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. The oxygen and hydrogen isotopic compositions of water in fluidinclusions in speleothems are important hydroclimate proxies because theyprovide information on the isotopic compositions of rainwater in the past.Moreover, because isotopic differences between fluid inclusion water and thehost calcite provide information on the past isotopic fractionation factor,they are also useful for quantitative estimation of past temperaturechanges. The oxygen isotope ratio of inclusion water (δ18Ofi), however, may be affected by isotopic exchange between thewater and the host carbonate. Thus, it is necessary to estimate the biascaused by this postdepositional effect for precise reconstruction ofpaleotemperatures. Here, we evaluate the isotopic exchange reactionbetween inclusion water and host calcite based on a laboratory experimentinvolving a natural stalagmite. Multiple stalagmite samples cut from thesame depth interval were heated at 105 ∘C in the laboratory from0 to 80 h. Then, the isotopic compositions of the inclusion water weremeasured. In the 105 ∘C heating experiments, theδ18Ofi values increased from the initial value by0.7 ‰ and then remained stable after ca. 20 h. Thehydrogen isotope ratio of water showed no trend in response to the heatingexperiments, suggesting that the hydrogen isotopic composition of fluidinclusion water effectively reflects the composition of past drip water. Wethen evaluated the process behind the observed isotopic variations using apartial equilibration model. The experimental results are best explainedby the assumption that a thin CaCO3 layer surrounding the inclusionreacted with the water. The amount of CaCO3 that reacted with the wateris equivalent to 2 % of the water inclusions in molar terms. These resultssuggest that the magnitude of the isotopic exchange effect has a minorinfluence on paleotemperature estimates for Quaternary climatereconstructions.

Experimental evaluation of oxygen isotopic exchange between inclusion water and host calcite in speleothems