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Karst springs in the Main Range of the Crimean Mountains and the Crimean Piedmont show a restricted range of values (δ18O = –10.5 to –8.0 ‰, δ2H = –72 to –58 ‰), somewhat more negative than the weighted mean of meteoric precipitation. This suggests preferential recharge at higher elevations during winter months. Groundwater tapped by boreholes splits in three groups. A first group has isotopic properties similar to those of the springs. The second group shows significantly lower values (δ18O = –13.3 to –12.0 ‰, δ2H = –95 to –82 ‰), suggesting recharge during colder Pleistocene times. The third group has high isotope values (δ18O = –2.5 to +1.0 ‰, δ2H = –24 to –22 ‰); the data points are shifted to the right of the Local Meteoric Water Line, suggesting water–rock exchange processes in the aquifer. These boreholes are located in the Crimean Plains and discharge mineralized (ca. 25 g L−1) thermal (65°C) water from a depth of 1600–1800 m. Groundwater associated with mud volcanoes on the Kerch peninsula have distinct isotope characteristics (δ18O = –1.6 to +9.4 ‰, δ2H = –30 to –18 ‰). Restricted δ2H variability along with variable and high δ18O values suggest water–rock interactions at temperatures exceeding 95 °C.

In the tropics, geochemical records from stalagmites have so far mainly been used to qualitatively reconstruct changes in precipitation, but several new methods to reconstruct past temperatures from stalagmite material have emerged recently: i) liquide vapor homogenization of fluid inclusion water ii) noble gas concentrations in fluid inclusion water, iii) the partitioning of oxygen isotopes between fluid inclusion water and calcite, and iv) the abundance of the 13C18O16O(‘clumped’) isotopologue in calcite. We present, for the first time, a direct comparison of these four paleo-thermometers by applying them to a fossil stalagmite covering nearly two glaciale interglacial cycles (Marine Isotope Stages (MIS) 12 e 9) and to two modern stalagmites, all from northern Borneo. The temperature estimates from the different methods agree in most cases within errors for both the old and recent samples; reconstructed formation temperatures of the recent samples match within 2-sigma errors with measured cave temperatures. However, slight but systematic deviations are observed between noble gas and liquide vapor homogenization temperatures. Whereas the temperature sensitivity of fluid inclusion d18O and clumped isotopes is currently debated, we find that the calibration of Tremaine et al. (2011) for fluid inclusion d18O and a synthetic calcite-based clumped isotope calibration (Ziegler et al., in prep.) yield temperature estimates consistent with the other methods. All methods (with the potential exception of clumped isotopes) show excellent agreement on the amplitude of glaciale interglacial temperature change, indicating temperature shifts of 4-5 C°. This amplitude is similar to the amplitude of Mg/Ca-based regional sea surface temperature records, when correcting for sea level driven changes in cave elevation. Our reconstruction of tropical temperature evolution over the time period from 440 to 320 thousand years ago (ka) adds support to the view that climate sensitivity to varying greenhouse forcing is substantial also in the deep tropics.

Karst springs in the Main Range of the Crimean Mountains and the Crimean Piedmont show a restricted range of values (δ18O = –10.5 to –8.0 ‰, δ2H = –72 to –58 ‰), somewhat more negative than the weighted mean of meteoric precipitation. This suggests preferential recharge at higher elevations during winter months. Groundwater tapped by boreholes splits in three groups. A first group has isotopic properties similar to those of the springs. The second group shows significantly lower values (δ18O = –13.3 to –12.0 ‰, δ2H = –95 to –82 ‰), suggesting recharge during colder Pleistocene times. The third group has high isotope values (δ18O = –2.5 to +1.0 ‰, δ2H = –24 to –22 ‰); the data points are shifted to the right of the Local Meteoric Water Line, suggesting water–rock exchange processes in the aquifer. These boreholes are located in the Crimean Plains and discharge mineralized (ca. 25 g L−1) thermal (65°C) water from a depth of 1600–1800 m. Groundwater associated with mud volcanoes on the Kerch peninsula have distinct isotope characteristics (δ18O = –1.6 to +9.4 ‰, δ2H = –30 to –18 ‰). Restricted δ2H variability along with variable and high δ18O values suggest water–rock interactions at temperatures exceeding 95 °C.

Coarsely crystalline cryogenic cave carbonates (CCCcoarse) are widely used indicators of past permafrost conditions. Nevertheless, details of their formation with respect to macroscopic morphology, stable isotope evolution and potential metastable precursor phases are poorly understood. CCCcoarse were found in 5 caves located along a north-south transect of the Ural Mountains, Russia. A comprehensive data set was generated including results of carbonate stable isotope composition, stable isotopic composition of fluid inclusion water, stable oxygen isotope thermometry, trace element composition, X-ray diffraction and transmission properties and U/Th disequilibrium dating. Detailed petrographic characterization of the samples allows for the proposal of a morphological classification scheme for CCCcoarse. Non-crystallographic branching of crystallites, also referred to as crystal splitting, is identified as the mechanism enabling morphological variety in CCCcoarse. Splitting propensity is likely related to physico-chemical properties of the mineral forming solution such as Mg++ concentration and supersaturation with respect to carbonate minerals. Numerical modelling of C and O stable isotope evolution shows that open system style degassing of CO2 during CCCcoarse formation accounts for 10 – 20% of the observed isotopic trends. These results support a model of slow and continuous degassing of CO2 via nucleation of gas bubbles in co-precipitating ice in compliance to freezing experiments. Fluid inclusion analyses suggest that early stages of CCCcoarse formation take place at isotopic equilibrium between carbonate minerals and the parent solution. However, oxygen isotope fractionation ( 𝛼𝑐𝑐−𝑤𝑎𝑡𝑒𝑟 18 = 1.0318 ±0.0005) appears to be smaller than expected from literature values extrapolated to 0°C. Diffraction properties of a CCCcoarse specimen indicate that it formed via non-classical crystallisation pathways. Crystallisation by particle attachment (CPA) of poorly crystalline, or even amorphous precursor phases can explain diffraction data.

The paper presents a detailed study demonstrating the hypogenic origin of the Corona 'e Sa Craba quartzite cave in SW Sardinia (Italy). Although the quartzite host-rock of this cave derived from silicification of Cambrian dolostones and dissolution of carbonate remnants could have had a role in the speleogenesis, detailed morphologic and petrographic investigation revealed clear evidence of quartz dissolution without signs of mechanical erosion by running waters. Thin section microscopy and scanning electron microscope (SEM) images show pervasive dissolution morphologies, such as pits and notches on quartz crystals causing the deep arenization of the cave walls, suggesting that the dissolution of quartz had a primary role in the formation of the void. The study of secondary cave minerals and the sulfur isotopic composition of sulfates and sulfides, coupled with data on fluid inclusions, allowed reconstruction of the peculiar speleogenetic history of this hypogenic hydrothermal quartzite cave. The cave formed by reduced hydrothermal fluids, probably under basic neutral pH in phreatic conditions. The presence of abundant cations of Ba2+ in reduced Cl-rich fluids enhanced the quartz dissolution rate, allowing the formation of the voids in deep settings. During the Late Oligocene uplift of the area, the hydrothermal fluids in the cave reached oxygen-rich conditions, thus a minerogenetic phase started with the deposition of barite when the temperature of the fluid was