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

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

Cosmogenic burial dating of in cave-deposited alluvium: unravelling long-term incision rates and complex speleogenesis in multi-level cave systems

Peer reviewe

Plio-Quaternary incision rates in the Meuse catchment vs long-term uplift rates of the Ardennes (E Belgium): new insights from 26Al/10Be burial dating of in cave-deposited alluvium

peer reviewe

A multiple dating-method approach applied to the Sanabria Lake moraine complex (NW Iberian Peninsula, SW Europe)

New evidence in the NW region of the Iberian Peninsula (c. 42º N 6 ºW) of a glacial advance coeval with the global Last Glacial Maximum (LGM) of the Marine Isotope Stage 2 has been identified through a dataset of exposure ages based on 23 10Be concentration measurements carried out on boulder samples taken from a set of latero-frontal moraines. Results span the interval 19.2e15.4 10Be ka, matching the last deglaciation period when Iberia experienced the coldest and driest conditions of the last 25 ka, and are consistent with Lateglacial chronologies established in other mountain regions from SW Europe. The extent of the LGM stade identified in this work is similar to the local maximum ice extent stade recorded and dated as prior to 33 ka using radiocarbon and optically stimulated luminescence. This work showcases how multiple-dating approaches and detailed geomorphological mapping are required to reconstruct realistic palaeoglacier evolution models

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

Implications of 36Cl exposure ages from Skye, northwest Scotland for the timing of ice stream deglaciation and deglacial ice dynamics

The French national AMS facility ASTER (CEREGE, Aix en Provence) is supported by the INSU/CNRS, the ANR through the "Projets thématiques d’excellence" program for the "Equipements d’excellence" ASTER-CEREGE action, IRD and CEA. The authors would like to thank Shasta Marrero for helpful and informative discussion on the CRONUScalc online calculator. DS was supported by a SAGES studentship and fieldwork by funds from the QRA and BSG.Geochronological constraints on the deglaciation of former marine based ice streams provide information on the rates and modes by which marine based ice sheets have responded to external forcing factors such as climate change. This paper presents new 36Cl cosmic ray exposure dating from boulders located on two moraines (Glen Brittle and Loch Scavaig) in southern Skye, northwest Scotland. Ages from the Glen Brittle moraines constrain deglaciation of a major marine terminating ice stream, the Barra-Donegal Ice Stream that drained the former British-Irish Ice Sheet, depending on choice of production method and scaling model this occurred 19.9 ± 1.5–17.6 ± 1.3 ka ago. We compare this timing of deglaciation to existing geochronological data and changes in a variety of potential forcing factors constrained through proxy records and numerical models to determine what deglaciation age is most consistent with existing evidence. Another small section of moraine, the Scavaig moraine, is traced offshore through multibeam swath-bathymetry and interpreted as delimiting a later stillstand/readvance stage following ice stream deglaciation. Additional cosmic ray exposure dating from the onshore portion of this moraine indicate that it was deposited 16.3 ± 1.3–15.2 ± 0.9 ka ago. When calculated using the most up-to-date scaling scheme this time of deposition is, within uncertainty, the same as the timing of a widely identified readvance, the Wester Ross Readvance, observed elsewhere in northwest Scotland. This extends the area over which this readvance has potentially occurred, reinforcing the view that it was climatically forced.PostprintPeer reviewe

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)

Moraine crest or slope: An analysis of the effects of boulder position on cosmogenic exposure age

Terrestrial cosmogenic nuclide dating of ice-marginal moraines can provide unique insights into Quaternary glacial history. However, pre- and post-depositional exposure histories of moraine boulders can introduce geologic uncertainty to numerical landform ages. To avoid geologic outliers, boulders are typically selected based on their depositional context and individual characteristics but while these criteria have good qualitative reasoning, many have not been tested quantitatively. Of these, boulder location is critical, as boulders located on moraine crests are prioritised, while those on moraine slopes are typically rejected. This study provides the first quantitative assessment of the relative utility of moraine crest and moraine slope sampling using new and published 10Be and 36Cl ages (n = 19) and Schmidt hammer sampling (SH; n = 635 moraine boulders, ∼19,050 SH R-values) in the northern and southern Pyrenees. These data show that for many of the studied moraines, the spatial distribution of “good” boulders is effectively random, with no consistent clustering on moraine crests, ice-proximal or -distal slopes. In turn, and in contrast to prior work, there is no clear penalty to either moraine crest or moraine slope sampling. Instead, we argue that landform stability exerts a greater influence on exposure age distributions than the characteristics of individual boulders. For the studied landforms, post-depositional stability is strongly influenced by sedimentology, with prolonged degradation of matrix-rich unconsolidated moraines while boulder-rich, matrix-poor moraines stabilised rapidly after deposition. While this pattern is unlikely to hold true in all settings, these data indicate that differences between landforms can be more significant than differences at the intra-landform scale. As ad hoc assessment of landform stability is extremely challenging based on geomorphological evidence alone, preliminary SH sampling, as utilised here, is a useful method to assess the temporal distribution of boulder exposure ages and to prioritise individual boulders for subsequent analysis

A Comparison Of New Calculations Of The Yearly 10Be Production In The Earths Polar Atmosphere By Cosmic Rays With Yearly 10Be Measurements In Multiple Greenland Ice Cores Between 1939 And 1994 - A Troubling Lack Of Concordance Paper #2

We have compared the yearly production rates of 10Be by cosmic rays in the Earths polar atmosphere over the last 50-70 years with 10Be measurements from two separate ice cores in Greenland. These ice cores provide measurements of the annual 10Be concentration and 10Be flux levels during this time. The scatter in the ice core yearly data vs. the production data is larger than the average solar 11 year production variations that are being measured. The cross correlation coefficients between the yearly 10Be production and the ice core 10Be measurements for this time period are <0.4 in all comparisons between ice core data and 10Be production, including 10Be concentrations, 10Be fluxes and in comparing the two separate ice core measurements. In fact, the cross correlation between the two ice core measurements, which should be measuring the same source, is the lowest of all, only ~0.2. These values for the correlation coefficient are all indicative of a "poor" correlation. The regression line slopes for the best fit lines between the 10Be production and the 10Be measurements used in the cross correlation analysis are all in the range 0.4-0.6. This is a particular problem for historical projections of solar activity based on ice core measurements which assume a 1:1 correspondence. We have made other tests of the correspondence between the 10Be predictions and the ice core measurements which lead to the same conclusion, namely that other influences on the ice core measurements, as large as or larger than the production changes themselves, are occurring. These influences could be climatic or instrumentally based. We suggest new ice core measurements that might help in defining more clearly what these influences are and-if possible-to correct for them.Comment: 24 pages, 6 figure