Cryogenic cave carbonates

Cryogenic cave carbonates (CCC for short) are a unique type of cave deposits, forming in freezing pools of water on cave ice during times when caves are in permafrost conditions. Because CCCs can be precisely dated by the U-Th method, they represent a key archive allowing studies of changing spatial distribution of permafrost on continents in response to past climate changes. Despite increasing appreciation of the unique character of this paleoclimate/paleoenvironmental archive, many fundamental questions about CCC origin, composition and mode of formation remain as yet unanswered. Particularly important is that the formation of CCC may involve metastable precursor phases (e.g., vaterite, monohydrocalcite, ikaite or amorphous calcium carbonate). Complex and possibly non-unique crystallization pathways may affect geochemical properties of CCC and their robustness as paleoclimate/paleoenvironmental proxy. This international collaborative project aims at finding mineralogical, crystallographic and morphological indicators of different mineral precursors and thus deciphering the CCC crystallization pathways. The study design involves analyses of natural (collected in caves) and synthetized in the laboratory CCC precipitates by a suite of both well-established and novel methods (XRPD, TEM, FTIR, solid state NMR, LA-ICP MS). Geochemical modeling will be employed to interpret analytical results.

Cryogenic cave carbonates (CCC) are a rare and remarkable type of mineral deposit that forms when pools of water freeze on cave ice under permafrost conditions. As the water gradually freezes, dissolved minerals precipitate and accumulate, creating distinctive carbonate formations. Because CCC can be dated with high precision using the uranium-thorium (U-Th) method, they represent an exceptional archive of past environmental change. In particular, they enable scientists to reconstruct how the spatial extent of permafrost across Eurasia responded to major climate shifts over the last half-million years. The prevailing model of CCC formation-developed largely from European case studies-proposes that these deposits form during periods of climate warming. In this scenario, warming thaws the rock above still-frozen caves, allowing groundwater to infiltrate the cave and freeze upon contact with the ice. Within this framework, CCC are interpreted as markers of degrading permafrost. Our project provides results that challenge this established paradigm. We conducted the first systematic investigation of CCC in Eastern Siberia, a region presently characterized by discontinuous permafrost. Surprisingly, we found that CCC in this region formed 500 to 1,000 years after major climate warming events-at a time when permafrost was expanding rather than retreating. Instead of recording permafrost degradation, CCC here appear to mark phases of permafrost aggradation. This unexpected result calls for a fundamental reassessment of how CCC are interpreted and highlights the complexity of permafrost dynamics in a changing climate. A second major objective of the project addressed a longstanding question in CCC research: do these deposits crystallize directly as calcite, or do they initially form as metastable precursor minerals that later transform? Resolving this issue is crucial for understanding how CCC grow and how reliably they record environmental conditions. In this international collaboration, involving researchers from Hungary, we searched for mineralogical, crystallographic, and morphological fingerprints of possible precursor phases in order to reconstruct CCC crystallization pathways. As a reference material, we investigated glendonite-a calcite pseudomorph after ikaite (calcium carbonate hexahydrate), a mineral that forms at temperatures close to 0C. Our analyses revealed characteristic features of the ikaite-to-calcite transformation, including aligned mesopores, frequent crystal twinning, fine grain size, abundant aqueous inclusions, and pronounced intergranular porosity at micrometer to submicrometer scales. We also identified abundant nanoscale SiO inclusions in many natural CCC samples. These micro- and nanoscale signatures provide strong evidence that precursor minerals likely play an important role in CCC formation. Together, these findings offer new insight into the crystallization history of CCC and pave the way for a more process-based and physically grounded interpretation of these valuable paleoclimate archives.