Open-system alpine speleothems: implications for U-series dating and paleoclimate reconstructions

Open-system alpine speleothems: implications for U-series dating and paleoclimate reconstructions -The present work is a critical review of 130 alpha-spectrometric U/Th analyses of speleothems from north-eastern Italy. About 33% of measurements display activity ratios (230 Th/ 234 U)\u3e1 while maintaining (234 U/ 238 U) close to unity, thus falling outside the closed system U-series disequilibrium fi eld. The open-system behaviour was mostly observed in speleothems that, based on geological data and petrographic observations, have experienced dramatic environmental changes, such as fl ooding of caves by glacial meltwater or dissolution by undersaturated waters. The paper discusses possible causes of the open-system behaviour in the U-Th system, and evaluates evidence for the U leaching vs. 230 Th introduction. By considering only samples which do not show the open-system behaviour and are not contaminated by detrital thorium, fi ve periods of active speleothem deposition have been identifi ed for north-eastern Italy: 0 to 10 ka, 40 to 63 ka, 88 to 100 ka, 140 to 160 ka, and 190 to 250 ka. Even in these, apparently good, data set, the ages of 140 to 160 ka might be affected by undetected 230 Th introduction, since they appear to be 20 to 40 ka older than the data obtained for the same paleoclimate period from European speleothems