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.