The use of aragonite stalagmites as paleoclimate proxies has become increasingly common because of their often exceptional uranium-series dating precision and fast growth rates, which allow near-annual climate resolution. However, aragonite is known to be susceptible to open-system behavior and may recrystallize to calcite under typical cave conditions. As a result, the fidelity of the oxygen isotopic variations preserved in aragonite stalagmites is not yet as firmly established as for calcite stalagmites, and few studies have investigated oxygen isotope replication and equilibrium with cave drip waters. I present new data and review findings from the literature that show that aragonite stalagmites may pass both the equilibrium and replication tests in some cases, but that other aragonites are clearly precipitated out of equilibrium with cave drip waters. As a case study, δ18O data for three exceptionally well-dated stalagmites from Juxtlahuaca Cave, Mexico, are presented. Two of the stalagmites are aragonite, and the third was measured on a diagenetically calcitized aragonite stalagmite. These results show that the modern aragonite precipitates in apparent oxygen isotopic equilibrium with drip waters, and the two aragonite speleothems replicate strongly over the past 530 yr, thus confirming a common paleoclimate signal. Observations of δ18O systematics in previously published aragonite stalagmite δ18O records from China and Belize show that at least two samples clearly precipitated modern aragonite out of isotopic equilibrium with cave drip waters. Because the equilibrium test only applies to modern precipitation for which temperature and drip-water δ18O values are known, both equilibrium and replication tests are required for confident interpretation of aragonite stalagmite δ18O time series, and studies that fail or do not attempt such tests should be considered cautiously.

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