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

M Luetscher,G E Moseley,C Spötl,R L Edwards,M Borreguero

doi:10.5194/tc-7-1073-2013

Abstract

Abstract. Coarse crystalline cryogenic cave carbonates (CCCcoarse) dated to the last glacial period are common in central European caves and provide convincing evidence of palaeo-permafrost during this time. Little is known, however, about the exact nature of the environment in which CCCcoarse formed as no modern analogue setting is known. Here, we report the first findings of sub-recent, albeit inactive, CCCcoarse from a cave of the Western Alps which is located in the present-day permafrost zone. The globular shape and the presence of ubiquitous euhedral crystal terminations are comparable to previously reported aggregates from the last glacial period and strongly suggest that these aggregates formed subaqueously in pools lacking agitation. Furthermore, stable isotope values of mm-sized spheroids point to calcite precipitation in a closed system with respect to CO2, strongly supporting the hypothesis of a cryogenic origin associated with the freezing of water ponds. U-series analyses revealed three clusters of late Holocene calcite precipitation intervals between 2129 and 751 a b2k. These ages correlate with known periods of elevated summer temperatures, suggesting that warming and thawing of the frozen catchment above the cave allowed water infiltration into the karst system. The growth of CCCcoarse resulted from the re-freezing of this water in the still cold karst cavities.

Highlights

The distribution of alpine permafrost and its evolution in a changing climate is being extensively studied to identify potential hazards associated with instable debris slopes and rock-wall activity (e.g. Huggel et al, 2010; Fischer et al, 2012)
While several of these features could be caused by processes other than ice, a new class of carbonate deposits, cryogenic cave carbonates (CCC), has recently emerged as the most reliable indicator ofglacial processes which can be dated by U-series methods (Zak et al, 2004, 2008, 2012)
This paper extends for the first time the record of CCCcoarse into the Holocene using samples from a partly deglaciated alpine cave located in the present-day permafrost zone

Summary

Introduction

The distribution of alpine permafrost and its evolution in a changing climate is being extensively studied to identify potential hazards associated with instable debris slopes and rock-wall activity (e.g. Huggel et al, 2010; Fischer et al, 2012). About the past evolution of permafrost (Harris et al, 2009; French, 2011; Stoffel and Huggel, 2012) and a better identification of freeze and thaw cycles could contribute to the interpretation of geomorphic and ecological responses to specific climatic events Caves represent unique environments to identify present and past cryogenic activity because they are connected to atmospheric processes but are well protected from surface erosion. While several of these features could be caused by processes other than ice, a new class of carbonate deposits, cryogenic cave carbonates (CCC), has recently emerged as the most reliable indicator of (palaeo)glacial processes which can be dated by U-series methods (Za ́k et al, 2004, 2008, 2012)

Methods

Results

Discussion

Conclusion