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C and O stable isotopic signatures of fast-growing dripstones on alkaline substrates: reflection of growth mechanism, carbonate sources and environmental conditions

Secondary carbonate precipitates (dripstones) formed on concrete surfaces in four different environments – Mediterranean and continental open-space and indoor environments (inside a building and in a karstic cave) – were studied. The fabric of dripstones depends upon water supply, pH of mother solution and carbonate-resulting precipitation rate. Very low δ13C (average−28.2‰) and δ18O (average−18.4‰) values showed a strong positive correlation, typical for carbonate precipitated by rapid dissolution of CO2 in a highly alkaline solution and consequent disequilibrium precipitation of CaCO3. The main source of carbon is atmospheric or biogenic CO2 in the poorly ventilated karstic cave, which is reflected in even lower δ13C values. Statistical analysis of δ13C and δ18O values of the four groups of samples showed that the governing factor of isotope fractionation is not the temperature, but rather the precipitation rate.