Physicochemical model for the generation of the isotopic composition of the carbonate travertine produced by the Tokhana spring, mount Elbrus area, northern Caucasus

The isotopic composition of calcite from travertine deposits of the Tokhana-Verkhnii hot spring in the Elbrus area shows broad variations in δ^sup 13^C and δ^sup 18^O (from +3.8 to +16.3[per thousand] and from +24.6 to +28.1[per thousand], respectively). The δ^sup 13^C and δ^sup 18^O values increase toward the sole of the travertine dome. The isotopically heaviest carbonates (δ^sup 13^C of up to +16.3[per thousand]) were found near the bottom of the dome and composed ancient travertine, which are now not washed by mineral water. The scatter of the δ^sup 13^C values of the fresh sample is slightly narrower: from +3.8 to +10[per thousand]. Calculations indicate that all carbonates of the Tokhana dome were not in equilibrium with spontaneous carbon dioxide released by the spring ([delta]^sup 13^C^sub CO2^ = -8[per thousand]). To explain the generation of isotopically heavy travertine, a physicochemical model was developed for precipitation of Ca carbonates during the gradual degassing of the mineral water. The character of variations in the calculated δ^sup 13^C values (from +5.5 to +13[per thousand]) is in good agreement with the tendency in the variations of the δ^sup 13^C in the carbonate samples. The calculated and measured pH values are also consistent. Our results demonstrate that the isotopic composition of large travertine masses can be heterogeneous, and this should be taken into account during paleoclimatic and paleohydrogeological reconstruction.[PUBLICATION ABSTRACT]