Characterization and Correction of Evaporative Artifacts in Speleothem Fluid Inclusion Isotope Analyses as Applied to a Stalagmite From Borneo

Abstract

Fluid inclusion water isotope measurements in speleothems have great potential for paleoclimate studies as they enable the reconstruction of precipitation dynamics and land temperatures. Several previous observations, however, suggest that inclusion waters do not always reflect the isotopic composition of surface precipitation. In such cases, dripwaters are thought to be modified by evaporation in the cave environment that results in more positive δ 2H and δ 18O values and shallow δ 2H/δ 18O slopes. Although evaporation can occur in cave systems, water can also be lost to evaporation during analysis but before water extraction. Here, we examine the likelihood of this possibility with a stalagmite from Borneo. We demonstrate that many samples lose water, and that water loss is controlled by the type and size of inclusions. With multiple replicate measurements of coeval samples, we calculate an evaporative δ 2H/δ 18O slope of 1.0 ± 0.6 (2SE). This value is consistent with model predictions of evaporative fractionation at high analytical temperature and low humidity. Finally, we propose a method to correct for this effect. We find that fluid–calcite δ 18O paleotemperatures calculated with corrected δ 18O data show excellent agreement with recent microthermometry temperature estimates for Borneo, supporting the validity of our approach and implying limited stalagmite δ 18O disequilibrium variations. Corrected fluid inclusion δ 18O and δ 2H values follow the expected hydroclimate response of Borneo to periods of reduced Atlantic Ocean meridional overturning circulation. Our results suggest that careful petrographic examination and multiple replicate measurements are necessary for reliable paleoclimate reconstructions with speleothem fluid inclusion water isotopes.