Inferring climate change from stable isotope compositions of ancient speleothems on Earth: possible implications for climatic reconstructions elsewhere in the solar system

摘要

The stable isotopic compositions of Late Pleistocene and Holocene cave deposits (speleothems) are valuable proxies for high-resolution climate reconstructions on Earth, in particular with respect to changes in vegetation, temperature, atmospheric composition, and monsoonal precipitation. On the basis of U-Th dating and annual growth bands, researchers have shown that many speleothems can grow continuously for thousands to hundreds of thousands of years. Depending on the growth rate of a given speleothem, and its sensitivity to environmental changes on the surface, it may record climatic shifts on timescales ranging from weeks to years. This has made modern speleothems particularly useful for applications ranging from paleotempestology to studies of glacial-interglacial transitions. It stands to reason, then, that ancient speleothems might hold valuable information about climatic change in the deep geologic past - at resolutions that have been previously unattainable using other materials (e.g. paleosols). Here we report carbon and oxygen isotopic signatures obtained from an Early Permian flowstone that was extracted from a shallow paleocave in Oklahoma. We interpret the stable isotope data to reflect progressive aridification and devegetation, possibly as a result of CO_2 forcing. This interpretation is consistent with independent paleoclimatic data from coeval sediments in the region, and is also broadly consistent with similar records from modern, low-latitude speleothems. The fact that isotopic signatures are preserved in speleothems up to a few hundred million years old indicates that it might be possible to retrieve similar climatic data from ancient carbonate deposits elsewhere in the solar system.