MULTIPROXY PALEOCLIMATE ANALYSIS OF LATE QUATERNARY SPELEOTHEM ISOTOPE RECORDS FROM REED'S CAVE, SOUTH DAKOTA AND RAT'S NEST CAVE, ALBERTA

Speleothems are important paleoclimate proxy records because they are formed in very stable cave environments and the isotope records can be precisely dated with U-series mass spectrometry. Speleothems from Reed's Cave, South Dakota and Rat's Nest Cave, Alberta are interpreted as part of paleoclimate reconstructions for Western North America. Carbon and oxygen isotope records from multiple speleothems within these caves reflect the complexity of interpreting the paleoclimate signal. Isotopic variation is controlled by temporal changes in cave seepage water and temperature dependent oxygen isotope fractionation in the calcite-water system. Dominant controls on the isotope ratios can vary between drip sites and this may result in discordant signals for coeval speleothems within the same cave. Records from Eastern North America and Western Europe show evidence of cooling and warming events and millennial cycles.

Combining oxygen isotope data with hydrogen isotope data from fluid inclusions provides a useful multi-proxy approach to paleoclimate modeling. Besides the water present in inclusions, additional water is obtained on heating the speleothem calcite to over 800°C; this suggests that water is strongly bound to the calcite, a type of lattice water. Sub-zero FTIR (Fourier-transform infrared) spectroscopy experiments show that the water that is visible at 3400 cm-1 is not liquid, fluid inclusion water. This implies a possible isotopic fractionation between structural water and macroinclusion water. Current work shows a shift of ~ 20 to 30 per mil in the D/H when measured in high temperature calcination experiments (and compared to fluid inclusions). This shift could represent fractionation of lattice bound water.