Pleistocene uplift, climate and morphological segmentation of the northern Chile coasts (24°S-32°S): Insights from cosmogenic 10Be dating of paleoshorelines

International audienceWe present new cosmogenic (10Be) exposure ages obtained on Pleistocene marine abrasion shore terraces of Northern Chile between 24°S and 32°S in order to evaluate the temporal and spatial variability of uplift rates along the coastal forearc. Both the dispersion of cosmogenic concentrations in samples from the same terrace and data obtained in vertical profiles show that onshore erosion rates, following emergence of paleoshorelines, approached 1 m/Myr. Therefore, minimum ages calculated without considering onshore erosion may be largely underestimated for Middle Pleistocene terraces. The elevation of the last interglacial (MIS-5) paleoshoreline is generally between 25 and 45 m amsl, suggesting that the entire coast of the study area has been uplifting during the Upper Pleistocene at rates approaching 0.3 mm/yr. Available ages for Middle Pleistocene terraces suggest similar uplift rates, except in the Altos de Talinay area where uplift may have been accelerated by the activity of the Puerto Aldea Fault. The maximum elevation of Pleistocene paleoshorelines is generally close to 250 m and there is no higher older Neogene marine sediment, which implies that uplift accelerated during the Pleistocene following a period of coastal stability or subsidence. We observe that the coastal morphology largely depends on the latitudinal climatic variability. North of 26.75°S, the coast is characterized by the presence of a high scarp associated with small and poorly preserved paleoshorelines at its foot. The existence of the coastal scarp in the northern part of the study area is permitted by the hyper-arid climate of the Atacama Desert. This particular morphology may explain why paleoshorelines evidencing coastal uplift are poorly preserved between 26.75°S and 24°S despite Upper Pleistocene uplift rates being comparable with those prevailing in the southern part of the study area

Long-term dispersion of river gravel in a canyon in the Atacama Desert, Central Andes, deduced from their 10Be concentrations

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Geomorphological evidence and 10Be exposure ages for the Last Glacial Maximum and deglaciation of the Velká and Malá Studená dolina valleys in the High Tatra Mountains, central Europe

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Design and performance of an automated chemical extraction bench for the preparation of 10Be and 26Al targets to be analyzed by accelerator mass spectrometry

International audienceThe growing number of studies based on data provided by the accelerator mass spectrometry (AMS) technique from increasingly diverse matrices implies that developing very effective and reproducible sample processing techniques is crucial to meet the demand generated. After being dissolved, the isolation of the cosmogenic nuclide to be measured and its chemical purification imply complex processes throughout successive stages The presented automatic system was designed to perform the final steps of the Be-10 and Al-26 chemical extraction protocol from natural matrices (sediments, pure mineral phases, ...), previously purified, decontaminated and dissolved in the case of in situ-produced cosmogenic nuclides, or leached or dissolved in the case of meteoric cosmogenic nuclides. Driven by Labview software, the developed automatic system, in addition to freeing the operator from tedious tasks, allows reproducibly processing simultaneously up to eight samples of different volumes in roughly 40 min. This article presents the design of the automated extraction bench and its performance based on the recovery efficiency of the stable element and the reproducibility of the measurements of the cosmogenic nuclide concentrations by AMS

Post‐Last Glacial Maximum glacier fluctuations in the southern Écrins massif (westernmost Alps): insights from 10 Be cosmic ray exposure dating

International audienceOnly a few chronological constraints on Lateglacial and Early Holocene glacier variability in the westernmost Alps have hitherto been obtained. In this paper, moraines of two palaeoglaciers in the southern ecrins massif were mapped. The chronology of the stabilization of selected moraines was established through the use of Be-10 cosmic ray exposure (CRE) dating. The equilibrium line altitude (ELA) during moraine deposition was reconstructed assuming an accumulation area ratio (AAR) of 0.67. Ten pre-Little Ice Age (LIA) ice-marginal positions of the Rougnoux palaeoglacier were identified and seven of these have been dated. The Be-10 CRE age of a boulder on the lowermost sampled moraine indicates that the landform may have been first formed during a period of stable glaciers at around 16.2 +/- 1.7 ka (kiloyears before AD 2017) or that the sampled boulder experienced pre-exposure to secondary cosmic radiation. The moraine was re-occupied or, alternatively, shaped somewhat before 12.2 +/- 0.6 ka when the ELA was lowered by 230 m relative to the LIA ELA. At least six periods of stable ice margins occurred thereafter when the ELA was 220-160 m lower than during the LIA. The innermost dated moraine stabilized at or before 10.9 +/- 0.7 ka. Three Be-10 CRE ages from a moraine of the Prelles palaeoglacier indicate a period of stationary ice margins at or before 10.9 +/- 0.6 ka when the ELA was lowered by 160 m with respect to the end of the LIA. The presented Be-10 CRE ages are in good agreement with those of moraines that have been attributed to the Egesen stadial. Assuming unchanged precipitation, summer temperature in the southern ecrins massif at ~12 ka must have been at least 2 degrees C lower relative to the LIA

The rapid deglaciation of the La Vega gorge (Sierra de Gredos, Spain) following the Last Glacial Maximum

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Unstable ice stream in Greenland during the Younger Dryas cold event

International audiencePast, present, and future ice sheet stability is closely linked to the dynamic behavior of major draining ice streams and surrounding ice shelves. While short observational records document the recent variability and acceleration of ice streams, the long-term dynamics of ice streams remain poorly documented. Here, we date the Pjetursson’s Moraine on Disko Island, Greenland, to 12.2 ± 0.6 ka and demonstrate that the Jakobshavn Isbræ (JI) ice stream collapsed during the middle of the Younger Dryas (YD) cold interval. We suggest that this collapse was due to the incursion of warm subsurface water under the ice shelf fronting the JI ice stream, as well as increased surface-air temperature and sea-surface temperature seasonality starting at the beginning of the YD cold interval. The triggered acceleration of the land-based JI and the delivery of icebergs into Disko Bugt potentially contributed to Heinrich Event 0 at the end of the YD. The collapse of the JI ice stream 12.2 ± 0.6 ka ago demonstrates that calving marine-based ice margins can respond rapidly to environmental changes. It provides a new benchmark for marine-terminating ice stream models

Late Holocene sea-cliff retreat recorded by 10Be profiles across a coastal platform: Theory and examples from the English Channel and Korea

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Dominance of tectonics over climate in Himalayan denudation

International audienceLandscape denudation in actively deforming mountain ranges is controlled by a combination of rock uplift and surface runoff induced by precipitation. Whereas the relative contribution of these factors is important to our understanding of the evolution of orogenic topography, no consensus currently exists concerning their respective influences. To address this question, denudation rates at centennial to millennial time scales were deduced from 10Be concentrations in detrital sediments derived from 30 small basins (10-600 km2) in an [~]200-km-wide region in central Nepal. Along a northward, strike-perpendicular transect, average denudation rates sharply increase from <0.5 mm/yr in the Lesser Himalayas to [~]1 mm/yr when crossing the Physiographic Transition, and then accelerate to 2-3 mm/yr on the southern flank of the high peaks in the Greater Himalayas. Despite a more than five-fold increase in denudation rate between the southern and northern parts of this transect, the corresponding areas display similar precipitation rates. The primary parameter that presents a significant co-variation with denudation is the long-term rock-uplift rate that is interpreted to result from the ramp-flat transition along the Main Himalayan Thrust. We propose that, in this rapidly uplifting mountain range, landscapes adjust quickly to changing climatic conditions, such that denudation is mainly limited by the rate at which material is pushed upward by tectonic processes and made available for removal by surface processes. In this particular context, variations in precipitation appear to have only a second-order role in modulating the denudation signal that is primarily set by the background rock-uplift rate

Quantifying incision rates since the early Miocene: novelties, potentialities and limitations

International audienceThe rates and chronologies of valley incision are closely modulated by the tectonic uplift of active mountain ranges and were controlled by repeated climate changes during the Quaternary. The Pyrenees are a continental collision between the Iberian and Eurasian plates induced a double vergence orogen, which has been considered as a mature mountain range in spite of significant seismicity and evidence of neotectonics. Nevertheless, recent studies indicate that the range may have never reached a steady state. One option for resolving this controversy is to quantify the incision rates since the Miocene by reconstructing the vertical movement of geometric markers. However, the few available ages from the Pyrenean terrace systems do not exceed the middle Pleistocene. To enlarge the time span of this dataset, we studied alluvium-filled horizontal epiphreatic passages in limestone karstic networks, which represent former valley floors. They record the transient position of former local base levels during the process of valley deepening. We used various suitable geochronological methods (26Al/10Be, 10Be/21Ne, ESR and OSL burial durations on quartz) on intrakarstic alluvial deposits from three valleys of the central and eastern Pyrenees, as well as on a recent analogue. In the Pyrenean context, under particular conditions, these geochronometers allow us to document incision processes since ~ 16-13 Ma, and to study influences of external forcing and eustatism. In comparison with other studies, it appears that incision rates are higher in the central Pyrenees and for the Spanish slope. However, the density of horizontal levels on an altimetric range, the geodynamical and paleoclimatic contexts, the reorganization of the drainage networks can make the filling stories of the networks more complex than expected. Indeed, these radiometric approaches may be limited when some formations are reworked inside and/or outside the karst. The validity of dosimetric methods in a mountainous context, and quartz bleaching conditions at the time of deposition will also be discussed. Acknowledgments: this work was financed by the BRGM within the framework of the RGF-Pyrénées program