Cryogenic Cave Pearls in the Periglacial Zone of Ice Caves
The Carpathian Mountains across Slovakia and Romania are home of several ice caves located at elevations between 700 and 1,200 m above sea level (asl). Although the mean surface annual temperature is above the freezing point, perennial ice deposits are common in caves and shafts with certain morphologies (large entrances followed by steep vertical or downward-sloping passages), into which the dense cold winter air sinks and remains trapped all year round. A particular type of cave pearls (cryogenic cave pearls, CCPs) occur in spatially restricted accumulations or extensive pearl fields (layers locally up to 0.5 m in thickness) within the scree covering the cave floor in the periglacial zone of these caves. The temperature in the periglacial zone oscillates around the freezing point, promoting seasonal ice formation. A similar type of pearl is observed in the entrance section of other caves that experience temporary freezing conditions during the cold season. When compared to pearls of non-iced caves, those from ice caves always occur at sites where liquid water cannot accumulate. CCP formation in nests with drips or in cave pools surrounded by rimstone dams is therefore excluded. Freezing–thawing processes are responsible for moving the pearls, preventing their cementation to the floor or between them. Results of U-series and radiocarbon dating indicate that the pearls are Holocene in age, with their growth continuing into the present. Pearls show high porosity, ranging from 7.6 to 22.6%. In the center they frequently contain radial aggregates of larger carbonate crystals, and their concentric layering is less obvious compared to normal cave pearls. In addition, polygonal pearls and pearl aggregates are common. Stable carbon and oxygen isotope study indicates the formation of an initial crystal aggregate by cryogenic precipitation during freezing of drip water. After that, pearls grow by addition of new carbonate layers, either at water freezing conditions or at temperature above 0°C. Overall, the pearl isotope data plot along a mixing line in the δ13C vs. δ18O diagram, connecting the field of normal speleothems of non-iced caves of the area, with the field of fine-grained cryogenic carbonate powder, formed in the glaciated parts of the ice caves. Seasonal water freezing and frost action in the cave periglacial zone is a necessary condition for the formation of these pearls.