Using triple oxygen isotopes to constrain speleothem paleorecord interpretation in western USA cave systems

Abstract:

Speleothem oxygen isotope (δ¹⁸O) records provide key insight into the rate and timing of terrestrial paleoclimate changes during the late Quaternary. However, it can be difficult to deconvolve the δ¹⁸O signal into its individual components, which include numerous processes related to temperature, rainfall, evaporation, infiltration, and the cave environment. Here, we develop a geochemical framework for using triple oxygen isotope analyses (Δ’¹⁷O) to constrain interpretation of δ¹⁸O speleothem records. This framework identifies dominant hydrologic processes by their characteristic (although not necessarily unique) trends in δ¹⁸O vs. Δ’¹⁷O space. We apply this framework in case studies of western USA speleothems. First, we investigate the Cave of the Bells, Arizona, record. Data define a horizontal δ¹⁸O vs. Δ’¹⁷O trend consistent with Rayleigh distillation, which agrees with published interpretations invoking changing temperature and precipitation (amount, intensity, source, or seasonality) as drivers of the changes in the d¹⁸O record. Notably, results likely exclude within-cave kinetic effects as a driver. Second, we compare two Nevada speleothems, from Leviathan and Lehman Caves, which formed under (near-)equilibrium and kinetic conditions, respectively; Lehman Caves δ¹⁸O mimics that of Leviathan Cave δ¹⁸O but has 6 ‰ variability in δ¹⁸O that is not likely due to changes in δ¹⁸O of regional precipitation alone. The combined data set defines a negative δ¹⁸O vs. Δ’¹⁷O trend and points to the importance of evaporative effects, either at the moisture source or during seasonal infiltration, on the Leviathan Cave δ¹⁸O record. Our work shows that δ¹⁸O records from these three cave systems cannot result from one process, something that is generally difficult to validate with δ¹⁸O records alone. Triple oxygen isotope distributions are valuable because they constrain the viable interpretations of individual records and provide a pathway to more accurate multi-record synthesis.