Cryogenic carbonate synthesis by controlled solution freezing

Cave carbonates offer insights into past environmental and climate change. A unique type of these deposits, cryogenic cave carbonates (CCCs), form by a mechanism known as cryogenic crystallization. CCCs may form through: (1) rapid freezing of thin water films on ice surfaces, creating small crystals (typically, <1 mm), and (2) slow freezing of water in pools, creating larger crystals (up to several cm in size). These two types of CCC show distinct stable isotope compositions. From a paleoclimatological point of view, CCCs of the second type are an indicator of past permafrost conditions¹. However, the details of their formation are still not fully understood, as no actively forming CCCs of this type have been observed in nature.

To study how the freezing proceeds and how it influences the geochemical signature and morphology of the cryogenic crystallization products, we employed several methods for forming cryogenic carbonates under controlled conditions in the laboratory. (1) Cryogenic carbonates were produced via bottom-up solution freezing, by lowering a plastic bottle filled with a Ca-bicarbonate solution into a -15 °C medium. The freezing times for the bottom and top layers varied between three and ten hours. (2) Cryogenic carbonates were also precipitated from a saturated Ca-bicarbonate solution via slow (several days) and uniform freezing at -2 °C in a freezer. To control the direction of freezing and enhance the top-down freezing process, the flask containing the bicarbonate solution was placed in an insulated box. After the experiments, the first- and last-formed carbonates were separated by sampling of the formed ice. Marked differences in the crystal size and the oxygen and carbon isotope compositions of the first- and last-formed carbonates were observed. The δ¹³C and δ¹⁸O values of synthetic cryogenic carbonates align with the field of the fast-forming natural CCCs. However, when compared with parent solutions, they are closer to the values of the field of CCCs² forming in freezing pools. The results highlight the importance of knowing the C isotopic composition of the solution’s dissolved inorganic carbon in isotope-based classification of CCCs, and are relevant for understanding the environment in which CCCs form.

We acknowledge the financial support of the NKFIH ANN141894 grant.

References:

¹ Žák, K., Onac, B.P., Kadebskaya, O., Filippi, M., Dublyansky, Y., Luetscher, M. (2018): Cryogenic mineral formation in caves, in: Perşoiu, A., Lauritzen, S.-E. (Eds.), Ice Caves, 123-162, Elsevier, Amsterdam.

² Spötl, C., Koltai G. & Dublyansky Y. (2023) Mode of formation of cryogenic cave carbonates: Experimental evidence from an Alpine ice cave. Chemical Geology, 638, 121712. DOI: 10.1016/j.chemgeo.2023.121712.