Abstract:
Progressive freezing of calcium- and bicarbonate- bearing cave waters can give rise to high supersaturation and the subsequent precipitation of microscopic calcite crystals and aggregates thereof. These particles are disseminated in the ice of ice caves and may later be concentrated by sublimation or melting of ice to form thin carbonate beds in layered ice (cryogenic carbonates). Such white to light brown, silty to fine sandy layers occur in the rear of the ice-bearing part of the Eisriesenwelt cave (Werfen, Salzburg) and were previously regarded as finely disintegrated limestone powder derived from the cave ceiling. Studies using scanning electron microscopy show that this material consists of 30-200 ?m aggregates of euhedral crystals, which, according to powder Xray diffraction analyses, are near-stoichiometric low-Mg calcite. The crystal aggregates commonly show a conspicuous flat top and resemble larger floating calcite rafts known from calcite-precipitating pools in ice-free caves. There are gradual transitions between these aggregates and skeletal crystal aggregates and (hemi)spherulitic forms, respectively. The small particle size and the skeletal crystal habit strongly argue in favour of rapid crystal growth during freezing of shallow puddles of icy water. This interpretation is corroborated by the highly positive C isotope values, which, in conjunction with the O isotope data, prove the cryogenic origin of these carbonates. The proportion of detrital contamination is very low. This study is the first report of fine-grained cryogenic speleothems in an eastalpine cave. It is supposed that such sediments are more wide spread and thicker layers may represent important paleoenvironmental marker horizons.