A high resolution authigenic 10Be/9Be record of geomagnetic moment variations over the last 300 ka from sedimentary cores of the Portuguese margin.

A high resolution study of authigenic Be isotopes (10Be and 9Be) combined with continuous relative paleointensity records has been performed along the same marine sedimentary sequences from the Portuguese margin (N.E. Atlantic) covering the past 300 ka in order to assess relationships between geomagnetic moment variations and 10Be production rate variations. A carefull examination of the various ways of taking into account environmental disturbing effects on the authigenic 10Be concentration leads to the conclusion that the most reliable proxy of cosmonuclide production rates is presently the authigenic 10Be/9Be ratio. Eight intervals of significant authigenic 10Be/9Be enhancement evidence geomagnetic moment drops related to global paleomagnetic excursions, some being already admitted, others being proposed as new geomagnetic features. Since, contrarily to sedimentary magnetic remanence, the authigenic 10Be/9Be records dipole moment variations without significant acquisition delay, it provides better constraints on their timing. Comparison of 10Be/9Be and benthic δ18O records from the same cores suggests that dipole moment lows preferentially occured during or at the end of interglacial episodes, with a quasi-period of 100 ka

Un modèle de recul des falaises pour évaluer les fréquences d'éboulement

International audienceQuantitative assessment of diffuse rock fall hazard needs rock fall inventories, which are difficult to obtain for small or large rock fall volumes. In the case of localized hazard, a quantitative assessment is not possible in the present state of knowledge. A power law relation between rock fall frequency and volume is proposed for a better estimation of the frequencies. This relation has been used to build a rock wall retreat model, which can determine the age of the rock surface. The model has been applied to the calcareous cliffs of the Grenoble area. The calculated age is of the same order of magnitude than the mean age measured using the cosmogenic nuclides. The rock fall frequencies given by the power law relation can thus be used for a better estimation of diffuse and localized hazard, for the whole range of rock fall volumes

What are the main factors that trigger the giant-landslides in the Peruvian western Andes? The Aricota giant-landslide case study

The central part of western Andes concentrates giant paleo-landslides that mobilized large volumes of rock, well preserved due to hyper arid climate of this region (Audin and Bechir, 2006; Pinto et al., 2008; Crosta et al., 2014; Crosta et al., 2015; Crosta et al., 2017; Mather et al., 2014; Zerathe et al., 2017) and bring us the opportunity to study the sequence of ancient events. The main goal of this study is to identifiy the role of climate and tectonics phenomena’s on the triggering of giant-landslides in Peruvian western Andes. We realized a multidisciplinary study on the Aricota landslide by using high-resolution DEM derived from Pléiades satellite image (Lacroix et al, 2016. Zerathe et al, 2016.) and coupled with cosmogenic nuclide methodology, We realize dating on the main body of landslides and main scarps in order to obtain chronological constraints of the gravitational destabilization events. We identified two events: (1) a first event with an estimated volume of ~2 km3 that event generated an impressive dam in the main valley and formed a lake approximately 6 km long upstream; and (2) a second event of rocks avalanche with an estimated volume of ~0.3 km3. 10Be dating (17 samples) suggest ages of 17.9 ± 0.7 ka and 12.1 ± 0.2 ka (weighted mean and 1 sigma uncertainty of the weighted mean) for these two events. Comparing those mean ages with local paleoclimatic reconstructions, we noticed that the failures match well with two important wet events of the Altiplano: Heinrich 1 (14 - 18 ka) and Younger Dryas (11 - 13 ka), which correspond to the formation of the Tauca and Copiasa lakes respectively (Placzek et al., 2013). This suggests that the climate probably played an important role in the triggers of landslides. However, as several neotectonic faults also affect this area (in the 50 km of radius, e.g. Incapuquio fault or Purgatorio fault), we can’t rule out a possible seismic trigger or even an earthquake/climate coupling. On the western flank of the Andes, the Aricota landslide is one of the few dating events. Although there are numerous recognized paleo-landslides. So, other new landslides ages, should also allow us to know more about the origin of these phenomena in this arid region

Toward the feldspar alternative for cosmogenic 10Be applications

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 10Be, 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 10Be measurements in such environments. Our strategy was to cross-calibrate the total production rate of 10Be in feldspar (P10fsp) against the total production rate of 3He in pyroxene (P3px) by measuring 3He and 10Be in cogenetic pyroxene (3Hepx) and feldspar (10Befsp). 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°S, 70°W), Southern Peru. Along with data recently published by Blard et al. (2013a) at a close latitude (22°S) but higher elevation (ca. 4000 m), the samples yield a remarkably tight cluster of 3Hepx - 10Befsp total production ratios whose weighted-mean is 35.6 ± 0.5 (1s). The obtained weighted-mean 3Hepx - 10Befsp total production ratio combined with the local 3Hepy total production rate in the high tropical Andes published by Martin et al. (2017) allows to establish a total SLHL 10Be in situ-production rate in feldspar mineral (P10fsp) 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 3Hepx - 10Befsp 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 10Be 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

Toward the feldspar alternative for cosmogenic 10Be applications in mafic environment

In situ-produced 10Be is one of the most commonly used TCN in quantitative geomorphology due to the fact that its production rate is relatively well constrained in the ubiquitous quartz mineral whose integrity minimizes the possibility of contamination by meteoric 10Be. Easily decontaminated from meteoric 10Be, it is in addition reliably measured using the Accelerator Mass Spectrometry technique for which its detection limit is lower than 104 at.g-1. However, volcanic or mafic areas are generally quartz free, which hamper the routine use of 10Be. In the case of a quartz poor lithology, an alternative possibility is to rely on 10Be - feldspars. Two preliminary studies (Kober et al., 2005 and Blard et al., 2013a) already provided promising results, demonstrating that (1) the decontamination protocol classically applied to quartz (Brown et al., 1991) efficiently removes all the meteoric 10Be contamination from the feldspar grains and (2) the total production rate of 10Be in feldspar is 8 to 10 % lower than that in quartz. However, only two samples were analyzed in both studies. In order to better constrain the 10Be in situ-production rate within feldspars, the number of samples analyzed needs to be increased. In this study, we developed a new chemical protocol for the 10Be extraction from feldspar matrices, and to cross-calibrate the total 10Be in situ-production rate in feldspar (P10fsp) against the total 3He production rate in pyroxene (P3px). The cosmogenic 3He and 10Be concentrations were measured, respectively, in pyroxene and feldspar extracted from eight samples of ignimbrite boulders from a giant landslide located between 800 and 2500 m in the high central Andes of Southern Peru. This area is ideally located, since two studies have already determined the local total 3He production rate in pyroxene on the nearby Altiplano (Blard et al., 2013b; Delunel et al., 2016)

Cuantificación de la tasa de denudación de los últimos 20 Ka utilizando nucleídos cosmogénicos (10Be) en el suroeste de los Andes Peruanos: Laguna Aricota

Existe un debate actual sobre el principal factor que controla los procesos de erosión en los Andes occidentales: McPhillips et al., 2014 proponen que la erosión depende fuertemente de la ocurrencia de terremotos mientras que otros estudios, reportan correlaciones positivas entre las variaciones climáticas y la erosión de la cuenca (Carretier et al., 2014; Reber et al., 2017) basadas en dataciones con nucleídos cosmogénicos (10Be). En el sector de Curibaya el deslizamiento Aricota, rellenó el valle y formó un gran dique que dio origen a la laguna Aricota, utilizando nucleídos cosmogénicos (10Be), Delgado et al., (2016) fecharon el deslizamiento (dique) en ~17 ka. Nosotros en este estudio tenemos la oportunidad de comparar (1) la tasa de erosión a partir del volumen de sedimentos acumulados en la laguna desde su formación, y (2) la tasa de denudación a partir de la concentración de isotopos de 10Be. Los resultados obtenidos en general muestran una importante correspondencia, indicando de esta manera que no hay influencia de productos volcánicos y de las superficies cuaternarias heredadas (morrenas)

Pleistocene Glaciations in the Northern Tropical Andes, South America (Venezuela, Colombia and Ecuador)

International audienceThis article presents an overview of glaciation studies in the northern tropical Andes (Venezuela, Colombia and Ecuador) mostly based on glacial geochronological data. The oldest dated evidences of glaciations are recorded in the Colombian Andes at the Bogotá Plain between 3000-3500 m a.s.l., dated between 1-3 Ma. Maximum extent of former glaciers in the northern Andes seems to have occurred prior to the global Last Glacial Maximum (gLGM). In the Venezuelan-Mérida Andes, former glaciers mostly reached the lowest recorded elevations during MIS 5-MIS 4, whereas in the Colombian Andes Andes, the maximum extents are recorded prior to 38 ka. In the Ecuadorian Andes the most extensive glacial cover probably occurred during MIS 8. In the northern tropical Andes, studied glacier advances are mainly related to MIS 2 period between the final gLGM to the Oldest Dryas (~18 ka-15 ka). Glacier advances during the Younger Dryas (~ 12.7 ka-11.7 ka) are not extensively evidenced and mainly restricted to elevations higher than ~ 3500 m a.s.l