Speleothem deposits are among the most valuable continental formations in paleoclimate research, as they can be dated using absolute dating methods, and they also provide valuable climate proxies. However, alteration processes such as post-depositional mineralogical transformations can significantly influence the paleoclimatic application of their geochemical data. An innovative sampling and measurement protocol combined with scanning and transmission electron microscopy, X-ray diffraction and Fourier transform infrared spectroscopy is presented, demonstrating that carbonate precipitating from drip water in caves at ~10 °C contains amorphous calcium carbonate (ACC) that later transforms to nanocrystalline calcite. Stable oxygen isotope fractionations among calcite, ACC and water were also determined, proving that ACC is 18O-depleted (by >2.4 ± 0.8‰) relative to calcite. This, in turn, has serious consequences for speleothem-based fluid inclusion research as closed system transformation of ACC to calcite may induce a negative oxygen isotope shift in fluid inclusion water, resulting in deterioration of the original compositions. ACC formation increases the speleothems’ sensitivity to alteration as its interaction with external solutions may result in the partial loss of original proxy signals. Mineralogical analysis of freshly precipitating carbonate at the studied speleothem site is suggested in order to determine the potential influence of ACC formation.

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Several Jurassic tectonic units of the Rudabánya Hills (Inner Western Carpathians) were studied by structural geological, metamorphic petrological and geochronological methods, in order to unravel its deformation history and nappe stacking. Three series showing pronounced lithostratigraphic similarities (black shale, sandstone turbidite and olisthostrome) are interpreted to be subunits of the Telekesoldal nappe. It is questionable whether a Triassic basement can be associated with these Jurassic rocks. Metamorphic petrological data indicate metamorphism at the transition between the high-temperature anchizone and epizone (300-350°C and 2-2.5 kbar pressure). These series were affected by three events of ductile deformation including in order 1) layer-parallel foliation, 2) folding with axial plane foliation and 3) kink bands. K-Ar ages suggest that the main detected metamorphic event took place about 137-117 Ma ago. In contrast, the uppermost Triassic-Jurassic Telekesvölgy, and the Triassic Bódva series were only affected by diagenetic (partly anchimetamorphic) alteration. The metamorphosed Jurassic complexes thrust upon the Telekesvölgy-Bódva sequence as a separate nappe at 87-94 Ma as suggested by K-Ar data. The metamorphism and deformation of these Jurassic series are suggested to appear simultaneously with the Middle Jurassic-earliest Cretaceous subduction of the Meliata branch of the Neotethys Ocean. Post-metamorphic deformation, including nappe emplacement seems to be coeval with mid-Cretaceous deformations of other Inner Western Carpathian units. The latter is probably related to the closure of the Penninic-Vahic Ocean farther to the north.