Prospecting cold based summit glaciers in the Austrian Alps: A novel approach to past glacier extent

Mountain glaciers can serve as climate archives over periods and in regions for which other climate information is extremely scarce. Almost all information on Holocene glacier extent in the Alps is based on moraine datings, dendrochronology and exposure age studies, referring to valley glacier tongue positions, but not to the high Alpine area. In contrast to the pioneering work done in the Western Alps, the existence of cold ice at summit glaciers has not been investigated systematically in Austrian Alps to date. The overarching project aim is to research minimal glacier extent of summit glaciers during the Holocene period. To this end, potentially cold based summit glaciers will be identified and subsequently explored for their geometry, englacial temperatures, as well as for their surface mass and energy balance. The glacier stratigraphy and ice build-up process is investigated based on analysis of water isotopes, chemical and physical properties, along with radiometric dating methods. Key research questions are: i) what are the maximum age constraints of cold base ice at the highest elevations? ii) what are energy fluxes from and to the present day cold ice bodies at their interfaces with the atmosphere and with possibly frozen bedrock? Ten test sites in exposed summit ranges with relatively low precipitation have been selected for this purpose. These sites will be thoroughly investigated for snow and ground temperatures, internal stratigraphy, surface flow velocity and ice body geometry based on probing and ground penetrating radar. These data will be used to identify the most promising sites for sampling basal ice by lateral access and by ice core drilling. An automatic weather station will record englacial and basal temperatures, BTS temperatures of the surrounding, energy and mass balance, as well as repeat photography of the surface properties. Data from historical archives will be collected and analysed to find changes in the topography of the summit glaciers since the first documentation, which usually was done during the Little Ice Age (LIA). We neither expect a continuous stratigraphy nor continuous climate information in the ice samples. The analysis of chemical and physical properties of the ice cores includes major ions, melt water conductivity and insoluble micro particle profiles, in combination with d18O and dD; the physical parameters cover ice density, grain size and bubble density. While impurities and stable water isotope species serve as general stratigraphic markers, the analysis of physical ice properties is focused on characterizing basal ice conditions, e.g. by indicating stagnant basal ice. The investigation of radionuclides includes the bomb tritium time horizon, from which the possible recent ablation rate is inferred. 210Pb and radiocarbon analyses of the particulate carbon fraction (>5g) will be deployed to constrain the maximum age down on the decadal and the millennial time scale, respectively. In this way the project will archive whatever information is left in the cold ice of the Austrian summit glaciers today.

The investigations at the summit of Weißseespitze in the Tyrolean Kauner valley and at the Großvenediger summit in Hohe Tauern National Park revealed that patches of cold ice -still exist in the Eastern Alps, are a valuable climate archive for the last millennia -are severely threatened by the current high rates of ice melt even at the highest summits Mountain glaciers can serve as climate archives of periods and in regions for which other climate information is extremely scarce. The ice core at Weißseespitze summit glacier is only 11 m long, but contains an archive for the last 6000 years, the period during which the summit has been continuously covered by ice. Under current melt rates, the ice will be gone in just over a decade even without any further warming. If we want to preserve the information in the cores for the next generations, we have to save these archives as soon as possible. So far, almost all information on Holocene glacier extent in the Alps has been based on moraine datings, dendrochronology and exposure age studies, related to valley glacier tongue positions, but not to the high Alpine area. In contrast to the pioneering work done in the Western Alps, the existence of cold ice at summit glaciers has not been investigated systematically in the Austrian Alps to date. The overarching aim of our project was to find minimal glacier extent of summit glaciers during the Holocene period.Our findings confirmed that the ice at the highest elevations was already gone at least once during the Holocene, in the so-called Holocene Climate Optimum. At the end of this warm period, Ötzi the ice man was buried under snow and ice at the nearby Tisenjoch, where he was exposed by melt in 1991. Although melt affected this lower elevation body of ice a few decades earlier than Weißseespitze, the ice at the highest summits will not last much longer than a decade: The ice at the highest peaks is less thick than at the glacier tongues, but has been affected by high melt rates since 2003 - with ever more frequent extreme years. Currently, ice temperatures at the base are still -3C, but with decreasing ice thickness we can expect rapid warming and even the destabilization of the cold ice as yet frozen to the ground. In this respect, the project made a vital timely appeal to archive whatever information is left in the cold ice of the Austrian summit glaciers today before it is too late.