Cryogenic cave carbonates: New insights from alpine ice caves

Cryogenic cave carbonates (CCC) are speleothems that form under freezing conditions and provide crucial insights into past cold environments and cryogenic processes. Traditionally, CCC have been classified into two categories: fine-grained CCC (hereafter CCC-f) and coarse-grained CCC (hereafter CCC-c), primarily based on particle size and stable isotopic composition. However, recent observations from alpine ice caves in Austria suggest the existence of an intermediate class, termed CCC-i, characterized by small grain sizes (<1 mm) and stable isotope values that overlap with CCC-c. This study documents the occurrence, morphology, and stable isotopic composition of CCC-i found in seven ice caves, providing insights into their formation mechanism. CCC-i are associated with small pools in ice and form in a cave climate that is transitional between the fully heterothermic regime close to the entrance and the homothermic regime of the inner cave passages characterized by high thermal stability. The stable isotopic signatures of CCC-i indicate a Rayleigh-type fractionation process similar to CCC-c but at a smaller spatial and temporal scale. This study highlights the paleoclimatic significance of CCC-i, as they may improve reconstructions of past cave glaciations. However, the ongoing retreat of cave ice due to climate warming threatens the preservation of CCC deposits, emphasizing the urgency of further research.