Investigation of the stable isotope fractionation in speleothems with laboratory experiments

Many speleothems show evidence for calcite precipitation under disequilibrium conditions. To improve the understanding of these kinetic processes, several laboratory experiments were performed to study the fractionation of stable oxygen and carbon isotopes during the precipitation of calcite. Carbonate was precipitated under controlled conditions from both a body of standing water (beaker experiments) and a solution flowing along a channel (channel experiments) at a relative humidity of 100%. Slow degassing of CO₂, simulated by the beaker experiments, results in δ¹⁸O values in equilibrium with the solution. In contrast, the δ¹³C values show a significant enrichment, inversely proportional to the height of the solution in the beakers. Fast degassing of CO₂, simulated by the channel experiments, showed an enrichment of both δ¹³C and δ¹⁸O and a slope of Δδ¹³C/Δδ¹⁸O of 1.4±0.6. These results represent experimental evidence for the Hendy effect, which is manifested in (i) a progressive increase in δ¹⁸O and δ¹³C away from the growth axis and (ii) a positive correlation between δ¹⁸O and δ¹³C along a single growth layer of a stalagmite.