Investigation of the stable isotope fractionation in speleothems with laboratory experiments

摘要

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 CO_2, simulated by the beaker experiments, results in δ~(18)O values in equilibrium with the solution. In contrast, the δ~(13)C values show a significant enrichment, inversely proportional to the height of the solution in the beakers. Fast degassing of CO_2, simulated by the channel experiments, showed an enrichment of both δ~(13)C and δ~(18)O and a slope of Δδ~(13)C/Δδ~(18)O of 1.4±0.6. These results represent experimental evidence for the Hendy effect, which is manifested in (ⅰ) a progressive increase in δ~(18)O and δ~(13)C away from the growth axis and (ⅱ) a positive correlation between δ~(18)O and δ~(18)C along a single growth layer of a stalagmite.