Unusual internal structure of cm-sized coldwater calcite: Weichselian spars in former pools of the Zinnbergschacht Cave (Franconian Alb/SE Germany)

The investigation of the internal structure of calcite crystals is a new focus in speleothem science, especially in the range of crystallization temperatures close to 0°C. Recently found calcite spars from Zinnbergschacht Cave of the Franconian Alb (SE Germany) are ideal for multi-method investigation. The elongated calcites (up to 6 cm in length) with three to six lateral faces and basal triangular faces at the ends are observed in collapse-zones in the cave. 230Th/U-ages of 38.9 ka suggest formation during the periglacial Weichselian, between the Scandinavian and Alpine Glaciations. The δ18O and δ13C values of the calcite spars vary from -11.18 to -16.11‰ V-PDB and from -4.78 to -6.13‰ V-PDB, respectively. The exceptionally low δ18O values of these calcites appear to be due to precipitation in pools on ice. The values deviate considerably from those of conventional interglacial speleothems (δ18O = -7.21 to -7.55‰, δ13C = -9.77 to -10.86‰) and also from true Weichselian cryogenic calcites (composite spherulites and rhombohedral chains with δ18O = -15.06 to -18.04‰ and with δ13C = -3.52 to -4.13‰). The δ18O values of the latter calcites is typical of cryogenesis of calcites with extensive oxygen isotope fractionation (preferred incorporation of 18O into the co-occurring ice). Thus, the δ18O values of the calcites suggest cold conditions up to the beginning of cryogenesis. Cathodoluminescence (CL) and backscattered electrons (BSE) indicate the distribution of impurities within the calcite spars as pigmented triangles surrounded by clear calcite, with a higher density of the triangles in the outer areas. Three hierarchies of triangles can be distinguished by BSE, documenting a filigreed primary structure of the spars. Electron backscatter diffraction (EBSD) reveals a divergent orientation of the triangular subcrystals from the center to the outer corners of the calcites. Thus, their internal structure reflects an example of fascicular optic fibrous calcites (FOFC), frequently discussed in carbonate petrology