Investigation of OSL surface exposure dating to reconstruct post-LIA glacier fluctuations in the French Alps (Mer de Glace, Mont Blanc massif)

Providing quantitative constraints on late Pleistocene glacier fluctuations remains an important challenge for understanding glacier response to past and future climate changes. In most mountainous settings, paleo-glacier reconstructions are limited because they often lack precise temporal constraints. Different geochronological methods have been developed and applied to date specific geomorphological or sedimentological markers for paleo-glacier dynamics. Recently, OSL (Optically Stimulated Luminescence) surface exposure dating has been introduced and provides us with an opportunity to improve paleo-glacier reconstructions. This method is based on the sensitivity of the OSL signal from rock minerals to light, resulting in bleaching of the OSL signal within the upper first millimeters of the exposed rock surface, a process that depends on the exposure age, the rock type and the local setting (e.g. topographic shielding, bedrock orientation etc.). Here, we investigate the potential of OSL surface exposure along a vertical cross-section of polished bedrock surfaces with known post-LIA (Little Ice Age) exposure ages (from 3 to 137 years) along the Mer de Glace glacier (Mont Blanc massif, France). The infrared stimulated luminescence (IRSL) signals from rock slices exhibit increasingly deep bleaching profiles with elevation and thus exposure age, which is consistent with progressive glacier thinning since the LIA. Our results show that OSL surface exposure dating can be applied to periglacial environments, and is a promising tool for high-resolution reconstruction of ice extent fluctuations, both in space and time.