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

Abstract

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.

Keywords:

• carbon-13
• concrete
• dripstones
• isotope ecology
• isotope hydrogeology
• oxygen-18
• secondary carbonates
• trace elements

Acknowledgements

The study was financially supported by the Slovenian Research Agency (ARRS) research programme P-0143 and the Ministry of Science, Education and Sports of the Republic of Croatia under Projects 098-0982934-2720 S. Hydrometeorological data were kindly provided by the Environmental Agency of Slovenia and National Hydrometeorological Survey of Croatia. Sincere thanks are due to the management of the Postojna Kras Turizem enterprise and in particular to Mr Stanislav Glažar for assistance and support during sampling. The authors also thank Dr Anthony R. Byrne for linguistic revision and two anonymous reviewers for thoughtful comments and suggestions.