SPATIAL AND TEMPORAL CONTROLS ON THE GEOCHEMICAL EVOLUTION OF CAVE DRIPWATERS AND MODERN SPELEOTHEM CALCITE

A study of the physical, chemical, and meteorological variations at 13 drip-water and pool sites in two cave systems in the Edwards aquifer of central Texas was undertaken to assess the controls on the evolution of vadose karst waters. Key parameters analyzed include O and C isotopes, Mg, Sr, and Ca concentrations, calcite saturation index, alkalinity, drip rate, and water and cave-air CO₂ concentrations. These parameters reveal complementary information about climatic, hydrogeologic and in-cave processes. Calcite grown from three of these drips was analyzed for its O and C isotopic composition to assess the relationship between the calcite and its associated dripwater.

Drip water C and elemental geochemistry range from less evolved compositions, similar to that for soil waters, to compositions that reflect in-cave processes including CO₂ degassing and calcite precipitation along cave roofs and stalactites. Evolved waters have higher δ¹³C, Mg/Ca, and Sr/Ca, and while they have lower Ca and dissolved inorganic C concentrations.

Seven of the thirteen sites have a seasonal geochemical response as evidenced by increased δ¹³C values and decreased Ca concentrations during the winter. Based on this response and the decreased water CO₂ concentrations during the winter, the main processes affecting the vadose water evolution are increased CO₂ degassing and calcite precipitation upflow of the drip site during winter, when cave CO₂ concentrations are low.

The C isotopic compositions of the precipitated calcite reflect that of their associated dripwaters, although some calcite samples have lower δ¹³C than their predicted equilibrium value (deviations up to 1.3‰). The O isotopic compositions of the calcites have higher δ¹⁸O than the predicted equilibrium value in most cases (deviations up to 1.4‰). The deviation from isotopic equilibrium values is most likely due to kinetic isotope effects resulting from rapid CO₂ degassing.

These results have general implications for the analysis of karst groundwater and speleothem geochemistry as they indicate processes of chemical evolution that may occur during transmission to and within the cave in addition to environmental change external to the cave.