Cryogenic and non-cryogenic pool calcites indicating permafrost and non-permafrost periods: a case study from the Herbstlabyrinth-Advent Cave system (Germany)

摘要

Weichselian cryogenic calcites collected in what is referred to as theR?tselhalle of the Herbstlabyrinth-Advent Cave system are structurallyclassified as rhombohedral crystals and spherulitic aggregates. The carbonand oxygen isotopic composition of these precipitates (δ13C = +0.6 to ?7.3‰ δ18O = ?6.9 to ?18.0‰) corresponds to those ofknown slowly precipitated cryogenic cave calcites under conditions ofisotopic equilibrium between water and ice of Central European caves. Thecarbon and oxygen isotopic composition varies between different caves whichis attributed to the effects of cave air ventilation before the freezingstarted.By petrographic and geochemical comparisons of Weichselian cryogenic calcitewith recent to sub-recent precipitates as well as Weichselian non-cryogeniccalcites of the same locality, a model for the precipitation of thesecalcites is proposed. While the recent and sub-recent pool-calcitesisotopically match the composition of interglacial speleothems (stalagmites,etc.), isotope ratios of Weichselian non-cryogenic pool-calcites reflectcooler conditions. Weichselian cryogenic calcites show a trend towards lowδ18O values with higher carbon isotope ratios reflecting slowfreezing of the precipitating solution. In essence, the isotope geochemistryof the Weichselian calcites reflects the climate history changing fromoverall initial permafrost conditions to permafrost-free and subsequently torenewed permafrost conditions. Judging from the data compiled here, the lastpermafrost stage in the R?tselhalle is followed by a warm period(interstadial and/or Holocene). During this warmer period, the cave icemelted and cryogenic and non-cryogenic Weichselian calcite precipitates weredeposited on the cave ground or on fallen blocks, respectively.