High-resolution stable isotopic and trace element data of cryogenic cave carbonates from an alpine cave: preliminary results

Coarsely crystalline cryogenic cave carbonates (CCC for short) are unique speleothems that precipitated in small water pools in cave ice via slow freezing. Their formation requires cave air temperatures very close to the 0°C isotherm. CCC have become increasingly important over the last decade and are one of the best terrestrial archives providing highly precise chronological constraints on paleo-permafrost degradation. Although a growing number of studies use CCCs as a time-marker of permafrost thawing, the details of their formation mechanism remain unanswered.

CCC found at over a dozen sites in the ice-free parts of Eisriesenwelt (Tennengebirge, Eastern Alps) cover a wide spectrum of morphologies from skeletal crystals to more complex beak-like and hemispheric structures. At some sites one or two morphologies are dominant, while at other sites several different types occur together. The mechanisms of formation of many of the morphologies remain enigmatic.

Here we present first results of high-resolution trace element (Mg, Sr, Ba, and U) and stable isotope (C and O) analyses of a white pea-like spherulite (~8 mm) and two beak-like aggregates (~12 and 15 mm in diameter) from Eisriesenwelt. The latter type shows an amber-coloured inner core, overgrown by more porous light brown to almost whitish calcite.

Oxygen isotope ratios of one of the beak-like aggregates reduce progressively by 4.8‰ from the core towards the rim, while carbon isotope values increase by 2.1‰, reflecting the progressive freezing of the water. The dark brown calcite is characterized by low Mg, Sr, and Ba but high U values compared to the light brown calcite. Trace element analyses also indicate an increase in Mg and Sr towards the rim, whereas U shows the opposite trend. In the white hemispheric spherulite sample, oxygen isotope ratios show a larger (up to 2‰) intracrystalline variability, and a 5.8‰ decrease over the last 2 mm of the sample. In contrast, carbon isotope values vary only by 0.5‰. Mg values show an increasing trend through time, from core to rim. Sr, U, and Ba concentrations do not exhibit a clear pattern, except for a sharp change before the cessation of CCC growth. This is likely related to the presence of a thin (hundreds of 100µm), translucent calcite rim, expressed as an increase (decrease) in Mg, Sr, U (and Ba) concentrations. In both morphologies Ba follows a similar pattern as Sr, however Ba values decrease progressively before the cessation of CCC growth.

Formation of CCC of the coarse crystalline variety has not been observed in-situ, limiting their significance as robust indicators of thawing conditions in ice caves. Studying individual CCC samples using a combination of high-resolution stable isotope and trace-element analyses is an important, but currently underdeveloped, approach to improving our understanding of CCC formation.