Cosmogenic 3He paleothermometry on post-LGM glacial bedrock within the central European Alps

Abstract. Diffusion properties of cosmogenic 3He in quartz at Earth’s surface temperatures offer the potential to reconstruct the evolution of past in-situ temperatures directly from formerly glaciated areas, information important for improving our understanding of glacier-climate interactions. In this study, we apply cosmogenic 3He paleothermometry on rock surfaces gradually exposed since the Last Glacial Maximum (LGM) to the Holocene period along two deglaciation profiles in the European Alps (Mont Blanc and Aar massifs). Laboratory experiments conducted on one representative sample per site indicate significant variability in 3He diffusion kinetics between the two sites, with quasi linear Arrhenius behavior observed in quartz from the Mont Blanc site and complex Arrhenius behavior observed from the Aar site, which we interpret to indicate the presence of multiple diffusion domains (MDD). Assuming that same diffusion kinetics apply to all quartz samples along each profile, predictive simulations indicate that 3He abundance in all the investigated samples should be at equilibrium with present-day temperature conditions. However, measured natural 3He concentrations in samples exposed since before the Holocene indicate an apparent 3He thermal signal significantly colder than today. This observed 3He thermal signal cannot be explained with a realistic post-LGM mean annual temperature evolution in the European Alps at the study sites. One hypothesis is that the diffusion kinetics and MDD model applied may not provide sufficiently accurate, quantitative paleo-temperature estimates in these samples; thus, whereas pre-Holocene 3He thermal signal is indeed preserved in the quartz, the helium diffusivity would be lower at Alpine surface temperatures than our diffusion models predict. Alternatively, if the modeled helium diffusion kinetics is accurate, the observed 3He abundances may reflect complex geomorphic/paleoclimatic evolution with much more recent ground temperature changes associated with the degradation of alpine permafrost.