Stable isotope fractionation in speleothems: Laboratory experiments

Abstract

In recent years, stalagmites have become important archives for paleoclimate. Several studies applying carbon and oxygen isotopes of stalagmites show a simultaneous increase of δ 13C and δ 18O 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 δ 13C and δ 18O values of the precipitated calcite for varying temperature, drip interval and initial SI CaCO 3 . All experiments show an isotopic enrichment of both δ 13C and δ 18O 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 δ 18O and δ 13C is lower for higher temperatures indicating faster oxygen isotope exchange between the water reservoir and the bicarbonate in the solution. In case of δ 13C, the fractionation factor between the precipitated calcite and the bicarbonate in the solution, 13 α CaCO 3 − HCO 3 − , 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, τ p, and the oxygen isotope exchange time between the bicarbonate and the water, τ b, yields faster reaction rates in comparison to previously published time constants, particularly at higher temperature (23 °C).