Stable isotope fractionation in speleothems: Laboratory experiments

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• 作者：Daniela Polag ; Denis Scholz ; Christian Mü ; hlinghaus ; Christoph Spö ; tl ; Andrea Schrö ; der-Ritzrau ; Monika Segl ; Augusto Mangini
• 关键词：Stable isotopes ; Stalagmite ; Stable isotope fractionation ; Laboratory experiments
• 刊名：Chemical Geology
• 出版年：2010
• 出版时间：3 December 2010
• 年：2010
• 卷：279
• 期：1-2
• 页码：31-39
• 全文大小：1078 K

文摘

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 SI_CaCO3.

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, ¹³α_{CaCO3 − 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, 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).