Evaluation of Speleothem Oxygen Isotope Fractionation from a Tropical Cave on the Island of Guam

Recent studies in the tropics apply oxygen isotope variations in speleothems as proxies for changes in past hydroclimate, yet little work has been done to study the modern tropical cave environment. Oxygen isotope time series in speleothems are commonly used to interpret past climate based on the assumption of equilibrium fractionation. When re-constructing paleoclimate, there are limited tools available to assess potential non-equilibrium isotope effects. One approach is to test for agreement of oxygen isotope values between parallel-milled tracks from a stalagmite. A stalagmite collected from Jinapsan Cave on the island of Guam supports the argument for equilibrium as multiple overlapping transects along the growth axis show consistent replication. However, to fully evaluate the fractionation processes relating to calcite growth, we have also studied the relationship between modern drip water and modern speleothem calcite. To accomplish this, we collected monthly dripwater samples and deployed artificial substrates under active drips to measure calcite growth rates and to assess the state of isotopic equilibrium between speleothem calcite and drip water. We evaluate the processes controlling the δ¹⁸O values of calcite formed on substrates relative to values for the dripwater feeding the site of the speleothem in Jinapsan Cave. The studied drip site shows seasonal variability in dripwater δ¹⁸O of ~1‰, with more negative composition occurring during the wet season. The δ¹⁸O of the substrate calcite sampled at the point of first growth (analogous to the growth axis in a speleothem) also shows a seasonal cycle of about 1‰, yet the values are more positive than predicted by the equilibrium equation of Kim & O'Neil 1997. In addition, carbon and oxygen isotopic composition between substrates strongly covary. Correlation of carbon and oxygen isotopes between substrates at the point of first growth may be explained by prior calcite precipitation. Carbon isotope values from the substrate calcite are strongly correlated with the Mg-Ca ratios of dripwater. Oxygen isotope values and the Mg-Ca ratio in drip waters are also well correlated, which may be due to coherence between the seasonal cycle of dripwater δ¹⁸O and the drier conditions that would cause prior calcite precipitation. The δ¹³C and Mg-Ca correlation is consistent with prior calcite precipitation affecting δ¹³C. Ongoing research is examining the relationship between calcite growth rate and the departure from equilibrium. When evaluated in the context of similar studies, the data from Guam show reasonable agreement with the fractionation factor of Tremaine et al., 2011, which was constructed from results for cave environments.