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In recent years, stalagmites have become an important archive for paleoclimate. Several studies about stable isotope records in stalagmites show a simultaneous enrichment of d18O and d13C along individual growth layers, which is associated with kinetic isotope fractionation. However, to deduce paleoclimatic information from calcite which is precipi...
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Despite the paleoclimatic relevance of primary calcite to aragonite transitions in stalagmites, the relative role of fluid Mg/Ca ratio, supersaturation and CO32- concentration in controlling such transitions is still incompletely understood. Accordingly, we have monitored the hydrochemistry of 50 drips and 8 pools that are currently precipitating c...
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... During periods, where the correlation coefficient is near 1, the slope between d 13 C and d 18 O (X vs Y) is also near and even above 1. Such values are extremely large when compared to laboratory and cave experiments, which have investigated stable isotopes along the path of carbonate precipitation (Mickler et al., 2006;Wiedner et al., 2008;Polag, 2009;Polag et al., 2010). Also studies, which investigated correlation coefficients between both stable isotopes for short periods along the growth axis of stalagmites, only rarely obtained values above 1 for the slope (Mickler et al., 2006;Boch and Sp€ otl, 2011 then represent the pure drip water oxygen isotope signal. ...
... The correlation coefficient is never below the negative 99% significant level (lower dashed line). The corresponding 15-pt running slope between d 13 C and d 18 O values (e) shows a large variability and often exceeds the experimentally found range (grey shaded rectangle) determined by laboratory experiments(Wiedner et al., 2008;Polag, 2009;Polag et al., 2010; for more details see Sec. 5.2). ...
The strength of the North Atlantic Oscillation (NAO) is considered to be the main driver of climate changes over the European and western Asian continents throughout the last millennium. For example, the predominantly warm Medieval Climate Anomaly (MCA) and the following cold period of the Little Ice Age (LIA) over Europe have been associated with long-lasting phases with a positive and negative NAO index. Its climatic imprint is especially pronounced in European winter seasons. However, little is known about the influence of NAO with respect to its eastern extent over the Eurasian continent.
Here we present speleothem records (δ¹³C, δ¹⁸O and Sr/Ca) from the southern rim of Fergana Basin (Central Asia) revealing annually resolved past climate variations during the last millennium. The age control of the stalagmite relies on radiocarbon dating as large amounts of detrital material inhibit accurate ²³⁰Th dating. Present-day calcification of the stalagmite is most effective during spring when the cave atmosphere and elevated water supply by snow melting and high amount of spring precipitation provide optimal conditions. Seasonal precipitation variations cause changes of the stable isotope and Sr/Ca compositions. The simultaneous changes in these geochemical proxies, however, give also evidence for fractionation processes in the cave. By disentangling both processes, we demonstrate that the amount of winter precipitation during the MCA was generally higher than during the LIA, which is in line with climatic changes linked to the NAO index but opposite to the higher mountain records of Central Asia. Several events of strongly reduced winter precipitation are observed during the LIA in Central Asia. These dry winter events can be related to phases of a strong negative NAO index and all results reveal that winter precipitation over the central Eurasian continent is tightly linked to atmospheric NAO modes by the westerly wind systems.
... These mechanisms depend on the degree of supersaturation, which would also explain the observed correlation between the calcite precipitation rate and the initial SI CaCO 3 of the solution. Similar observations were also made by in-situ cave experiments using the same glass channel set-up (Polag (2009)). This suggests that the impact of the drop does not lead to a complete CO 2 equilibration of the solution with the cave atmosphere as it is often assumed in theoretical models. ...
In recent years, stalagmites have become important archives for paleoclimate. Several studies applying carbon and oxygen isotopes of stalagmites show a simultaneous increase of δ13C and δ18O along individual growth layers, which is interpreted as being indicative of isotope fractionation under disequilibrium conditions. In order to obtain robust paleoclimatic information from calcite precipitated under these non-equilibrium conditions it is important to improve the quantitative understanding of the corresponding isotope fractionation processes. Here we present laboratory experiments simulating calcite precipitation under cave-analogue conditions. The major focus was the investigation of the temporal evolution of the δ13C and δ18O values of the precipitated calcite for varying temperature, drip interval and initial SICaCO3.All experiments show an isotopic enrichment of both δ13C and δ18O with increasing distance from the point of drip water impinge. Longer drip intervals and higher temperatures result in a larger enrichment. In addition, the slope between δ18O and δ13C is lower for higher temperatures indicating faster oxygen isotope exchange between the water reservoir and the bicarbonate in the solution.In case of δ13C, the fractionation factor between the precipitated calcite and the bicarbonate in the solution, 13αCaCO3-HCO3-, shows a larger increase with higher temperatures in comparison to previous studies. This possibly indicates an increasing contribution of disequilibrium isotope fractionation processes for increasing temperatures, which are not accounted for by the equilibrium isotope fractionation factors. Furthermore, a quantitative determination of the calcite precipitation time, Τp, and the oxygen isotope exchange time between the bicarbonate and the water, Τb, yields faster reaction rates in comparison to previously published time constants, particularly at higher temperature (23°C).
