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Mountain glaciers, tracers of continental palaeoclimates

This study consists of activities centred on the reconstruction of continental palaeoclimates through the study of the fluctuations of mountain glaciers during the Pleistocene and Holocene periods. These projects are intended to develop new, integrated, approaches for the quantification of changes in temperature and precipitation in continental areas, in order to better characterise the atmosphere as an actor in palaeoclimatic dynamics.

A major challenge of these new projects is to test the possibility of exploiting the spatial variability of mountain glaciers to replenish palaeoprecipitations during specific episodes during the Quaternary period. This task requires the bringing together of complementary expertise in geomorphology, geochronology, and numerical modelling. The recruitments recently made at CRPG strengthened its expertise in glacial geomorphology and numerical modelling. The base of the CRPG in these projects remains the determination of geochronology by cosmogenic isotopes, either by the quality of facilities and the expertise of the noble gases laboratory (cosmogenic noble gas analysis of 3He and 21Ne), or through our close collaboration with the CEREGE (analysis of the radionuclides 10Be, 26Al, and 36Cl). It is also important to note that, within this study, our fruitful collaborations with the LGGE glaciologists and the climatologist modellers from LSCE-LOCEAN will be continued and strengthened.

The main project of this study will be GALAC, which has obtained, in 2011, support from the young researcher programme at ANR to unite several geomorphologists (CRPG), geochronologists (CRPG), and glaciologists (LGGE). This project will complete the production of cosmogenic 3He dating of ancient glaciers in Altiplano, with special emphasis on the period of deglaciation (20-15 ky BP), to better characterise the spatial variability of glaciers during the wet period of Lake Tauca (17-15 ky). By coupling numerical modelling, the water balance of lakes, and a glacier mass balance, our project will map the palaeoprecipitations during this very particular climatic episode. Our preliminary work shows that the temporal correlation of these two objects of the hydrosphere (lakes and glaciers) can accurately and reliably reconstruct the spatial field of precipitations and temperatures in the mountainous regions. This project will test and therefore pave the way for a new method of palaeoclimate reconstruction under continental conditions. Our results on the tropical Andes will better characterise the major atmospheric processes that were associated with climatic event Heinrich 1, an episode characterised by the cessation of the thermohaline circulation for several hundred years between 17,000 and 15,000 years ago. The work on palaeoglaciation will also be conducted on non-tropical regions, in high-latitude regions characterised by very different climatic contexts. Thus, we will establish a geochronological framework for the deglaciation of the Vosges (in collaboration with the University of Nancy II) and the Kerguelen Islands (with V. Jomelli and LGGE). Although it is well established that these regions have experienced one or more cycles of ice ages, the absolute chronology of these episodes are still uncertain. These pioneering projects will therefore establish the first geochronological constraints on the dynamics of glaciers in these regions. Their palaeoclimatic significance will be quantified by numerical modelling and will be viewed in a global context, by comparing them with the behaviour of tropical glaciers, in particular, those of the Altiplano. These projects can only continue to produce solid results if we continue, in the coming years, to improve the reliability and accuracy of cosmogenic chronometers. Our recent data obtained on the websites of local calibration of the Altiplano are very promising in this regard ; in the most favourable cases, it is now possible to date mountain glaciers with an uncertainty of less than 5%. This fundamental prerequisite is a major advance because it offers the opportunity to compare these continental recordings to other palaeoclimatic series (ocean sediments and continental ice).

Several figures from the GALAC project, which aims to reconstruct the spatial variability of glaciers in the Altiplano during the wet phase of Lake Tauca (between 17,000 and 15,000 years BP)

Finally, in certain favourable cases, we also plan to refine the conclusions of these reconstructions based on palaeoglaciation by coupling them with geochemical tracers, such as the analyses of stable isotopes in speleothems and lake bioconstructions (δ18O, δ13C) or D/H in the organic matter of lake sediments. Indeed, these isotopic tools can potentially provide independent constraints on palaeo-temperature or precipitation and allow for the reduction of the uncertainties associated with the inversions of glacier equilibrium lines by numerical modelling.