ISOTOPIC COMPOSITION OF PALEOPRECIPITATION FROM FLUID INCLUSIONS IN SPELEOTHEM CALCITE

Improvements in the extraction of microinclusion water from speleothem calcite make possible multiple, higher resolution measurements of dD of water. This can be applied to paleoclimate reconstruction using d¹⁸O and d¹³C of speleothems. Additionally, analysis of lattice-bound water may provide an additional paleothermometer. Two speleothems from Reed's Cave, South Dakota that grew during the Wisconsin glacial period from marine isotope stages 4 to 2 (65 –23 kya) were analyzed to determine the isotopic composition of paleoprecipitation. Paleotemperatures are calculated using O-isotope fractionation between calcite and water. The oxygen isotopes of fluid inclusion water cannot be measured directly because of post-depositional exchange of oxygen molecules between the host calcite and the fluid inclusions. The isotopic compositions of cave dripwaters approximate average annual d¹⁸O and d D of precipitation (Yonge et al .1983), therefore d¹⁸O can be calculated from D/H of inclusion water using the MWL relationship d D=8 d¹⁸O + 10.

FTIR analysis shows that even after removal of microinclusions, a strong water signal still exists. High temperature calcination of calcite to CaO + CO₂ releases this crystal-bound water. Isotopic (D/H) fractionation between bound and inclusion water may yield a paleotemperature record. Also, the presence of this component in water released at high temperatures may explain a -20‰ offset observed by some researchers.