted from its output in river water) we calculate an annual weathering flux for the granitic catchments of the Rhône and Oberaar glaciers, Switzerland.

Here we have used a method to calculate weathering rates by quantifying the depth and abundance of etch pits on surfaces of apatite grains of known age by scanning electron microscopy (SEM). Using this technique weathering rates can be calculated for the individual minerals grains. This is a second method for calculating catchment denudation rates and a first order test to constrain our mass balance flux values, as apatite is the principle phosphate bearing mineral in granites. We have compared the degree of etching of apatites in glacial sediments of different ages taken from our two Swiss catchments. These include recent glacial-fluvial, lacustrine and melt-out sediment, little ice age (1600-1850) terminal moraines, and till dating from before the climate optimum (700-1300). Results from the oldest moraine show that the apatite is heavily etched. Etch pits have formed at sites on the grain surface that have a low activation energy for dissolution, including volumes of crystal structure surrounding fission tracks. It is possible that the degree of size and depth of fission track etch pits can be used as a semi-quantitative measure of the degree of weathering of a grain surface. This approach of combining chemical mass balance data with high-resolution electron microscopy should provide the data necessary to understand the controls on the flux of phosphorous in temperate alpine glaciers.