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Abstract: The Vil-car-1 flowstone core from Villars cave (SW France) provides one of the first European speleothem records extending back to 180 ka, based on U–Th TIMS and MC-ICP-MS measurements. The core offers a continuous record of Termination II and the Last Interglacial. The penultimate deglaciation is characterized by a prominent 5‰ depletion in calcite δ18O. Determining which specific environmental factors controlled such a large oxygen isotopic shift offers the opportunity to assess the impact of various factors influencing δ18O variations in speleothem calcite. Oxygen isotope analyses of fluid inclusions indicate that drip water δ18O remained within a very narrow range of ±1‰ from Late MIS6 to the MIS5 δ18O optimum. The possibility of such a stable behaviour is supported by simple calculations of various effects influencing seepage water δ18O. Although this could suggest that the isotopic shift in calcite is mainly driven by temperature increase, attempts to quantify the temperature shift from Late MIS6 to the MIS5 δ18O optimum by assuming an equilibrium relationship between calcite and fluid inclusion δ18O yield unreasonably high estimates of ∼20 °C warming and Late MIS6 cave temperatures below 0 °C; this suggests that the flowstone calcite precipitated out of thermodynamic equilibrium at this site. Using a method proposed by combining clumped isotope measurements, fluid inclusion and modern calcite δ18O analyses, it is possible to quantitatively correct for isotopic disequilibrium and estimate absolute paleotemperatures. Although the precision of these absolute temperature reconstructions is limited by analytical uncertainties, the temperature rise between Late MIS6 and the MIS5 optimum can be robustly constrained between 13.2 ± 2.6 and 14.6 ± 2.6 °C (1σ), consistent with existing estimates from Western Europe pollen and sea-surface temperature records. [Copyright &y& Elsevier]

Abstract: The oxygen isotope composition of speleothems is an important proxy of continental paleoenvironments, because of its sensitivity to variations in cave temperature and drip water δ18O. Interpreting speleothem δ18O records in terms of absolute paleotemperatures and δ18O values of paleo-precipitation requires quantitative separation of the effects of these two parameters, and correcting for possible kinetic isotope fractionation associated with precipitation of calcite out of thermodynamic equilibrium. Carbonate clumped-isotope thermometry, based on measurements of Δ 47 (a geochemical variable reflecting the statistical overabundance of 13C18O bonds in CO2 evolved from phosphoric acid digestion of carbonate minerals), potentially provides a method for absolute speleothem paleotemperature reconstructions independent of drip water composition. Application of this new technique to karst records is currently limited by the scarcity of published clumped-isotope studies of modern speleothems. The only modern stalagmite reported so far in the literature yielded a lower Δ 47 value than expected for equilibrium precipitation, possibly due to kinetic isotope fractionation. Here we report Δ 47 values measured in natural speleothems from various cave settings, in carbonate produced by cave precipitation experiments, and in synthetic stalagmite analogs precipitated in controlled laboratory conditions designed to mimic natural cave processes. All samples yield lower Δ 47 and heavier δ18O values than predicted by experimental calibrations of thermodynamic equilibrium in inorganic calcite. The amplitudes of these isotopic disequilibria vary between samples, but there is clear correlation between the amount of Δ 47 disequilibrium and that of δ18O. Even pool carbonates believed to offer excellent conditions for equilibrium precipitation of calcite display out-of-equilibrium δ18O and Δ 47 values, probably inherited from prior degassing within the cave system. In addition to these modern observations, clumped-isotope analyses of a flowstone from Villars cave (France) offer evidence that the amount of disequilibrium affecting Δ 47 in a single speleothem can experience large variations at time scales of 10kyr. Application of clumped-isotope thermometry to speleothem records calls for an improved physical understanding of DIC fractionation processes in karst waters, and for the resolution of important issues regarding equilibrium calibration of Δ 47 in inorganic carbonates. [Copyright &y& Elsevier]

Abstract: The last deglaciation and its climatic events, such as the Bølling–Allerød (BA) and the Younger–Dryas (YD), have been clearly recorded in the δ13C profiles of three stalagmites from caves from Southern France to Northern Tunisia. The three δ13C records, dated by thermal ionization mass spectrometric uranium–thorium method (TIMS), show great synchroneity and similarity in shape with the Chinese cave δ18O records and with the marine tropical records, leading to the hypothesis of an in-phase (between 15.5 and 16 ka ∼±0.5ka) postglacial warming in the Northern Hemisphere, up to at least 45°N. The BA transition appears more gradual in the speleothem records than in the Greenland records and the Allerød seems warmer than the Bølling, showing here close similarities with other marine and continental archives. A North–South gradient is observed in the BA trend: it cools in Greenland and warms in our speleothem records. Several climatic events are clearly recognizable: a cooler period at about 14ka (Older Dryas (OD)); the Intra-Allerød Cold Period at about ∼13.3ka; the YD cooling onset between 12.7 and 12.9±0.3ka. Similar to the BA, the YD displays a gradual climate amelioration just after its onset at 12.75±0.25ka, up to the Preboreal, and is punctuated by a short climatic event at 12.15ka. Even though the Southern Hemisphere stalagmite records seem to indicate that the postglacial warming started about ∼3ka±1.8ka earlier in New Zealand (∼41°S), and about ∼1 to ∼2ka earlier in South Africa (24.1°S), large age uncertainties, essentially due to slow growth rates, make the comparison still perilous. The overall δ13C speleothem record seems to follow a baseline temperature increase controlled by the increase in insolation and punctuated by cold events possibly due to the N-America freshwater lake discharges. [Copyright &y& Elsevier]