Calcium isotopes in a proglacial weathering environment: Damma glacier, Switzerland

Abstract

The biogeochemical cycling and isotopic fractionation of calcium during the initial stages of weathering were investigated in an alpine soil chronosequence (Damma glacier, Switzerland). This site has a homogeneous silicate lithology and minimal biological impacts due to sparse vegetation cover. Calcium isotopic compositions, obtained by TIMS using a 43Ca– 46Ca double spike, were measured in the main Ca pools. During this very early stage of weathering, the young soils which have formed ( δ 44 / 42 Ca = + 0.44 ‰ ) were indistinguishable to the rocks from which they were derived ( δ 44 / 42 Ca = + 0.44 ‰ ) and stream water ( δ 44 / 42 Ca = + 0.48 ‰ ) was also within error of the average rock. This lack of variation indicates that the dissolution of the bulk silicate rock does not strongly fractionate Ca isotopes. The only Ca pool which was strongly fractionated from bulk rock was vegetation, which exhibited an enrichment of light Ca isotopes. Significant Ca isotope fractionation between bulk rock and the dissolved flux of Ca is likely to only occur where the Ca biogeochemical cycle is dominated by secondary processes such as biological cycling, adsorption and secondary mineral precipitation.