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The Medieval and Roman Warm Periods in the Western Swiss Alps
Luetscher, M., Borreguero, M., Moseley,G.E., Spotl, C. and Edwards, R.L. 2013. Alpine permafrost thawing during the Medieval Warm Period identified from cryogenic cave carbonates. The Cryosphere 7: 1073-1081.

The authors write that a new class of carbonate deposits - coarse cryogenic cave carbonates (CCCcoarse) - "has recently emerged as the most reliable indicator of (palaeo)glacial processes which can also be dated by U-series methods (Zak et al. 2004, 2008, 2012)." And they say that their paper "extends for the first time the record of CCCcoarse into the Holocene using samples from a partly deglaciated alpine cave [Leclanche Cave (46°20'42"N, 7°15'47"E] located in the present-day permafrost zone" of the Sanetsch area of the western Swiss Alps, as described by Borreguero et al. (2009).

What was done
Working with seven individual CCCcoarse aggregates found in Leclanche Cave, Luetscher et al. analyzed them in terms of their petrography, isotopic composition and U/Th dating.

What was learned
In the area of petrography, the five researchers discovered pieces of evidence that "strongly suggest that these aggregates formed sub-aqueously in pools lacking agitation." In the area of isotopic composition, they found evidence for the "progressive freezing of ponded water." And in the area of U/Th dating, they determined that "two coeval samples were deposited at 751 55 and 823 58 a b2k," where a b2k = years before AD 2000, so that in these two cases the two dates were AD 1249 and AD 1177. In addition, four other samples provided an average age of AD 927, while one sample suggested "a significantly older age of 2129 235 a b2k," which equates to 129 BC.

What it means
Luetscher et al. state that the "230Th/234U dating indicates that all spheroids formed in the Late Holocene," and that six were "coeval with the Medieval Warm Period (MWP) characterized by elevated summer temperatures (Mangini et al., 2005; Buntgen et al., 2011)," while the other sample fell within "the Roman Warm Period, which was also characterized by a succession of warm climate episodes (Buntgen et al., 2011) and reduced glacier extents (Holzhauser et al., 2005)." And so they conclude that "CCCcoarse has the potential to provide precise chronologies of past warm episodes in areas where palaeo-environmental proxy data are scarce." And so we conclude that the Medieval and Roman Warm Periods were both likely warmer than the Current Warm Period has been to date in this part of the world, in light of the fact that this area today is overlain by permafrost, which had to have thawed sufficiently during these two earlier periods for water to seep down and into the cave, where it established the pools within which the CCCcoarse aggregates were created.

Borreguero, M., Pahud, A., Favre, G., Heiss, G., Savoy, L. and Blant, M. 2009. Lapi di Bou: Recherches et explorations speleologiques 1974-2009. Cavernes 70: 213 pp.

Buntgen, U., Tegel, W., Nicolussi, K., McCormick, M., Frank, D., Trouet, V., Kaplan, J.O., Herzig, F., Heussner, K.-U., Wanner, H., Luterbacher, J. and Esper, J. 2011. 2500 years of European climate variability and human susceptibility. Science 331: 578-582.

Holzhauser, H., Magny, M. and Zumbuhl, H.J. 2005. Glacier and lake-level variations in west-central Europe over the last 3500 years. The Holocene 15: 789-801.

Mangini, A., Spotl, C. and Verdes, P. 2005. Reconstruction of temperature in the Central Alps during the past 2000 yr from a δ18O stalagmite record. Earth and Planetary Science Letters 235: 741-751.

Zak, K., Onac, B. and Persoiu, A. 2008. Cryogenic carbonates in cave environments. A review. Quaternary International 187: 84-96.

Zak, K., Richter, D.K., Filippi, M., Zivor, R., Deininger, M., Mangini, A. and Scholz, D. 2012. Coarsely crystalline cryogenic cave carbonate - a new archive to estimate the Last Glacial minimum permafrost depth in Central Europe. Climate of the Past 8: 1821-1837.

Zak, K., Urban, J., Cilek, V. and Hereman, H. 2004. Cryogenic cave calcite from several Central European caves: age, carbon and oxygen isotopes and a genetic model. Chemical Geology 206: 119-136.

Reviewed 18 December 2013