Monitoring stable isotopes in caves over altitudinal gradients: fractionation behaviour and inferences for speleothem sensitivity to climate change

摘要

Large changes of the climate can dramatically affect the environment surrounding and within a cave. This variability, including temperature shifts, can change the amount of in-cave isotopic fractionation affecting speleothems, potentially leaving these records difficult to interpret. Here, caves located in steep altitudinal topography in the Northern Italian Alps are used to create a thermal gradient (3–12 °C) to study the effects of temperature on the speleothem isotope record. The data indicate that the amount of in-cave oxygen isotope fractionation is reduced to minimum values of around 0.25‰ with cooler temperatures and under faster dripping stalactites, which infers that such sites would be most suitable for palaeoclimate reconstruction. However, when considering the possible freezing periods, reduced saturation index and fluctuating drip rates, such sites appear less than ideal for study on long timescales. The importance of picking the best site(s) within a cave for palaeoclimate reconstruction is paramount, to reduce the in-cave fractionation and gain a record that reflects climate changes. A method for rapid determination of calcite fractionation is demonstrated here, through prior cave monitoring of dripwaters and calcite precipitates, permitting an informed choice of speleothems for further study.