Cryogenic carbonates in cave environments:: A review

Cryogenic cave carbonate (CCC) represents a specific type of speleothem. Its precipitation proceeds at the freezing point and is triggered by freezing-induced concentration of solutes. Compared to classical speleothems (stalagmites, flowstones), CCC occurs as accumulations of loose uncemented aggregates. The grain sizes range from less than 1 mu m to over 1 cm in diameter. Karst groundwater chemistry and its freezing rate upon entering the cave are responsible for highly variable grain morphology. Rapid freezing of water results in the formation of CCC powders with grain size typically below 50 mu m. Slow freezing of water in caves (usually in systems where the CO2 escape is partly restricted; e.g., ice covered water pools) results in the formation of large mineral grains, with sizes from less than 1 mm to about 20mm. The range of carbon and oxygen stable isotope compositions of CCC is larger than for a typical carbonate speleothem. Rapid freezing of water accompanied by a quick kinetic CO2 degassing results in large ranges of delta C-13 of the CCC powders (between -10 parts per thousand and + 18 parts per thousand PDB). Slow freezing of water, with a restricted CO2 escape results in gradual increase of delta C-13 values (from -9 parts per thousand to +6 parts per thousand PDB; data ranges in individual caves are usually much more restricted), accompanied by a delta O-18 decrease of the precipitated carbonate (overall range from -10 parts per thousand to -24 parts per thousand PDB). These unusual trends of the carbonate 6180 evolution reflect incorporation of the heavier 180 isotope into the formed ice. New isotope data on CCC from three Romanian ice caves allow better understanding of the carbon and oxygen isotope fingerprint in carbonates precipitated from freezing of bulk water. CCCs are proposed as a new genetic group of speleothems. (C) 2007 Elsevier Ltd and INQUA. All rights reserved.