Alpine permafrost thawing during the Medieval Warm Period identified from cryogenic cave carbonates

摘要

Coarse crystalline cryogenic cave carbonates (CCC_coarse) dated tothe last glacial period are common in central European caves and provideconvincing evidence of palaeo-permafrost during this time. Little is known,however, about the exact nature of the environment in whichCCC_coarse formed as no modern analogue setting is known. Here, wereport the first findings of sub-recent, albeit inactive, CCC_coarsefrom a cave of the Western Alps which is located in the present-daypermafrost zone. The globular shape and the presence of ubiquitous euhedralcrystal terminations are comparable to previously reported aggregates fromthe last glacial period and strongly suggest that these aggregates formedsubaqueously in pools lacking agitation. Furthermore, stable isotope valuesof mm-sized spheroids point to calcite precipitation in a closed system withrespect to CO₂, strongly supporting the hypothesis of a cryogenic originassociated with the freezing of water ponds. U-series analyses revealed threeclusters of late Holocene calcite precipitation intervals between 2129 and751 a b2k. These ages correlate with known periods of elevated summertemperatures, suggesting that warming and thawing of the frozen catchmentabove the cave allowed water infiltration into the karst system. The growthof CCC_coarse resulted from the re-freezing of this water in thestill cold karst cavities.