Settings of cryogenic cave carbonate formation

Cryogenic cave carbonates (CCCs) and, more generally, cryogenic cave minerals, form by solute rejection mechanisms when water enters and freezes in caves, or parts of caves, at temperatures below the freezing point. Negative temperatures may occur in caves in conjunction with climatic factors ranging from seasonal (e.g., sag-type cave entrances or ventilated parts of caves) to millennial (development of permafrost in karst massifs) timescales. When using CCCs for paleoclimate reconstructions, it is important to understand the environment in which they were formed.

Conventionally, two ‘end-members’ of cave microclimate settings are distinguished, in which water freezes either rapidly or slowly. These settings are responsible for the formation of fine-grained (typical sizes < 1 mm) vs. coarse-grained CCC (> 1 mm; the largest individual CCC encountered so far weighs 123 g). Besides difference in size, these two types of CCC show distinct stable isotope (O and C) patterns, currently considered as the most reliable (if not the only) feature that allows discriminating between the two types. The slowly formed CCCs that are coarse grained and show characteristic trends of ¹⁸O depletion and ¹³C enrichment (relative to non-cryogenic carbonates from the same cave) are thought to represent markers of past permafrost.   

Although both defining features, size and stable isotopic properties of CCCs, are controlled by the rate of freezing, the control may have different characteristic times. The rate of freezing is controlled by: (1) the overcooling of the site; (2) the amount of water entering the cave; and (3) the regime of water inflow (semi-continuous or pulse-like). Large amounts of water (even of low temperature) entering a cave entrain large amounts of heat that need to be dissipated before freezing commences. Further, the release of latent heat maintains the temperature of the freezing water body at 0 °C for the duration of the freezing process. Slow freezing leads to progressive depletion of the residual water in ¹⁸O and, as a result, CCCs are also increasingly depleted in ¹⁸O. We observed depletions of several permil in the time span of a few days in laboratory experiments.  

In caves located in relatively cold environments (e.g., Alpine or high-latitude caves) seasonal freezing conditions can be created by low-intensity influx of outside cold air into sections of caves located in already relatively cold rock. In such zones a small overcooling may be maintained for a sufficiently long time, producing CCCs with isotope properties mimicking (partly or entirely) those of CCCs forming in static settings of “true” permafrost.