The isotopic geochemistry of CaCO3 encrustations in Taylor Valley, Antarctica: Implications for their origin

Abstract

Calcium carbonate (CaCO₃) encrustations occur in most desert soils, including polar ones, and such encrustations preserve records of geochemical, hydrological, and atmosphere processes affecting these soils. We have collected a series of CaCO3 encrustations found underneath surface rocks in the soils and tills of Taylor Valley, McMurdo Dry Valleys (~78°S lat.), Antarctica. These encrustations were analyzed for 87Sr/86S and δ18O and δ13C to determine what relation they have with the underlying soils, and the material in which they are in contact, and to identify the processes that control their formation. In all but one case, the isotopic data indicate that the source of Sr to these encrustations is not from the rock on which it is associated. The primary source of Sr (and by analogy Ca) is either from dust that has been deposited through aeolian processes or from the aggregate of till material within the soils. The δ13C values for Taylor Valley encrustations ranged from 5.7 to 11.0‰, and are consistent with a carbon source from atmospheric CO₂. The δ18O values range from –8.1 to –11.2‰ and are heavier than expected for equilibrium calcite precipitation from Taylor Valley meteoric water. Taken together these results indicate that the CaCO₃ was formed by rapid evaporation of films beneath clasts that had become supersaturated with respect to CaCO₃.