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Numerous hyperthermal events have been documented through the Paleocene-Eocene transition. The best known hyperthermal event is the Paleocene-Eocene Thermal Maximum (PETM; around 56Ma), a period that led to surface and bottom water warming of about 5°C within a few millennia at tropical latitudes. It is therefore considered as one of the best analogues of current global warming. The PETM is also characterized by an abrupt 3-4 per mil negative δ13C excursion in deep marine core sediments and by a thin clay-rich layer associated with the PETM onset, most often interpreted as carbonate dissolution due to the shoaling of the CCD. The duration represented by these clays and carbonates is of peculiar interest to constrain the exported carbonate production dynamics of surface ocean and its dissolution throughout the water column. This is key to produce realistic carbon budgets across hyperthermal events.To this end, we generated a new 4 Ma (57.5-53.5) record of extraterrestrial 3He-derived sedimentation rates from pelagic sediments recording at least 10 hyperthermal events at ODP Site 1209 (North Pacific). Our main results indicate that carbonate sedimentation dropped drastically during the PETM onset (minimum of 0.02 cm/ka) and recovered rapidly during the recovery phase of the event (around 0.7 cm/ka). Surprisingly, the sedimentation rate is low (0.3 cm/ka) after the recovery until the Eocene Thermal Maximum 2 (ETM2; around 54Ma). After this major event, the sedimentation rate increased abruptly (0.7 cm/ka) over the last 500 ka of the studied interval due to the overabundance of Zygrhablithus bijugatus a large rod-shaped nannofossil whose ecology is poorly understood yet.Comparisons between the new record of extraterrestrial 3He-derived sedimentation rate and dissolution proxies from this and previous studies lead us to challenge the widely accepted model previously proposed for hyperthermal events, which assumes that the CaCO3 accumulation is mainly controlled by dissolution.

Although natural CO2-rich groundwaters of eastern Belgium have been known for centuries, the exact origin of their gas is still unclear. This paper presents the results of a sampling campaign in Belgium (Spa, Stoumont, Malmedy): 30 samples of both carbogaseous and non-carbogaseous groundwaters were analyzed for major elements, CO2 content and carbon isotopic composition. Among them, 13 samples were also analyzed for 3He/4He and 4He/20Ne ratios. The combination of δ13C (between ca. −9 ‰ VPDB1 and −2 ‰ VPDB), CO2/3He ratio (between 1.9×108 and 2.9×109) and 3He/4He (between 0.92 and 2.70 Ra) shows with a high level of confidence that the CO2 in the carbogaseous groundwater of Spa and Bru has a mantle origin. It can likely be attributed to the degassing of mantle from the neighboring Eifel volcanic fields, located at a distance of 100 km eastwards. The identity and nature of the deep-rooted fractures that act as CO2 transport pathways to the surface are still to be clarified, but several major thrust faults exist in the Rhenish Massif and could connect the Eifel volcanic fields with the studied area.

Terrestrial amplification (TA) of land warming relative to oceans is apparent in recent climatic observations. TA results from land-sea coupling of moisture and heat and is therefore important for predicting future warming and water availability. However, the theoretical basis for TA has never been tested outside the short instrumental period, and the spatial pattern and amplitude of TA remain uncertain. Here, we investigate TA during the Last Glacial Maximum (LGM; ~20 thousand years) in the low latitudes, where the theory is most applicable. We find remarkable consistency between paleotemperature proxies, theory, and climate model simulations of both LGM and future climates. Paleoclimate data thus provide crucial new support for TA, refining the range of future low-latitude, low-elevation TA to 1.37 − 0.23 + 0.27 (95% confidence interval), i.e., land warming ~40% more than oceans. The observed data model theory agreement helps reconcile LGM marine and terrestrial paleotemperature proxies, with implications for equilibrium climate sensitivity.

We use PRISM climatic data (1981–2010) and Landsat images (2012–2013) to establish an empirical relationship linking annual temperature and precipitation to the equilibrium line altitude (ELA) of glaciers in the Sierra Nevada (36–41°N, California, USA). For this, we determined the present-day ELAs of 57 glaciers and the local 0°C isotherms elevation Iso0, averaged over the 1981–2010 period. The difference, for each glacier, is Y, the normalized snowline altitude (Y = ELA – Iso0). We then empirically calibrated a logarithmic relationship between this normalized snowline altitude and mean annual precipitation using data from partially covered glaciers. Our calibration is statistically distinct from that previously established for the tropical and midlatitude Andes (Fox and Bloom [1994], Journal of Geography (Chigaku Zasshi), 103, 867–885; Condom et al. [2007], Global and Planetary Change, 59, 189–202). This new relationship for North America is an easy-to-use tool to permit paleoclimatic reconstructions from paleo-ELAs. For a specific paleoglacial site, paleotemperature can be computed knowing the paleoprecipitation range, and vice versa. We also performed a test showing that, if precipitation is well known, the uncertainty associated with paleotemperature is about 1°C (1σ).

The amplitude of the Last Glacial Maximum (LGM) cooling compared to pre-industrial has long been a topic of debate, which partly arises from the fact that this cooling is spatially heterogeneous. Paleotemperature reconstructions shows that this cooling is larger on land than over the oceans, a feature which is well captured by Global Climate Models. However the amplitude of the LGM cooling at high altitudes is still not well constrained, with available data showing an important disparity from a region to another (Blard et al., 2007; Tripati et al., 2014). Here we present a new compilation of glacier-based temperature reconstructions at high elevation (> 2500 m) for the LGM, which are compared to synchronous changes of sea surface temperatures (Pacific Ocean), along the American Cordillera, from 40°S to 40°N. This new reconstruction confirms that lapse rates were steeper during the LGM in the tropics and shows that this feature relates to a drier atmosphere. To further analyse this observation, we first use the IPSL global climate model PMIP4 results (Kageyama et al., 2021), which, in agreement with the reconstructions, yields a steeper tropical lapse rate in its LGM simulation, compared with the pre-industrial one. Next, we disentangle the impacts of the lower atmospheric CO2 concentration and of lower humidity using a single column radiative-convective equilibrium model (Kluft et al., 2019), and show the strong impact of changes in humidity in the tropical lapse rate steepening at the LGM.ReferencesBlard, P.-H., Lavé, J., Wagnon, P. and Bourlès, D : Persistence of full glacial conditions in the central Pacific until 15,000 years ago, Nature, 449, 591–594, https://doi.org/10.1038/nature06142, 2007.Tripati, A. K., Sahany, S., Pittman, D., Eagle, R. A., Neelin, J. D., Mitchell, J. L. and Beaucoufort, L.: Modern and glacial tropical snowlines controlled by sea surface temperature and atmospheric mixing, Nature Geoscience, 7, 205–209, https://doi.org/10.1038/ngeo2082, 2014.Kageyama, M., Harrison, S. P., Kapsch, M.-L., Lofverstrom, M., Lora, J. M., Mikolajewicz, U., Sherriff-Tadano, S., Vadsaria, T., Abe-Ouchi, A., Bouttes, N., Chandan, D., Gregoire, L. J., Ivanovic, R. F., Izumi, K., LeGrande, A. N., Lhardy, F., Lohmann, G., Morozova, P. A., Ohgaito, R., Paul, A., Peltier, W. R., Poulsen, C. J., Quiquet, A., Roche, D. M., Shi, X., Tierney, J. E., Valdes, P. J., Volodin, E., and Zhu, J.: The PMIP4 Last Glacial Maximum experiments: preliminary results and comparison with the PMIP3 simulations, Clim. Past, 17, 1065–1089, https://doi.org/10.5194/cp-17-1065-2021, 2021.Kluft, L., Dacie, S., Buehler, S. A., Schmidt, H., & Stevens, B. (2019). Re-Examining the First Climate Models: Climate Sensitivity of a Modern Radiative–Convective Equilibrium Model, Journal of Climate, 32(23), 8111-8125

Marl-limestone alternations are rhythmical inter-bedded deposits that commonly occur in many sedimentological environments. It is quite well established that these lithological variations originate from astronomically-driven climatic variations paced by the Milankovitch cycles of main periods 19, 23, 41, 100 and 405 ka. However, the sedimentological mechanisms involved are not clear: some models attribute these alternations to cyclic changes in the carbonate flux, while terrigenous siliciclastic input remained relatively constant. On the opposite, other models suggest that the carbonate flux was constant while the siliciclastic flux changed cyclically, or that both fluxes varied in antiphase. To disentangle these different scenarios, we collected marlstone and limestone samples from two sedimentary hemipelagic marl-limestone successions of Bajocian, Middle Jurassic (3 marl-limestone couplets over 3.4 m) and Valanginian, Lower Cretaceous (1 marl-limestone couplet over 0.9 m) from the Southern French Alps (Barles). We measured their concentrations in calcium carbonate, organic carbon, nannofossil, as well as in extraterrestrial 3He (3HeET). Carbonate contents range from 45% in marls to 86% in limestones. Importantly, the measured 3HeET concentrations of all samples remained nearly constant in the siliciclastic fractions, within uncertainties (< 20%). Hence, our results indicate that, at the astronomical timescale, sedimentation rates were mainly controlled by large changes in the CaCO3 net fluxes, leading to variable dilution of the terrigenous input. Nannofossil counting shows that pelagic CaCO3 fluxes of coccolithophores are inversely correlated to the total carbonate content along the marl-limestone alternations and represent less than 7% of the total carbonate content. Hence, in this setting, these marl-limestone alternations were driven by fluctuations in micritic CaCO3 supply from the nearby carbonate platform that variably diluted nannofossil and organic carbon particles. Finally, assuming a constant 3HeET flux of 100 pcc.cm-2.Ma-1, total 3HeET-derived sedimentation rates range from 20 to 30 m/Ma in the marl strata, while they reach up to 80 to 100 m/Ma in the limestone layers. These sedimentation rates are broadly compatible with local average rates estimated for the whole Bajocian and Valanginian stages by biostratigraphy.

This review article summarizes the state of the art of cosmogenic 3He (3Hec), with a focus on the most efficient methods for measuring this cosmogenic noble gas in terrestrial samples. After briefly reviewing the scientific applications and production pathways of cosmogenic 3He, I summarize the most important theoretical and practical aspects of 3He analyses and describe the best strategies for correcting for non-cosmogenic 3He components in minerals. I also review our knowledge of 3Hec production rates and explore potential new applications for future studies. Our ability to accurately and precisely measure cosmogenic 3He is mainly constrained by the level of the non-cosmogenic 3He background (i.e., magmatic, radiogenic, nucleogenic, and atmospheric 3He), and thus by the geological characteristics of the samples. Constructing 3He vs. 4He isochrons by analyzing several aliquots from the same sample constitutes a useful and overlooked method that is advantageous because it obviates the often-complicated step of vacuum crushing. This method also allows the direct and joint determination of cosmogenic 3He and the magmatic 3He/4He ratio. I perform numerical modeling to explore the impact of the non-cosmogenic 3He components on the final uncertainties and detection limits of 3He dating. Reducing the magmatic component by selecting phenocrysts in the 100–500 m size fraction improves the precision of cosmogenic 3He analyses. Moreover, it is important to measure U, Th, and Li concentrations in the analyzed minerals and their host rocks to ensure proper corrections for radiogenic 4He and nucleogenic 3He, improving both the accuracy and precision of the method. After summarizing the most important aspects of 3He analytical techniques, including the best 3Hec extraction techniques and the key parameters of noble gas mass spectrometry that result in accurate and precise helium isotopic measurements, I also review 3Hec production rates and their spatial variability. The global database of absolute calibration sites yields a world-wide average 3Hec production rate in olivine and pyroxene of 124 ± 11 at g−1 yr−1 using the LSD scaling and the online CREp calculator (https://crep.otelo.univ-lorraine.fr/#/). Cross-calibrations against 10Be indicate that the ratio of the production rate of 3Hec in olivine/pyroxene to that of 10Be in quartz is 33 ± 2 and increases by less than 7% between sea level and 5,000 m elevation. This important observation demonstrates that 3He in olivine/pyroxene and 10Be in quartz can be considered as synchronized chronometers. However, 3Hec/10Be cross-calibrations based on 3Hec in accessory minerals (zircon, garnet, kyanite, apatite) yield unexpectedly high 3He/10Be production ratios of 40–60 above 3,000 m elevation. As the capture of cosmogenic thermal neutrons by 6Li is unlikely to explain this excess, I discuss other plausible mechanisms that should be explored, such as 3Hec inherited from previous exposure episodes, unrecognized specific reaction pathways, or the impact of snow cover. New cross-calibration data obtained by measuring 3Hec against other cosmogenic nuclides in different settings will advance our understanding of cosmogenic nuclide production rates and improve the accuracy and precision of applications relying on cosmogenic 3He. Other improvements could extend the applicability of the 3He geoscientific toolbox; for example, coupling 3He with radioactive cosmogenic nuclides (10Be, 36Cl, 53Mn) will allow paleoaltimetry or the determination of burial ages or paleo-depths in intermediate and mafic terrains.

In global geochemical cycles, the solid flux from the continent to the ocean is usually reduced to the input of sediments from rivers [1]. However, regional studies have shown that the input of sediments from rocky coast erosion may be a significant part of this flux [2]. So, it is important to consider this input into global cycles and to quantify it over different timescales.On short-term timescales, from the year to the century, coastal erosion is currently quantified with direct measurement of the coastline retreat, between successive time intervals [3]. Extrapolating on timescales longer than a thousand years is difficult. This leads to a lack of data and therefore a gap in knowledge in longer term coastal erosion [4].A solution to quantify long-term erosion of rocky coast is to reconstruct the initial geometry of the coastline and to know the age of its formation. Volcanic islands are suitable objects for this method. Indeed, their initial shape is simple and can be easily reconstructed, and their maximum extension can be dated [5,6,7]. Thus, the topographic reconstruction of a volcanic island allows, by comparison with its current topography, the quantification of volumes lost by erosion. In turn, it becomes possible to obtain values of the rocky coast total retreat on timescales from thousands to hundreds of thousands of years [8].Another and complementary approach to quantify long-term erosion is the use of cosmogenic isotopes. This approach is widely used to date surface exposure and quantify long-term denudation rates in drainage basins [9]. The use of this approach to directly quantify sea-cliffs retreat is completely new. By sampling colluvial deposits resulting from coastal erosion of sea-cliffs, measuring cosmogenic 10Be and 3He concentrations, depending on the cliff lithology, may provide values on the retreat rate of the cliffs on the millennia timescale.It is interesting to test both approaches on volcanic islands. Indeed, their wide geographic distribution provides a diversity of climatic and geodynamic settings allowing to analyze the effects of various factors on long-term coastal erosion. We compare and discuss the results obtained by both methods on the island of Fogo (Cape Verde). [1] Milliman and Farnsworth (2013). Cambridge University Press.[2] Regard et al. (2022). Earth and Planetary Science Letters 579, 117356.[3] Bird (2011). John Wiley & Sons.[4] Prémaillon et al. (2018). Earth Surface Dynamics 6, 651-668.[5] Lahitte et al. (2012). Geomorphology 136, 148-164.[6] Ramalho et al. (2013). Earth-Science Reviews 127, 140-170.[7] Karátson et al. (2016). Geomorphology 253, 123-134.[8] Bossis et al. (in press).[9] Bierman et al. (1996). Earth Surface Processes and Landforms 21, 125-139.

Chlorine-36 (36Cl) is currently the only in situ cosmogenic nuclide applicable in carbonates, Ca- and K-rich feldspars and aphyric silicate rocks. Because the production reactions of 36Cl are more numerous and complex than those of other cosmogenic nuclides (e.g. 10Be, 3He), comprehensive and user-friendly calculators are essential for routine application of 36Cl to Earth surface research questions. However, the existing, commonly used calculators are affected by several drawbacks, such as oversimplification of the muon production model, or the absence of choice regarding production rate parameters. Here, we present the novel 36Cl exposure age calculation tool in development, “CREp 36Cl dev”, which we have implemented as a new functionality in the existing cosmic-ray-exposure program CREp for 10Be and 3He exposure ages. Taking advantage of the most recently developed scaling, muon and low-energy neutron models, “CREp 36Cl dev” allows calculating surface exposure ages, providing several choices between scaling and muon models and production parameters. “CREp 36Cl dev” can be accessed at https://crep-dev.otelo.univ-lorraine.fr/#/.

IntroductionMarl-limestone alternations are well known rhythmical inter-bedded deposits that commonly occur in many hemipelagic to pelagic deposits of the Phanerozoic. It is quite well established that the origin of these lithological variations are astronomically-driven climatic variations (22, 41, 100 and 405 ka being the main periods) e.g. [1]. However, the exact sedimentological control is not clear: several models attribute these alternations to cyclic changes in the carbonate flux, whereas the terrigenous silicoclastic flux remained relatively constant. On the opposite, some models suggest that the carbonate flux was constant while the silicoclastic flux changed cyclically.Material and methodsTo disentangle these different scenarios, we collected marlstone and limestone samples from two sedimentary successions of Bajocian, Middle Jurassic (3 marl-limestone- couplets over 3.5 m) and Valanginian, Lower Cretaceous (1 marl-limestone couplet over 1 m) age from the Southern French Alps (Barles). We measured their carbonate contents, the nannofossil proportion, as well as their extraterrestrial 3He (3HeET) concentrations in ~200 mg decarbonated aliquots.Results and discussionThe carbonate content ranges from 45% in marls to 86% in limestones. Importantly, for all samples, measured 3HeET concentrations are constant in the silicoclastic fractions, within uncertainties. Hence, our results indicate that sedimentation rates at the astronomical timescale in the examined examples were mainly controlled by large changes in the CaCO3 net fluxes, leading to variable dilution of the terrigenous and 3HeET fractions. Nannofossil counting shows that pelagic CaCO3 fluxes of coccolithophores are inversely correlated to the total CaCO3 along the marl-limestone alternations and represent less than 4% of the total carbonate content. Hence, in this setting, these marl-limestone alternations were more probably driven by variations in the CaCO3 supply from the nearby carbonate platform. Finally, assuming a constant 3HeET flux of 0.1 pcc/cm2/ka [2], and the whole thickness of Bajocian and Valanginian strata in this region, the measured 3HeET concentrations imply sedimentation rates that are broadly compatible with current duration estimates of these two stages. References:[1] Eldrett J. S. et al. (2015) Earth. Plan. Sci. Let., 423, 98-113.[2] Farley K.A. et al. (2012) GCA, 84, 314-328.

Glaciers are strong agents of erosion and play a key role in the evolution of mountain ranges. In order to improve our understanding of the influence of glacial erosion dynamics on landscape evolution and mountain building, it is essential to quantitatively constrain glacial erosion rates across multiple topographic and climatic settings.In situ cosmogenic 10Be concentrations measured in river sediments have been widely used over the last twenty years to infer denudation rates integrated at the catchment scale. This approach was mainly applied to fluvial settings because in this case, the 10Be concentration of detrital sediments is a simple function of denudation. In regions covered by glaciers, river sediments result from a mixture of material produced in the pure fluvial domain and sediments produced by glacier erosion. The 10Be concentration measured in such settings thus results from the mixture of these two sources. Here, we use a simple mass conservation approach to estimate pure glacial erosion rates from the 10Be concentration measured in watersheds combining glacial and fluvial domains. In practice, we first established an empirical power-law linking denudation rates to the mean slope of non-glaciated catchments. For each partially glaciated catchment, this law was used to constrain the pure fluvial 10Be end-member using slopes derived from a DEM. Finally, this input was used to compute the pure glacial erosion rate required to satisfy the 10Be concentration measured in rivers. This new approach was applied to 2 different datasets: Present-day glacier erosion in the Alps. We apply this approach to determine the erosion of modern glaciers across the entire Alps. We used previously published 10Be concentration measured in river sediments covering partially glaciated watersheds. The fluvial denudation power law was constrained from 148 fluvial – glacier free catchments. We then selected 11 watersheds with glaciers bigger than 5 km2 and a glacial cover of at least 5% of their total surface. The so-obtained glacial erosion rates from these 11 watersheds range from 0.2 to 1.5 mm.yr-1. Finally, we compare those values to satellite-derived glaciers' sliding velocity which is thought to be the main factor controlling glacial erosion rates. Paleo-erosion in the Var (Southern Alps) setting over the last 75 ka. We apply the same approach to the Var catchment (Southern French Alps) to estimate past glacial erosion rates over the last 75 ka (Mariotti et al., 2021). This basin has been deglaciated since the Holocene and 10Be modern denudation rates were estimated across 9 sub-basins (Mariotti et al., 2019) providing the required dataset to estimate the local fluvial denudation power law. 10Be concentrations were measured in two 75 ka sedimentary cores drilled in the Mediterranean Sea when the Var catchment was previously glaciated (Mariotti et al., 2021). Our findings show that during the LGM, the pure glacial erosion rates were 3 times higher (1.5 +/- 1 mm.yr-1) than during MIS 3-4 (0.4 +/- 0.5 mm.yr-1). This suggests a nonlinear forcing of climate on glacial erosion, mainly controlled by the interplay between glacier velocity, climate, and basin topography.

The impact of discrete extreme meteorological events on the long-term evolution of landscapes and sedimentary budget is poorly understood. We need quantitative estimates of the geomorphic change occurring during such events, of the sediment fluxes produced by landslides, flashfloods, and sediment remobilization. The frequency of such events at the geological and historical time scale and how they can be driven by climate change is also a major concern, especially for risk management. 10Be concentrations measured in river sediments produced during extreme events may provide a powerful tool to quantitatively study the geomorphic impact of the event. On October 2-3 2020, the Var catchment in the French Alps was struck by an extreme rainfall episode connected to the "Alex" storm (> 500 mm / 24h). This event resulted in flash floods in the Vésubie and Var valleys, mobilizing large volume of sediments and resulting in a 10 km long sedimentary plume at the Var outlet in the Mediterranean Sea. Fortunately, the Var catchment had been extensively studied before this event: 10Be had been measured in sediments to derive sub-catchment denudation rates and interannual variability of the 10Be signal (Mariotti et al., 2019). Moreover, paleo denudation rates over the last 75 ka for the whole catchment had also been measured using two sediments cores drilled in the Mediterranean Sea (Mariotti et al., 2021), providing a high-resolution record of past sedimentary dynamics. This extreme rainfall event of October 2020 and our previous 10Be dataset offer the unique opportunity to assess the sensibility of a sedimentary system and its capacity to relay extreme events in a source-to-sink system. This is also a great opportunity to characterize the 10Be geochemical signature of such events. This step is important to interpret paleo-10Be signals in sedimentary archives, with the aim to better assess the frequency of extreme events at the geological time scale. In order to characterize the response of the Var system to the Alex event, we compare 10Be concentrations in samples taken in 2016, 2017 and 2018 with 10Be concentrations in samples taken at the same locations after the 2020 storm at +7 days, +21 days, +4 months and +7 months. We use also use samples taken within each sub-catchments to constrain the evolution of the 10Be signal over time. This dataset permits to define the background of the 10Be concentrations and compare these concentrations to the ones measured after the storm. The 10Be concentrations measured at the outlet of the Var catchment at +7 days and +21 days are similar to those measured before the storm. However, the sample taken +4 months later shows a 20% decrease in 10Be concentration from pre-storm values. The Vésubie sub-basin is the only one to exhibit a 10Be decrease at +21 days. Hence, the delayed depletion observed at the outlet probably reflects the transfer of a 10Be-depleted sediment-wave from the Vésubie valley, where most of the landslides and terraces reworking happened during the storm.

Natural CO2-rich groundwaters of eastern Belgium have been known for centuries although the exact origin of the gas they contained was still unclear. This paper presents the results of a sampling campaign in the area (Spa, Stoumont, Malmedy) where 30 samples of both carbogazeous and non-carbogazeous groundwaters have been analyzed for major elements, CO2 content and carbon isotopic composition. Among them, 13 samples were analyzed for 3He / 4He and 4He / 20Ne ratios. The combination of δ13C and 3He / CO2 ratios have shown with a high level of confidence that the CO2 in groundwater from springs and boreholes has a mantellic origin, and can very likely be attributed to the degassing of the neighboring and still buoyant Eifel mantle plume, located at a distance of 100 km eastwards. The identity and nature of the deep-rooted fractures that act as CO2 transport pathway to the surface are still to be clarified, but several major thrust faults exist in the Rhenish Massif and could connect the Eifel volcanic field with the studied area.

Marine sedimentary archives are well dated and often span several glacial cycles; cosmogenic 10Be concentrations in their detrital quartz grains could thus offer the opportunity to reconstruct a wealth of past denudation rates. However, these archives often comprise sediments much finer ( µ m) than typically analyzed in 10Be studies, and few studies have measured 10Be concentrations in quartz grains smaller than 100 µ m or assessed the impacts of mixing, grain size, and interannual variability on the 10Be concentrations of such fine-grained sediments. Here, we analyzed the in situ cosmogenic 10Be concentrations of quartz grains in the 50–100 and 100–250 µ m size fractions of sediments from the Var basin (southern French Alps) to test the reliability of denudation rates derived from 10Be analyses of fine sands. The Var basin has a short transfer zone and highly variable morphology, climate, and geology, and we test the impact of these parameters on the observed 10Be concentrations. Both analyzed size fractions returned similar 10Be concentrations in downstream locations, notably at the Var's outlet, where concentrations ranged from ( 4.02 ± 0.78 ) × 10 4 to ( 4.40 ± 0.64 ) × 10 4 atoms g −1 of quartz. By comparing expected and observed 10Be concentrations at three major river junctions, we interpret that sediment mixing is efficient throughout the Var basin. We resampled four key locations 1 year later, and despite variable climatic parameters during that period, interannual 10Be concentrations were in agreement within uncertainties, except for one upper subbasin. The 10Be -derived denudation rates of Var subbasins range from 0.10±0.01 to 0.57±0.09 mm yr −1 , and spatial variations are primarily controlled by the average subbasin slope. The integrated denudation rate of the entire Var basin is 0.24±0.04 mm yr −1 , in agreement with other methods. Our results demonstrate that fine-grained sediments (50–250 µ m) may return accurate denudation rates and are thus potentially suitable targets for future 10Be applications, such as studies of paleo-denudation rates using offshore sediments.

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International audience; We measured concentrations of Ne isotopes in quartz in a 27.6-meter sandstone core from a low-erosion-rate site at 2183 m elevation at Beacon Heights in the Antarctic Dry Valleys. Surface concentrations of cosmogenic 21 Ne indicate a surface exposure age of at least 4.1 Ma and an erosion rate no higher than ca. 14 cm Myr 1. 21 Ne concentrations in the upper few centimeters of the core show evidence for secondary spallogenic neutron escape e↵ects at the rock surface, which is predicted by first-principles models of cosmogenic-nuclide production but is not commonly observed in natural examples. We used a model for 21 Ne production by various mechanisms fit to the observations to distinguish cosmic-ray-produced 21 Ne from nucleogenic 21 Ne produced by decay of trace U and Th present in quartz, and also constrain rates of subsurface 21 Ne production by cosmic-ray muons. Core samples have a quartz (U-Th)/Ne closure age, reflecting cooling below ⇠95°C, near 160 Ma, which is consistent with existing apatite fission-track data and the 183 Ma emplacement of nearby Ferrar dolerite intrusions. Constraints on 21 Ne production by muons derived from model fitting are consistent with a previously proposed value of 0.79 mb at 190 GeV for the cross-section for 21 Ne production by fast muon interactions, but indicate that 21 Ne production by negative muon capture is likely negligible.

Goal – The reconstruction of past topographies remains challenging and only a few methods allow accurate determination of past surface elevations. We propose here a new technique for deriving paleo-elevations, in which multiple cosmogenic nuclides are measured in the same geological sample exposed at the Earth's surface. This method relies on the altitude dependence of the cosmogenic nuclides' production rates combined with the radioactive decays of nuclides with different half-lives. Theory – The position of the two cosmogenic nuclide exposure curves ( 26 Al/ 10 Be vs 10 Be or 10 Be/ 21 Ne vs 10 Be) depends on the altitude of exposure. If the studied surfaces have been exposed for sufficiently long durations (>500 ka), or have been affected by low erosion rates (500 ka), or low erosion rates (, SCOPUS: ar.j, info:eu-repo/semantics/published

Methods based on cosmic-ray produced nuclides are key to improve our understanding of the Earth surface dynamic. Measuring multiple cosmogenic nuclides in the same rock sample has a great potential, but data interpretation requires rigorous and often complex mathematical treatments. In order to make progress on this topic, this paper presents two easy-to-use MATLAB© programs permitting to derive information from pairs of cosmogenic nuclides (26Al-10Be or 10Be-21Ne) measured in rock samples that have been exposed to cosmic rays in the past: “Paleoaltitude.m” and “Burial.m” Codes available here as supplementary material. • “Paleoaltitude.m” computes paleoelevations from a sample whose burial age is known. This new paleoaltimetry method is presented in detail in Blard et al. [1]. The present article also develops the mathematical approach. • Since the elevation of exposure may affect the accuracy of a burial age [1], the second MATLAB© script “Burial.m” is designed to compute burial ages from 26Al-10Be or 10Be-21Ne pairs, taking into account the position of a sample (elevation and latitude) during its preburial exposure history., MethodsX, 6, ISSN:2215-0161

The Kerguelen Archipelago (49°S, 69°E) is an excellent location for the study of multi-millennial glacier fluctuations, since it is the largest still glaciated emerged area (552 km2 in 2001) in the sub-Antarctic sector of the Indian Ocean, where many glacio-geomorphological formations such as moraines may be dated. To investigate the so-far little-known Late Glacial and the Holocene glacier fluctuations in Kerguelen, we apply cosmogenic nuclide dating of moraines in 3 glacial valleys: Val Travers valley, Ampere glacier valley and Arago glacier valley. We use in situ 36Cl dating of the basaltic moraine boulders at the first two sites, and 10Be dating of the quartz-bearing syenite boulders at the third site. The new 36Cl and 10Be exposure ages provide time constraints over the last 17,000 years. A glacial advance was highlighted during the Late Glacial at 14.4 ± 1.4 ka ago, probably linked to the Antarctic Cold Reversal event. These results are consistent with those previously obtained on the archipelago (Jomelli et al., 2017, 2018; Charton et al., 2020) and more generally those from other the sub-Antarctic regions (e.g. Sagredo et al., 2018). This suggests that all glaciers at this latitude were broadly sensitive to this specific climatic signal. No Early nor Mid Holocene advances were evidenced in Kerguelen glacier evolution during the Holocene due to missing moraines that may have formed in these specific periods. Radiocarbon-dated peat, published in the 1990s, provides evidence of less extensive glacier extents during the Early Holocene than during the Late Holocene (Frenot et al., 1997). Finally, glaciers seem to have re-advanced only during the Late Holocene, especially within the last millennium, at ⁓1 ka, ⁓620 years and ⁓390 years (Verfaillie et al., submitted). A comparison of this new dataset with the available 10Be ages from other sub-Antarctic regions allows for the identification of 3 different glacier evolution patterns during the Holocene. The glacial fluctuations experienced by Kerguelen glaciers seems particularly uncommon, and are likely due to its singular location in the Southern Indian Ocean. Finally, climatic factors that may explain the Kerguelen glacier evolution (temperature, precipitation) are discussed. To this end, we investigate the chronology of glacier advance/retreat periods with (i) the variation in atmospheric temperatures recorded in ice cores in Antarctica and (ii) the variation in precipitation (Southern Westerly Winds, Southern Annular Mode).Charton et al., 2020 : Ant. Sci. 1-13Frenot et al., 1997 : C.R. Acad. Sci. Paris Life Sciences 320, 567-573Jomelli et al., 2017 : Quat. Sci. Rev. 162, 128-144Jomelli et al., 2018 : Quat. Sci. Rev. 183, 110-123Sagredo et al., 2018 : Quat Sci. Rev. 188, 160-166Verfaillie et al., submitted

Reconstructing the spatial and temporal variabilities of the vertical atmospheric temperature gradient (lapse rate, LR) is key to predict the evolution of glaciers in a changing climate. Variations in this parameter may amplify or mitigate the future warming at high elevation, implying contrasted impacts on the stability of glaciers. Several regional studies suggested that the tropical LR was steeper than today during the last glacial maximum (LGM) (Loomis et al., 2017; Blard et al., 2007), while another study concluded that the LGM lapse rate was similar than today (Tripati et al., 2014).Here we combine published LGM sea surface temperatures (SSTs) data and LGM moraines dated by cosmogenic nuclides to reconstruct the lapse rate along the American Cordillera. To do so, we combined paleo-Equilibrium Line Altitudes (ELAs) of glaciers with independent precipitation proxies to derive high latitude atmospheric temperatures. The whole dataset includes 34 paleo-glaciated sites along a North-South transect in the American Cordillera, ranging in latitude from 40°N to 36°S. Our reconstruction indicates that the lapse rate (LR) was steeper than today in the tropical American Cordillera (20°N – 11°S). The average ΔLR (LGM – Modern) for this Tropical Andes region (20°N – 11°S) is ~-1.5 °C.km-1 (20 sites). At higher latitude, in both hemispheres (Central Andes, 15°S – 35°S (8 sites); Sierra Nevada and San Bernardino mountains (40°N – 34°N) (6 sites), the LR was constant during the LGM. Our results show that a drier climate during the LGM is systematically associated with a steeper LR. Modification of LR during LGM was already observed from other tropical regions, in Hawaii-Central Pacific (Blard et al 2007), and in Eastern Africa (Loomis et al., 2017). Similarly, in these regions, precipitation did not increase during the LGM. With this multi-site exhaustive synthesis, we make a case that drier Tropical LGM conditions induce a steeper LR. This corresponds to an amplification of cooling at high altitude during the LGM. These results highlight the necessity to consider LR variations in modelling future climate. In a warmer and wetter Earth, temperature increase may be amplified at high elevation, due to smoother LR. If valid, this mechanism implies that tropical glaciers are more vulnerable than predicted by current climate modelling. ReferencesBlard, P.-H., Lavé, J., Pik, R., Wagnon, P., & Bourlès, D. (2007). Persistence of full glacial conditions in the central Pacific until 15,000 years ago. Nature, 449(7162), 591.Loomis, S. E., Russell, J. M., Verschuren, D., Morrill, C., De Cort, G., Damsté, J. S. S., … & Kelly, M. A. (2017). The tropical lapse rate steepened during the Last Glacial Maximum. Science advances, 3(1), e1600815.Tripati, A. K., Sahany, S., Pittman, D., Eagle, R. A., Neelin, J. D., Mitchell, J. L., & Beaufort, L. (2014). Modern and glacial tropical snowlines controlled by sea surface temperature and atmospheric mixing. Nature Geoscience, 7(3), 205.

The climatic reorganizations that occurred in the Southern and Northern hemispheres during the last deglaciation are thought to have affected the continental tropical regions. However, the respective impact of North and Southern climatic changes in the Tropics are still poorly understood. In the Norhtern Tropical Andes, moraines records indicate that the Antarctic Cold Reversal (ACR, 14.3-12.9 ka BP) stage was more represented than the Younger Dryas (12.9-11.7 ka BP) (Jomelli et al., 2014). However, further South, in the Altiplano basin (Bolivia), two cold periods of the North Hemisphere (Heinrich Stadial 1a (16.5-14.5 ka) and Younger Dryas) are synchronous with (i) major advances or stillstands of paleo-glaciers and with (ii) the highstands of the giant palaeo-lakes Tauca and Coipasa (Martin et al., 2018). Therefore, additional geochronological records of paleoglaciers fluctuations are necessary to address the respective impacts of North and South Hemisphere on the glacial dynamics in the region.We present new Cosmic Ray Exposure (CRE) ages from glacial landforms of the Bolivian Andes that extend pre-existing datasets for four different sites spreading from 16 to 21°S. We reconstruct the Equilibrium Line Altitudes (ELA) associated with each moraine with the AAR method and use them in an inverse algorithm that combines both the palaeo-glaciers and palaeo-lake budgets to derive temperature and precipitation reconstructions. Our temperature reconstruction (ΔT vs. Present) shows a consistent trend through the four glacial sites with a progressive warming from ΔT= -5°C (17 ka BP) to –2.5°C (15-14.5 ka BP, at the end of the Tauca highstand). This is followed by a return to colder conditions, around -4°C, during the ACR (15.5-12.9 ka BP). The Coipasa highstand is coeval with another warming trend followed by ΔT stabilization at the onset of the Holocene (circa 10 ka BP), around -3°C. Precipitation is mainly characterized by increases during the lake highstands, modulated by the distance from the glacial sites to the center of the paleolakes that are moisture sources (recycling processes).These new results highlight the decorrelation of the glacier dynamics to the temperature signal in regions that are characterized by high precipitation variability. They also provide a theoretical frame to explain how both regional and global forcings can imprint the paleo-glacial records. Our results strongly suggest that during the last deglaciation (20 – 10 ka BP), in the Tropical Andes, atmospheric temperatures follow the Antarctic variability, while precipitation is driven by the changes occurring in the Northern Hemisphere.ReferencesJomelli et al., Nature, 2014; Martin et al., Sc. Advances, 2018

Understanding the history of the Greenland Ice Sheet (GrIS) is critical for determining its sensitivity to warming and contribution to sea level; however, that history is poorly known before the last interglacial. Most knowledge comes from interpretation of marine sediment, an indirect record of past ice-sheet extent and behavior. Subglacial sediment and rock, retrieved at the base of ice cores, provide terrestrial evidence for GrIS behavior during the Pleistocene. Here, we use multiple methods to determine GrIS history from subglacial sediment at the base of the Camp Century ice core collected in 1966. This material contains a stratigraphic record of glaciation and vegetation in northwestern Greenland spanning the Pleistocene. Enriched stable isotopes of pore-ice suggest precipitation at lower elevations implying ice-sheet absence. Plant macrofossils and biomarkers in the sediment indicate that paleo-ecosystems from previous interglacial periods are preserved beneath the GrIS. Cosmogenic 26 Al/ 10 Be and luminescence data bracket the burial of the lower-most sediment between 0.7 to 1.4 Ma. In the upper-most sediment, cosmogenic 26 Al/ 10 Be data require exposure within the last 1.0 ± 0.1 My. The unique subglacial sedimentary record from Camp Century documents at least two episodes of ice-free, vegetated conditions, each followed by glaciation. The lower sediment derives from an Early Pleistocene GrIS advance. 26 Al/ 10 Be ratios in the upper-most sediment match those in subglacial bedrock from central Greenland, suggesting similar ice-cover histories across the GrIS. We conclude that the GrIS persisted through much of the Pleistocene but melted and reformed at least once since 1.1 Ma., info:eu-repo/semantics/published

International audience; Denudation of volcanic ocean islands creates remarkable landscapes and contributes to Earth's carbon cycle, since the chemical alteration of basalts is a CO2 sink. Because many volcanic islands have large climate gradients and relatively low variations in lithology and tectonic history, they represent excellent natural laboratories for studying climatic effects on landscape evolution. However, little is known about the control of denudation rates in ocean islands and the respective influences of climatic gradients and morphological parameters. Here, we present new measurements of long-term denudation rates from Santo Antão Island, Cape Verde (17°N). In this 779 km 2 island, mean annual precipitation ranges from 40 mm.yr-1 in the southwest to 1100 mm.yr-1 in the north. To constrain the spatial distribution of denudation rates, we measured the cosmogenic 3 He concentration in river-transported pyroxene grains from 23 river bedload samples. We obtained basin wide denudation rates ranging from 2.7 ± 0.1 to 57.5 ± 0.3 m/Ma. The denudation rates display a significant spatial variability, with the highest rates in catchments located in the northeast side of the island where modern precipitation are the highest and low denudation rates in the southern and western dry basins. Our study shows that precipitation is the main control on denudation and landscape development of the Santo Antão volcanic island. This study provides for the first time the spatial distribution of denudation rates across a volcanic island located in a tropical zone.

Denudation is one of the main processes that shapes landscapes. Because temperature, precipitation and glacial extents are key factors involved in denudation, climatic fluctuations are thought to exert a strong control on this parameter over geological timescales. However, the direct impacts of climatic variations on denudation remain controversial, particularly those involving the Quaternary glacial cycles in mountain environments. Here we measure in situ cosmogenic 10Be concentration in quartz in marine turbidites of two high-resolution cores collected in the Mediterranean Sea, providing a near-continuous (temporal resolution of ~1–2 kyr) reconstruction of denudation in the Southern Alps since 75 kyr ago (ka). This high-resolution palaeo-denudation record can be compared with well-constrained climatic variations over the last glacial cycle. Our results indicate that total denudation rates were approximately two times higher than present during the Last Glacial Maximum (26.5–19 ka), the glacial component of the denudation rates being $$1.5_{ - 1.0}^{ + 0.9}$$ mm yr−1. However, during moderately glaciated times (74–29 ka), denudation rates were similar to those today (0.24 ± 0.04 mm yr−1). This suggests a nonlinear forcing of climate on denudation, mainly controlled by the interplay between glacier velocity and basin topography. Hence, the onset of Quaternary glaciations, 2.6 million years ago, did not necessarily induce a synchronous global denudation pulse. Constraints on the denudation of the Southern Alps over the last glacial cycle indicate a nonlinear influence of climate on landscape evolution in glaciated areas, according to a beryllium isotope record measured from quartz in a sequence of Mediterranean turbidites.

The Golo River drains a steep catchment (average gradient of 30 m km−1, surface of 1214 km2) in the northeast part of Corsica Island, delivering sediments to the Ligurian Sea. In this study, we review and revise the geologic map and constrain the extent of the Golo coastal alluvial plain formations and their relative and absolute chronology. To update the surface extent of each formation, we performed a geomorphologic analysis with DEMs and satellite imagery data coupled with an extensive pedogenic and sedimentary field observations database, including a new borehole of 117,4 m depth. Additionally, we performed in-situ cosmogenic 10Be analysis from a depth-profile in the well-preserved alluvial terrace Fy2, yielding a minimum age of 70 ka for its emplacement. Our new chronology, based on cosmogenic 10Be and soil chronosequences, implies older ages than those previously obtained with luminescence methods. Soil mixing by bioturbation is proposed as a possibility to explain differences between luminescence and 10Be ages. In this scenario, 10Be dates the original deposition of the alluvial terrace, while luminescence dates a later soil development. We highlighted at least five outcropping alluvial terraces in the Golo coastal plain, which are controlled by sea-level fluctuations and were most likely deposited during past sea-level highstands (close to present-day sea-level). Moreover, we identified from a borehole more than 117 m of coarse fluvial sediments in the plain, that do not outcrop at the surface. New cosmogenic 26Al/10Be burial ages suggest that this sedimentary unit results from a thick accumulation of fluvial material filling a zone significantly affected by subsidence, probably accommodated by a normal fault during the Early Quaternary.

International audience; The respective impacts of Northern and Southern Hemispheric climatic changes on the Tropics during the last deglaciation remain poorly understood. In the High Tropical Andes, the Antarctic Cold Reversal (ACR, 14.3-12.9 ka BP) is better represented among morainic records than the Younger Dryas (12.9-11.7 ka BP). However, in the Altiplano basin (Bolivia), two cold periods of the Northern Hemisphere (Heinrich Stadial 1a, 16.5-14.5 ka BP, and the Younger Dryas) are synchronous with (i) major advances or standstills of paleoglaciers and (ii) the highstands of giant paleolakes Tauca and Coipasa. Here, we present new cosmic ray exposure (CRE) ages from glacial landforms of the Bolivian Andes that formed during the last deglaciation (Termination 1). We reconstruct the equilibrium line altitudes (ELA) associated with each moraine and use them in an inverse algorithm combining paleoglaciers and paleolake budgets to derive temperature and precipitation during the last deglaciation. Our temperature reconstruction (ΔT relative to present day) yields a consistent regional trend of progressive warming from ΔT = −5 to −2.5°C during 17-14.5 ka BP, followed by a return to colder conditions around −4°C during the ACR (14.5-12.9 ka BP). The Coipasa highstand (12.9-11.8 ka BP) is coeval with another warming trend followed by ΔT stabilization at the onset of the Holocene (ca. 10 ka BP), around −3°C. Our results suggest that, during the last deglaciation (20-10 ka BP) atmospheric temperatures in the Tropical Andes mimicked Antarctic variability, whereas precipitation over the Altiplano was driven by changes in the Northern Hemisphere.

This paper reviews current understanding of deglaciation in North, Central and South America from the Last Glacial Maximum to the beginning of the Holocene. Together with paleoclimatic and paleoceanographic data, we compare and contrast the pace of deglaciation and the response of glaciers to major climate events. During the Global Last Glacial Maximum (GLGM, 26.5-19 ka), average temperatures decreased 4° to 8°C in the Americas, but precipitation varied strongly throughout this large region. Many glaciers in North and Central America achieved their maximum extent during the GLGM, whereas others advanced even farther during the subsequent Heinrich Stadial 1 (HS-1). Glaciers in the Andes also expanded during the GLGM, but that advance was not the largest, except on Tierra del Fuego. HS-1 (17.5-14.6 ka) was a time of general glacier thickening and advance throughout most of North and Central America, and in the tropical Andes; however, glaciers in the temperate and subpolar Andes thinned and retreated during this period. During the Bølling-Allerød interstadial (B-A, 14.6-12.9 ka), glaciers retreated throughout North and Central America and, in some cases, completely disappeared. Many glaciers advanced during the Antarctic Cold Reversal (ACR, 14.6-12.9 ka) in the tropical Andes and Patagonia. There were small advances of glaciers in North America, Central America and in northern South America (Venezuela) during the Younger Dryas (12.9-11.7 ka), but glaciers in central and southern South America retreated during this period, except on the Altiplano where advances were driven by an increase in precipitation. Taken together, we suggest that there was a climate compensation effect, or ‘seesaw’, between the hemispheres, which affected not only marine currents and atmospheric circulation, but also the behavior of glaciers. This seesaw is consistent with the opposing behavior of many glaciers in the Northern and Southern Hemispheres.

Reconstructing the spatial and temporal variabilities of the vertical atmospheric temperature gradient (lapse rate, LR) is key to predict the evolution of glaciers in a changing climate. Variations in this parameter may indeed amplify or mitigate the future warming at high elevation, implying contrasted impacts on the stability of glaciers. Several regional studies suggested that the tropical LR was steeper than today during the last glacial maximum (LGM) (Loomis et al., 2017; Blard et al., 2007), while another study concluded that the LGM lapse rate was similar than today (Tripati et al., 2014).Here we combine published LGM sea surface temperatures (SSTs) data and LGM moraines dated by cosmogenic nuclides to reconstruct the lapse rate along the American Cordillera. To do so, we combined paleo-Equilibrium Line Altitudes (ELAs) of glaciers with independent precipitation proxies to derive high latitude atmospheric temperatures. The whole dataset includes 34 paleo-glaciated sites along a North-South transect in the American Cordillera, ranging in latitude from 40°N to 36°S. Our reconstruction indicates that the lapse rate (LR) was steeper than today in the tropical American Cordillera (20°N – 11°S). The average ΔLR (LGM – Modern) for this Tropical Andes region (20°N – 11°S) is ~-2 °C.km-1 (20 sites). At higher latitude, in both hemispheres, the LR was constant or decreased during the LGM. More precisely, this ΔLR change in the Central Andes (15°S – 35°S) is between 0 and 1°C.km-1 (8 sites), while it is ~1 °C.km-1 in Sierra Nevada and San Bernardino mountains (40°N – 34°N) (6 sites). Our results show that a drier climate during the LGM is systematically associated with a steeper LR. Modification of LR during the LGM was already observed from other tropical regions, in Hawaii-Central Pacific (Blard et al 2007), and in Eastern Africa (Loomis et al., 2017). Similarly, in these regions, precipitation did not increase during the LGM. With this multi-site exhaustive synthesis, we make a case that drier Tropical LGM conditions induce a steeper LR. This corresponds to an amplification of cooling at high altitude during the LGM. These results highlight the necessity to consider LR variations in modelling future climate. In a warmer and wetter Earth, temperature increase may be amplified at high elevation, due to smoother LR. If true, this mechanism indicates that tropical glaciers are more threatened by climate change than predicted by current climate modelling.ReferencesBlard, P.-H., Lavé, J., Pik, R., Wagnon, P., & Bourlès, D. (2007). Persistence of full glacial conditions in the central Pacific until 15,000 years ago. Nature, 449(7162), 591.Loomis, S. E., Russell, J. M., Verschuren, D., Morrill, C., De Cort, G., Damsté, J. S. S., … & Kelly, M. A. (2017). The tropical lapse rate steepened during the Last Glacial Maximum. Science advances, 3(1), e1600815.Tripati, A. K., Sahany, S., Pittman, D., Eagle, R. A., Neelin, J. D., Mitchell, J. L., & Beaufort, L. (2014). Modern and glacial tropical snowlines controlled by sea surface temperature and atmospheric mixing. Nature Geoscience, 7(3), 205.

As the melting of the Greenland Ice Sheet (GrIS) accelerates, it is critical to improve our knowledge of its Pleistocene history in order to better understand its sensitivity to different climate states. The study of sediment from the base of the ice sheet offers valuable insights, since this material holds useful information about its history and origin. Here, we present various mineralogical and geochemical analysis from basal sediments of the NEEM ice core from northwestern Greenland (NEEM community, 2013), a complement to the first analysis of the basal ice made by Goossens et al. (2016).In an effort to specify the provenance and characterize the sediments in the basal ice of the NEEM ice core, strontium and neodymium isotopic ratios were measured in 7 bulk till samples located into the deepest part of the core. Laser granulometry and shape characterization by SEB images of the grains suggest a mixed origin of this material. The deepest sample yield in situ cosmogenic 10Be and 26Al concentrations lower than 104 at.g-1 and 21Ne concentration in the 107-108 at.g-1 range. These preliminary cosmogenic nuclides data suggest that several cycles of waning and waxing of the GrIS had occurred over the last 10 million years. Additional sample material is being processed to reduce the uncertainty of 26Al and 10Be measurements and refine this chronology.To better characterize the origin of the basal sediment and the duration of pre-burial exposure, measurements of meteoric cosmogenic 10Be in 7 samples distributed along the basal part of the core are currently in progress. These data will be combined with the measurement of total organic carbon and nitrogen in the same samples. C and N concentrations and isotopes bring useful information about the type of soil and till material in these basal sediments (Bierman et al., 2016). Bierman, P.R., Shakun, J.D., Corbett, L.B., Zimmerman, S.R., Rood, D.H., 2016. A persistent and dynamic East Greenland Ice Sheet over the past 7.5 million years. Nature 540, 256–260. https://doi.org/10.1038/nature20147Goossens, T., Sapart, C.J., Dahl-Jensen, D., Popp, T., El Amri, S., Tison, J.-L., 2016. A comprehensive interpretation of the NEEM basal ice build-up using a multi-parametric approach. The Cryosphere 10, 553–567. https://doi.org/10.5194/tc-10-553-2016NEEM community, 2013. Eemian interglacial reconstructed from a Greenland folded ice core. Nature, 493. doi:10.1038/nature11789

Determining the geographic footprint of past climate events is a fundamental step in identifying the mechanisms that drive and propagate these changes around the globe. Glacial deposits are a particularly robust source of such data; glaciers are sensitive indicators of climate that leave records of their past fluctuations on the landscape. Given precise chronologic control, glacial deposits can be used to reconstruct past climate variability. Recent advances in cosmogenic nuclide surface-exposure dating have established past glacial fluctuations as a key climate proxy. However, uncertainties in the application of cosmogenic nuclide production-rate-scaling frameworks hinder efforts to compare past glacial fluctuations with other records of past climate conditions. Production-rate scaling is particularly uncertain in the tropics, where the theorized impacts of changing magnetic field strength on the incoming cosmic ray flux are greatest. Here we present results in-progress from the CoNTESTA [Cosmogenic Nuclide Temporal and Elevation Scaling: Testing and Application] Project, which seeks to establish multiple nuclide production-rate calibration sites of varying age from the low latitudes in order to assess directly the impacts of changing magnetic field strength on nuclide production over time. We also report new data that address empirically the impacts of elevation on nuclide production. The results of this project will strengthen our understanding of cosmogenic nuclide production globally and will improve surface-exposure age calculations from all regions. This in turn will enable more robust assessment of the global phasing of glacial fluctuations and will forward our understanding of landscape dynamics and Earth surface history.

The paleoclimatic changes that occurred in the Southern and Northern hemispheres during the last deglaciation are thought to have affected the continental tropical regions. However, the respective impact of North and Southern climatic changes in the tropics are still poorly understood. In the High Tropical Andes, the Antarctic Cold Reversal (ACR, 14.3-12.9 ka BP) was reported to be more represented than the Younger Dryas (12.9-11.7 ka BP) among morainic records. However, in the Altiplano basin (Bolivia), two cold periods of the North Hemisphere (Heinrich Stadial 1a (16.5-14.5 ka) and Younger Dryas) are synchronous with (i) major advances or stillstands of paleo-glaciers and with (ii) the highstands of the giant palaeo-lakes Tauca and Coipasa. Therefore, additional results are needed to disentangle between potential North and South Hemisphere climatic influence on the glacial dynamics in the region.We present new Cosmic Ray Exposure (CRE) ages from glacial landforms of the Bolivian Andes that extend pre-existing datasets for four different sites spreading from 16 to 21°S. We reconstruct the Equilibrium Line Altitudes (ELA) associated with each moraine with the AAR method and use them in an inverse algorithm that combines both the palaeo-glaciers and palaeo-lake budgets to derive temperature and precipitation reconstructions. Our temperature reconstruction (ΔT vs. Present) shows a consistent trend through the four glacial sites with a progressive warming from ΔT= -5°C (17 ka BP) to –2.5°C (15-14.5 ka BP, at the end of the Tauca highstand). This is followed by a return to colder conditions, around -4°C, during the ACR (15.5-12.9 ka BP). The Coipasa highstand is coeval with another warming trend followed by ΔT stabilization at the onset of the Holocene (circa 10 ka BP), around -3°C. Precipitation is mainly characterized by increases during the lake highstands, modulated by the distance from the glacial sites to the center of the paleolakes that are moisture sources (recycling processes).These new results highlight the decorrelation of the glacier dynamics to the temperature signal in regions that are characterized by high precipitation variability. They also provide a theoretical frame to explain how both regional and global forcings can imprint the paleo-glacial records. Our results strongly suggest that during the last deglaciation (20 – 10 ka BP), in the Tropical Andes, atmospheric temperatures follow the Antarctic variability, while precipitation is driven by the changes occurring in the Northern Hemisphere.

The Greenland Ice Sheet (GrIS) is melting in response to a rapidly warming climate. It is imperative to understand GrIS sensitivity to past climate, especially during periods when the ice sheet was smaller than present or possibly absent. The Camp Century ice core from NW Greenland, collected in 1966 and the first ice core to be drilled to the bed of the GrIS, revolutionized our understanding of global paleoclimate since 125 ka. However, basal sediment from the ice core was not fully explored and then sat in storage for decades – until it was re-discovered two years ago. We are now investigating these unique samples from the sub-glacial environment using modern analyses. Here, we present initial results from two samples, the upper and lower portions of >4 m of basal sediment. We applied an array of geochemical analyses to characterize paleoenvironment (lipid biomarkers, δ13C, δ15N), to infer past climatic conditions (δ18O, δD) from frozen pore water, and to determine the exposure and burial history of the sediments below the ice sheet (optically stimulated luminescence [OSL], cosmogenic 10Be, 26Al, and 21Ne). The sub-glacial sediment consists of poorly sorted, reddish-brown, quartz-rich diamict, with paleo-permafrost features in some layers. This material contains woody macrofossils, fungal sclerotia (Cenococcum geophilum), and mosses (Tomenthypnum nitens, Polytrichum juniperinum) that yield a 14C age >55 ka. Woody tissue from the upper and lower samples yield stable δ13C ratios of -26.7±0.1‰ and -29.6±0.1‰ and δ15N ratios of 2.4±0.8‰ and -2.3±0.8‰. Leaf wax (n-alkanoic acid) distributions are similar to modern Arctic shrubs. Frozen pore water yielded δ18O ratios of -23.06±0.08‰ and -21.49±0.08‰, enriched relative to all overlying ice (600 ka. In situ 10Be concentrations in quartz decrease with depth from 7.7±0.1 x104 atoms/g (500-850 µm) and 6.6±0.2 x104 (250-500 µm) in the upper sediment to 1.6±0.1 x104 atoms/g (500-850 µm) and 1.8±0.1 x104 (250-500 µm) in the lower sediment. The 26Al/10Be ratio also decreases with depth. In the upper sediment, 26Al/10Be ratios range between 4.2 and 4.9 indicating > 900 ka of burial. In the lower sediment, 26Al/10Be ratios range from 1.4 to 2.0 indicating >2 Ma of burial. Measured 21Ne/10Be ratios in quartz exceed 1000, which could indicate long-term burial and/or the presence of nucleogenic 21Ne.These results demonstrate that Camp Century basal sediment was exposed under ice-free conditions that supported vegetation similar to today. Cosmogenic data indicate the deeper sediment has been buried for most of the Pleistocene and the OSL date rules out surface exposure of the deeper material at MIS 11. Cosmogenic analysis indicates that the upper sample experienced less burial or more recent re-exposure. These data are consistent with a growing body of evidence indicating a dynamic Pleistocene GrIS, even under a pre-industrial climate system in which atmospheric CO2 concentrations did not exceed ~300 ppm.

The Camp Century Ice core, NW Greenland, recovered a 4.5 m basal section consisting of frozen sediments and debris-rich ice. This material was recently re-discovered in Danish ice core storage and visually logged. As part of a multi-disciplinary effort to unlock the climatic and paleo-environmental signal of this unique record, we have analysed detrital mineral composition and metamorphic ages. Bulk mineral analyses were performed at the Geological Survey of Denmark and Greenland on grain mounts from 2 core intervals using a SEM automated quantitative mineralogy (AQM) approach coupled to laser ablation ICP-MS analyses. This setup allows us to gain a full mineral description together with single-grain U-Pb dates for a large population of metamorphic components, e.g. apatite, rutile, titanite and zircon. In addition, amphibole grains were picked for 40Ar-39Ar dating performed at the LDEO Argon Isotope Lab. Mineralogical characterization was completed by X-Ray diffraction analysis of the fine fraction to determine the presence and nature of potential clay weathering products, and single-crystal X-ray diffraction was utilized to characterize the atomic arrangements of minerals that occur in solid solutions. The AQM results indicate that metamorphic minerals are present in sufficient amounts (100’s) for gaining statistically valid provenance data. Preliminary results show ages in the 1900 – 1700 Ma range (amphibole, rutile) and around 2700 Ma (zircon). This, along with the presence of swelling clays in the sediments, is consistent with weathering of the local bedrock, and/or sediments transported from the Inglefield orogenic belt north of the site. To gain information on the youngest thermal events of sediment sources, potentially revealing deep glacial incision, (U-Th-Sm)/He dating of single apatite grains is underway. Preliminary work on the 125-250 µm size fraction yield abundant subhedral-to-subrounded, euhedral apatite suitable for thermochronology. Here we report the results from the different methods and discuss the implications for understanding erosional processes and potential transport pathways of the Camp Century basal ice sediments.

Volcanic flank collapses often result in giant debris avalanches that are capable of travelling tens of kilometres across the ocean floor and generating tsunamis that devastate distant communities. The San Andres Landslide on El Hierro, Canary Islands, represents one of the few places in the world where it is possible to investigate the landslide mass and fault planes of a volcanic collapse structure. In this study, a new conceptual model for the development of this enormous slump is presented on the basis of structural geological and geomorphological measurements, petrological and microstructural analyses, and cosmogenic radionuclide dating. Structural geological and geomorphological measurements indicate that the fault plane records two distinct events. Petrological and microstructural analyses demonstrate that a thin layer of frictionite covers the surface of the fault in contact with an oxidised tectonic breccia that transitions into the underlying undeformed basanite host rock. This frictionite comprises a heterogeneous cataclastic layer and a translucent silica layer that are interpreted to represent two separate slip events on the basis of their architecture and crosscutting relationships. Cosmogenic 3He dating reveals a maximum exposure age of 183 ± 17 ka to 52 ± 17 ka. Arguments are presented in support of the idea that the first slip event took place between 545 ka and 430 ka, prior to significant clockwise rotation of El Hierro, and the second slip event took place between 183 ka and 52 ka, perhaps in association with one of the giant debris avalanches that occurred around that time. This is the first time that more than one slip event has been recognised from the fault plane of the San Andres Landslide. It is also believed to be the first time a silica layer resulting from frictional melt has been described in a volcanic setting.

International audience; Despite increasing knowledge on the orogenic phases of the Pyrenees, the Neogene evolution of the range remains poorly constrained. The central Pyrenees, particularly the Ariège River valley and its terrace systems and glacial extensions, are key to reconstructing Pyrenean evolution during the Neogene. However, few terrace relics remain on the piedmont edges. To overcome this limitation and temporally extend the dataset obtained from terraces, we focus on alluvium-filled horizontal epiphreatic passages developed in limestone karstic networks. These landforms record the transient position of former local base levels during the process of valley deepening, similar to fluvial terraces. The alluvium fills of the studied caves in the Tarascon-sur-Ariège area, in the transition zone between the upper range and the piedmont, therefore enable the reconstruction of the geodynamic evolution of the Ariège River valley. All studied caves are developed on at least eight levels. Based on burial durations determined by 26Al/10Be and 10Be/21Ne terrestrial cosmogenic nuclide dating of samples from epiphreatic levels, our results indicate Langhian to Messinian (~13–5 Ma) incision rates averaging 48 m Myr−1. However, the obtained record does not allow us to accurately retrace the Pliocene evolution of the area due to the lack of known caves at corresponding levels. Moreover, raised local base levels during glacial phases both make the record more complex and call into question the methodological approach in terms of potential internal sediment remobilization and mixing related to implied re-flooding periods.

Proximal alluvial sediments represent a useful sedimentary archive to reconstruct the tectono-climatic history of continental rift basins. However, poor dating of coarse fluvial successions usually prevents high-resolution distinction of tectonic and climatic processes, and thus good determination of process rates. This paper presents a dating study of Plio-Pleistocene Kalavryta river system during the early history of the Corinth Rift (northern Peloponnese, Greece) based on magnetostratigraphy and palynology. This river system developed across several active normal fault blocks that are now uplifted along the southern rift margin. The detailed sedimentary record constrains alluvial architectures from the proximal basin to the river outlet where small deltas built into a shallow lake. In four magnetostratigraphy sections the correlation to the reference scale relies on the identification of the Gauss/Matuyama magnetic reversal and biostratigraphic elements. The river system developed between about 3.6 to 1.8 Ma, with sediment accumulation rates (SARs) ranging from 0.40 to 0.75 mm yr−1. SAR is lower in the alluvial fans than in the deltaic system, and higher at the centre of the normal fault depocentres than at the fault tip. By comparison with worldwide Cenozoic SARs, our values are higher but lie in the same range as those determined in coarse alluvial foreland basins. Moreover, in the context of overfilled intra-mountainous rift basins, these rates are minimum values and can be used as a proxy for accommodation rate. Therefore, early rift stratal wedges and growth synclines attest high sedimentation rates and also high rates of tectonic processes. Finally, in the distal river system, floral compositions and changes of vegetation deduced from palynological data are coherent with alternating fluvio-deltaic and shallow lacustrine deposits, which are linked to relative base level variations. Dry/cool climate is preferentially recorded during periods of low lake level, while the warm/moist climate is mainly recorded in prodelta deposits during periods of high lake level. This correlation suggests that, despite the dominant control of active faulting, climate is a key control of syn-rift stratigraphic architectures.

The sensitivity of tropical glaciers to paleoclimatic conditions that prevailed during the Antarctic cold reversal (ACR, ca. 14.5-12.9 ka) has been the subject of a wide debate. In 2014 a paper suggested that tropical glaciers responded very sensitively to the changing climate during the ACR (Jomelli et al., 2014). In this study, we reexamine the conclusions from this study by recalculating previous chronologies based on 226 10Be and 14 3He ages respectively, and using the most up-to date production rates for these cosmogenic nuclides in the Tropical Andes. 53 moraines from 25 glaciers were selected from the previous analysis provided by Jomelli et al. (2014) located in Colombia, Peru and Bolivia. We then focused on two distinct calculations. First we considered the oldest moraine and its uncertainty for every glacier representing the preserved evidence of the maximum glacier extents during the last deglaciation period, and binned the results into 5 distinct periods encompassing the Antarctic cold reversal and Younger Dryas (YD) chronozones: pre-ACR, ACR, ACR-YD, YD and post-YD respectively. Results revealed a predominance of pre-ACR and ACR ages, accounting for 60% of the glaciers. Second we counted the number of moraines per glacier according to the different groups. 21 moraines (40%) of the selected glaciers belong to the pre-ACR-ACR chronozones while 3 moraines only (5%) were dated to the YD and YD-Holocene groups. The rest was assigned to the ACR-YD. These results suggest that moraine records are a very good proxy to document the ACR signal in the Tropical Andes.

International audience; The possibility of quantifying surface processes in mafic or volcanic environment using the potentialities offered by the in situ-produced cosmogenic nuclides, and more specifically by the in situ-produced Be-10, is often hampered by the rarity of quartz minerals in the available lithologies. As an alternative to overcome this difficulty, we explore in this work the possibility of relying on feldspar minerals rather that on quartz to perform in situ-produced Be-10 measurements in such environments. Our strategy was to cross-calibrate the total production rate of Be-10 in feldspar (P-10fsp) against the total production rate of He-3 in pyroxene (P-3px) by measuring He-3 and Be-10 in cogenetic pyroxene (He-3(px)) and feldspar (Be-10(fsp)). The samples were collected from eight ignimbritic boulders, exposed from ca 120 to 600 ka at elevations ranging from 800 to 2500 m, along the preserved rock-avalanche deposits of the giant Caquilluco landslide (18 degrees S, 70 degrees W), Southern Peru. Along with data recently published by Blard et al. (2013a) at a close latitude (22 degrees S) but higher elevation (ca. 4000 m), the samples yield a remarkably tight cluster of He-3(px) - Be-10(fsp) total production ratios whose weighted-mean is 35.6 +/- 0.5 (1 sigma). The obtained weighted mean He-3(px) - Be-10(fsp) total production ratio combined with the local He-3(py) total production rate in the high tropical Andes published by Martin et al. (2017) allows to establish a total SLHL Be-10 in situ-production rate in feldspar mineral (P-10fsp) of 3.57 +/- 0.21 at.g(-1).yr(-1) (scaled for the LSD scaling scheme, the ERA40 atm model and the VDM of Lifton, 2016). Despite the large elevation range covered by the whole dataset (800-4300 m), no significant variation of the He-3(px) - Be-10(fsp) total production ratios in pyroxene and feldspar was evidenced. As an attempt to investigate the effect of the chemical composition of feldspar on the total Be-10 production rate, major and trace element concentrations of the studied feldspar samples were analyzed. Unfortunately, giving the low compositional variability of our dataset, this issue is still pending. (C) 2017 Elsevier B.V. All rights reserved.

International audience; In-situ cosmogenic 3 He is a robust tool for determining denudation rates or exposure ages of lavas bearing mafic phenocrysts. However, analyses are often complicated by the presence of several helium sources. In particular, in old magmatic rocks with high radiogenic 4 He contents, discriminating cosmogenic 3 He from magmatic 3 He is not straightforward since these varieties may vary largely between aliquots. We sampled sands from the Tekeze and Mile rivers, both draining the basaltic Ethiopian highlands, an area where erosion patterns are intimately linked to the development of the Western Afar margin and to heterogeneous monsoon precipitation. From each river we analyzed~15 aliquots of pyroxenes having variable grain sizes (0.3 mm up to N 1 mm). The total 3 He is both higher and more scattered in the bigger grains. Crushing of these largest grains and subsequent melting of the powder tends to produce more homogeneous 3 He values, suggesting that magmatic 3 He hosted in inclusions is responsible for most of the inter-aliquot variability. We also performed a Monte Carlo simulation based on a numerical denudation model of the two watersheds. The simulation confirms that cosmogenic 3 He variability cannot be responsible for the observed scatter since the cos-mogenic 3 He variability is averaged away and unobservable in aliquots of~200 grains. A compilation of previously published data also indicates that magmatic helium can be significantly variable, even between pre-crushed aliquots. Hence, magmatic helium, unlike cosmogenic 3 He, is highly variable, even in the case of aliquots of hundreds of grains. We suggest this is due to a strong nugget effect, possibly due to large fluid (or melt)-inclusions contained in phenocrysts. In addition, the fact that small and big grains have comparable radiogenic 4 He concentrations suggests that grain fragmentation during river transport is responsible for the lower magmatic helium content of the smallest grains. Therefore, one should preferably use small grain (0.2-0.5 mm) granulometry for in-situ cosmogenic 3 He analysis in mafic phenocrysts. Using the measured cosmogenic 3 He, we calculate basin-averaged denudation rates of 70 ± 20 and 57 ± 5 mm kyr −1 , for the Mile and for the Tekeze river, respectively. These values are coherent with long-term denu-dation rates previously proposed from low-temperature thermochronology.

International audience; In situ cosmogenic chlorine-36 (36Cl) is so far the only nuclide applicable in carbonate environments and in aphyric Ca-and K-rich silicate rocks. Because the production reactions of 36Cl are more numerous and complex than those of other cosmogenic nuclides (e.g. 10Be, 3He), comprehensive and user-friendly calculators are essential (Schimmelpfennig et al., Quat. Geochr. 4, 441-461, 2009; Marrero et al.,Quat. Geochr. 31, 160-187,2016). However, several drawbacks still affect these commonly used calculators, such as over simplification of the muon production model, which can be inaccurate for depth profile simulations, or the absence of choice regarding production rate parameters. Here, we present a new 36Cl exposure calculator, which we are currently implementing in the existing cosmic-ray-exposure program CREp (crep.otelo.univ-lorraine.fr; Martin et al., Quat. Geochr. 38, 25-49, 2017).So far, CREp allows calculating 10Be and 3He surface exposure ages and provides the possibility to choose between global or locally and regionally calibrated production parameters of these nuclides from the ICE-D-calibration data base. Taking advantage of the most recently developped scaling,muon and low-energy neutron models, the CREp-36Cl calculator allows calculating surface exposure ages as well as depth profiles and provides several choices between scaling and muon models and production parameters.

International audience; Because of its essential role in coupling climate and tectonics, denudation is a key parameter when constraining the history of Earth’s surface. This is particularly true at the Pliocene-Pleistocene transition, and the potential impact of the onset of Quaternary glaciations remains strongly debated. In the present study, we measured in situ cosmogenic 10Be within continuous late Cenozoic sedimentary sections that had already been dated using magnetostratigraphy. The new data were obtained from four sedimentary basins in the northern and southern Tianshan range (Central Asia). We first thoroughly discuss how in situ cosmogenic 10Be concentrations can be corrected for radioactive decay and for the contribution of postdepositional cosmogenic accumulation to derive the paleo–denudation rates. Our analysis shows that, in the four sedimentary records, the potential bias remains low enough to consider the derived denudation rates reliable. The four records, although likely influenced by local particularities due to lithological heterogeneity and local tectonics, display similar trends of continuously increasing denudation between ca. 9 Ma and the present. These rates have remained relatively high but steady since 4 Ma, ∼1.5 m.y. before the onset of the Quaternary glacial cycles. Though the rejuvenation of the Tianshan range since 11 Ma may explain most of the progressive increase (×5) in denudation, our data suggest that the Quaternary glaciations had only a limited impact on denudation in the Tianshan. Our data, however, indicate an increase in the spatial and high-frequency variability (