Accounting for kinetic isotope effects in Soreq Cave (Israel) speleothems

Abstract

There is growing evidence that speleothem calcite grows out of isotopic equilibrium with cave drip water, with clumped isotope analysis providing a sensitive indicator for disequilibrium. This disequilibrium is primarily the result of CO2 degassing from a thin film of water, leading to irreversible 13C enrichment and reversible 18O enrichment and Δ47 depletion. Here we examine isotopic disequilibrium in Soreq Cave (Israel) using multiple modern-day and late Holocene speleothems. The variability observed in Δ47 is small, within the analytical uncertainty, but the Δ47-derived temperature is offset from the modern cave temperature by ∼4°C, reflecting degassing related disequilibrium that is fairly constant spatially. δ18O is more heterogeneous, reflecting short-term variability in drip water δ18Ow combined with variability in the speleothem growth rates and related fractionation between dissolved carbonate species and the growing calcite mineral. This complexity, however, is markedly reduced by spatial or temporal averaging, enabling an interpretation of the cave paleoclimate record. We examine the Soreq Cave speleothems through a comparison with 2 types of thermometers: one is based on CaCO3 precipitation from a bulk solution and is typically used for calibration of the Δ47 and δ18O thermometers; a second that is based on CaCO3 precipitating at the surface of the solution thus focusing and amplifying the thin film characteristics of speleothem formation. Soreq Cave speleothems are intermediate between these thermometers, providing a cave-specific thermometer calibration.