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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.

A quantitative analysis was performed with the aim of identifying changes in the volume and thickness of the Brenva Glacier tongue (Mont Blanc Massif, Italy) in the second half of the 20th century. This analysis was based on the comparison of digital elevation models (DEMs) derived from historical records, specifically maps (1959, 1971, 1983, 2003) and photogrammetric surveys (1991, 1997). The DEMs were generated by means of a digital photogrammetric workstation, with semi-automatic and automatic procedures. Problems relating to the identification of the control points in the photos had to be resolved in order to define the external orientation. An unconventional photogrammetric methodology, based on the identification of homologous points in zones considered outside of the glacier area, was adopted to insert the surveys into a single reference system. Furthermore, along with the photogrammetric data, DEMs derived from digitized historical maps were generated and compared to define changes in the geometry of the glacier tongue. The results indicated a positive long-term glacier tongue balance. In fact, between 1959 and 2003, there was an increase of 22.6 × 106 m3, equal to an average thickness of ca.+ 34 m (+ 0.7 m a− 1 w.e.). Validation of the data obtained from comparison of the DEMs and the reliability of the results were discussed as well.

International audience; The behaviour of rare earth elements (REE) during fluid–rock interaction in mid-crustal shear zones has received little attention, despite their potential for mass balance calculation and isotopic tracing during deformation. In this study, several cases of large REE mobility during Alpine fluid-driven shear zone development in the pre-Alpine granitic basement of the Mont Blanc Massif are considered. On a regional scale, the undeformed granite compositions range within 5 wt% SiO2 (70.5–75.3 wt%) and magmatic chemical variations are of the order of 10–20%, ascribed to minor effects of crystal fractionation. Major and trace element mobility observed in shear zones largely exceeds these initial variations. Shear zones developed a range of mineral assemblages as a result of shearing at mid-crustal depths (at not, vert, similar0.5 GPa, 400°C). Five main shear zone assemblages involve muscovite, chlorite, epidote, actinolite and calcite, respectively, as major phases. In most cases, selective enrichments of light or heavy REE (and Y, Ta, Hf) are observed. REE mobility is unrelated to deformation style (cataclastic, mylonitic), the intensity of strain, and to the shear zone's major metamorphic mineral assemblages. Instead, the changes in REE concentrations are ascribed to the alteration of pre-existing magmatic REE-bearing minerals during deformation-related fluid–rock interaction and to the syntectonic precipitation of metamorphic REE-bearing minerals (mainly monazite, bastnäsite, aeschynite and tombarthite). Minor proportions (

To improve our understanding of present-day cosmogenic production systematics of 10Be, 3H and 3He, we exposed three sets of targets of purified water at altitudes of 620, 3810 and 4745 m in the Mont Blanc Massif of the French Alps. In addition, tanks were stored 1780 m underground to quantify 3He contributions from decay of “inherited” tritium initially present in the water. After analyses of 3He and 10Be, both the summit and tunnel 3H–3He tanks were re-degassed and stored underground for an additional year. The stored summit tanks were then analyzed to determine cosmogenic 3H levels by the 3He ingrowth method, and the tunnel tanks used to re-determine inherited tritium. Production rates (in atoms per g H2O per year) for direct production of 3He and 10Be were 1824±52 and 112±9; 1013±16 and 70±5; and 134±58 and 5.9±0.7 at the three elevations, respectively. We determined production ratios of 0.32±0.08 for 3H:3He and 20.2±1.5 for (3H+3He):10Be. Our 10Be production rates, when normalized for inter-laboratory calibration and for differences in geomagnetic latitude of exposure, are somewhat lower than results of a similar experiment undertaken by Nishiizumi et al. (1996). Our 3H:3He ratio is consistent with theoretical and meteorite estimates (Kruger and D. Heymann, 1968), but considerably lower than values assumed in many exposure age studies of igneous rocks (e.g., (Kurz, 1986; Trull et al., 1995)).

Foreland-propagating external thrust belts may be considered as essentially plane strain phenomena so that displacements can be correlated throughout their linked, three-dimensional fault geometry. This approach has been applied to part of the northwest external French-Swiss Alps, around the Mont Blanc basement massif. Imbricates of basement and cover sequences on the SW margin of this massif restore to a width in excess of 77 km with an implicit shortening of at least 67 km. These displacements can be correlated with those in the neighbouring Helvetic nappes by transferring movements, via lateral branch lines, onto the Mont Blanc thrust. By reappraising thrust geometries, the Helvetic/Ultrahelvetic nappe complex has been restored to a width of 114 km to the ESE of the Aiguilles Rouges basement massif. Displacements on the internal (SE) margin of the Mont Blanc massif, estimated by balanced sections and a restoration of the Ultrahelvetic klippen in the sub-alps, exceed 59 km. Thrust continuity, incorporating the restorations of nappes and imbricate geometries around the Mont Blanc massif, is illustrated on a crude, restored branch-line map which also serves as a preliminary palaeogeographic reconstruction. External thrust systems, to the east of the external Belledonne/Aiguilles Rouges massif, restore to a width of at least 140 km in the footwall to the Frontal Pennine thrust.

In 1980 and 1981 mercury was determined in the filtered air (0.45 μm filter pore size) over the Ligurian coast from Diano Marina to Genoa and over Tuscany from Florence to Rosignano Solvay after air sampling using an airship. From 1980 to 1982 air sampling was also performed 1–2 m over the ground in Genoa and Rosignano Solvay, in Fiascherino/La Spezia, in the ventilated air of the mercury mine of Abbadia San Salvatore (Mt. Amiata/ Tuscany), in the surroundings of the geothermal hydroelectric power plants in Larderello as well as in Piancastagnaio and Bagnore (Mt. Amiata region), at one small fumarole of Vesuvius, at high altitudes of the Mont Blanc massif and some other sites. Air samples were taken using gold and silver adsorbers for the mercury collection. Determinations of the electrothermally desorbed mercury were carried out by atomic adsorption spectrometry. Different local conditions resulted in a wide concentration range of total or prevailing elemental mercury (about 1–1200 ng/m3, STP). The background levels (1–6 ng/m3, STP) were lower over the sea than over land. A geochemical anomaly (Mt. Amiata) and civilization accounted for the regional enhancement of atmospheric mercury levels; industrial activities influenced only even more limited areas. As a rule several hundred meters away from the point sources the air was almost unpolluted by mercury.