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Poster; The Pilatte hut is located at 2572 m a.s.l. at Saint-Christophe-en-Oisans (south of the Ecrins Massif, France), ata 3-hours-walk from La Bérarde hamlet. Its capacity is 120 beds for hikers and climbers who are engaged in theascent of Les Bans (3669 m a.s.l.).Built on a rocky ledge on the right side of the Pilatte Glacier (A = 2.64 km2; L = 2.6 km), it currently dominatesthe glacier by about 150 m. This relief results from the retreat of the glacier since the end of the Little Ice Age, asthe till around the hut was deposited during this stage. The glacier has lost about 1.8 km in length during the sameperiod.A first wooden hut was built in 1925 and presently serves as a winter refuge. In 1954, the growth of mountainactivities led to the construction of a larger hut made of cemented stones. An extension to the west made ofreinforced concrete was built in 1994. But in the late 1980s, severe damages to the 1954 part of the building werealready recognized: vertical cracks lining the north and south facades, ,subsidence (c. 10 cm downstream) of theground floor, cracked interior walls.

Environmental magnetism of speleothems is still in its early stage of development. Here we report on our investigation of the environmental and paleomagnetic information that has been recorded in speleothems, and what are the factors that control its preservation and reliability. To address these issues, we used a multidisciplinary approach, including rock magnetism, petrography, scanning electron microscopy, stable carbon and oxygen isotope compositions, and major and trace element concentrations. We applied this to a set of samples from different stages of speleothem evolution: present-day dripwater (glass plates), a weathered stalactite, a fresh stalagmite, cave sediments, and terra rossa soils. These samples come from the Penico and Excentricas caves, located in two distinct aquifers of the Algarve region, South Portugal. Our results show that the main magnetic carriers of the speleothems under study are primary (detrital) and consist of maghemite (and magnetite?). Similarities in coercivity and temperature dependence of the studied set of samples suggest that iron oxides are inherited from the terra rossa soils that cap the cave and were transported to the speleothems by dripwater. Hence, they represent a regional environmental signature. Interestingly, a stable and probably detrital remanent magnetization could be isolated in the fresh stalagmite, whereas the weathered stalactite yielded chaotic magnetic directions and very low remanent intensities. We propose that these low intensities can be the result from(i) different remanence acquisition mechanisms between stalagmite and stalactite and/or (ii) iron dissolution by fungal activity. We also suggest that magnetic properties and color and the content in detrital elements in the fresh speleothem inform about environmental processes acting on the interface of rock (soil)-atmosphere, while oxygen isotope composition and alkaline-earth element concentrations inform about calcite-water interaction processes. These results provide a better understanding of how environmental information is recorded in speleothems and what the factors are that control the reliability of the paleomagnetic and paleo-environmental signal. info:eu-repo/semantics/acceptedVersion

International audience; Batch experiments were conducted to simulate abiotic weathering of the continental crust under high-CO2 atmospheric conditions during Precambrian times, i.e., corresponding to the general Archaean conditions as well as to the immediate aftermath of the Neoproterozoic Snowball Earth ice ages. Three types of rock (basalt, granodiorite and tonalite) representative of the Archaean to Proterozoic continental crust were reacted in the form of powders for 1 year at 40°C with pure water under various water/rock ratios, in oxic and anoxic atmospheres containing 10% CO2, that is, under conditions assumed to characterize the greenhouse effect which prevailed at that time. Chemical and mineralogical data collected during the course of the experiments reflect alteration phenomena occurring in two steps: (1) rapid leaching of the fresh surfaces, probably related to the proton-donor capacity of the CO2 and (2) a steady state reaction under near-neutral conditions. These observed dissolution rates can be satisfactorily modeled by kinetic data available in the literature. Using a 1-D weathering model reproducing the two steps, we evaluate the atmospheric CO2 consumption rate at the Snowball Earth aftermath, focusing on the relative contribution of surface leaching versus steady state reaction. The results show that the process is dominated by surface exchange.

This work is a multidisciplinary approach of Precambrian weathering and marine sedimentation in order to look for the proxys of the compositional evolution if the Earth oceans and atmosphere. Archaean BIFs (Carajás, Brazil) result from partial oxidation of dissolved oceanic iron, whereas Palaeoproterozoic pyrite bearing sandstones attest the low oceanic Precambrian sulphate levels. One year long atmosphere experiences of rock weathering under Precambrian conditions, supporting a numerical model, constraint the required time interval for the pCO2 decrease from 300 PAL (or Precambrian Atmospheric Level) to Present-day ones (1 PAL) in the Snowball Earth aftermath at a duration in between 200 000 and 500 000 yrs. The comparison between present-day CO2 consumption data and modelled ones underline the relatively low values for Precambrian times likely due to the lack of terrestrial plants. Finally, modelling of abiotic neoproterozoic cap carbonates precipitation display that enriched CO2 submitted to degassing and/or evaporation can explain their formation at the end of the global glaciations events.; Ce travail multidisciplinaire vise à exploiter les enregistrements de l'évolution des enveloppes fluides terrestres au Précambrien (altération continentale et de la sédimentation marine). Des BIFs archéens (Carajás, Brésil) attestent de l'oxydation incomplète du fer dissous océanique, alors que des grès à pyrites indiquent la pauvreté en sulfates océaniques. Des expériences annuelles d'altération de roches en conditions précambriennes, support d'un modèle numérique, encadrent le temps requis pour que la pCO2 atm diminue de 300 PAL (Present Atmospheric Level) à 1 PAL entre 200 000 et 500 000 ans suite aux glaciations globales néoprotérozoïques. La consommation précambrienne de CO2atm, inférieure à l'Actuel, s'explique par l'absence de végétaux terrestres. Enfin, la modélisation de la précipitation abiotique des cap carbonates néoprotérozoïques montre que le dégazage et/ou l'évaporation d'un océan riche en CO2 expliquent la formation de ces roches au sortir des glaciations globales.

This work is a multidisciplinary approach of Precambrian weathering and marine sedimentation in order to look for the proxys of the compositional evolution if the Earth oceans and atmosphere. Archaean BIFs (Carajás, Brazil) result from partial oxidation of dissolved oceanic iron, whereas Palaeoproterozoic pyrite bearing sandstones attest the low oceanic Precambrian sulphate levels. One year long atmosphere experiences of rock weathering under Precambrian conditions, supporting a numerical model, constraint the required time interval for the pCO2 decrease from 300 PAL (or Precambrian Atmospheric Level) to Present-day ones (1 PAL) in the Snowball Earth aftermath at a duration in between 200 000 and 500 000 yrs. The comparison between present-day CO2 consumption data and modelled ones underline the relatively low values for Precambrian times likely due to the lack of terrestrial plants. Finally, modelling of abiotic neoproterozoic cap carbonates precipitation display that enriched CO2 submitted to degassing and/or evaporation can explain their formation at the end of the global glaciations events.; Ce travail multidisciplinaire vise à exploiter les enregistrements de l'évolution des enveloppes fluides terrestres au Précambrien (altération continentale et de la sédimentation marine). Des BIFs archéens (Carajás, Brésil) attestent de l'oxydation incomplète du fer dissous océanique, alors que des grès à pyrites indiquent la pauvreté en sulfates océaniques. Des expériences annuelles d'altération de roches en conditions précambriennes, support d'un modèle numérique, encadrent le temps requis pour que la pCO2 atm diminue de 300 PAL (Present Atmospheric Level) à 1 PAL entre 200 000 et 500 000 ans suite aux glaciations globales néoprotérozoïques. La consommation précambrienne de CO2atm, inférieure à l'Actuel, s'explique par l'absence de végétaux terrestres. Enfin, la modélisation de la précipitation abiotique des cap carbonates néoprotérozoïques montre que le dégazage et/ou l'évaporation d'un océan riche en CO2 expliquent la formation de ces roches au sortir des glaciations globales.