Aspects taphonomiques de la stratigraphie et de la datation de charbons de bois dans les sols : exemple de quelques sols des Alpes

Les phénomènes d'enfouissement et de fragmentation des charbons de bois des sols de montagne sont mis en évidence par l'analyse anthracologique et par des datations absolues au14C de quatre profils pédologiques provenant des Alpes françaises. Ces dernières s'avèrent indispensables mais insuffisantes pour comprendre le mode de stratification. Les datations doivent être précédées d'une analyse des assemblages anthracologiques à la fois qualitative (composition taxonomique) et quantitative lorsque les sols sont suffisamment riche en charbons. Le processus de stratification dans les sols n'est pas comparable à celui que l'on observe dans un lac ou une tourbière; les racines, la pédofaune et les processus d'érosion contribuent à créer une vitesse différentielle d'enfouissement des charbon dans les sols. Cela conduit à une stratification dite « en écailles » des charbons appartenant à un même taxon. Les assemblages et les datations permettent d'identifier des phases d'incendies subies par la végétation sur la station MAUR 6 située en moyenne altitude (1770 m). Quant à QUEYRAS 2, en haute altitude (2670 m), il ne semble pas approprié pour révéler, de par sa composition en charbon, différentes phases d'incendies et surtout les plus anciennes; le réseau racinaire mais surtout les alternances de gel-dégel participent à la réduction de taille des fragments dans les sols.The burial and fragmentation processes are investigated by anthracological analysis and by absolute14C dating of four pedological profiles from the French Alps. Absolute14C dates are essential but insufficient for the understanding of the mode of stratification. The dates must be considered after a detailed qualitative (taxonomic composition) and quantitative analysis of anthracological assemblages. The soil stratification process does not compare with that in lake or peatbog sediments. Action of roots, soil fauna and erosion processes mean burial of wood charcoal in the soil occurs at different speeds. The result is a stratification in "scales". The assemblages and dates allow us to recognize three fire phases on the stand MAUR 6 at middle altitude (1770 m a.s.l.). On QUEYRAS 2, at high altitude (2670 m a.s.l.), wood charcoal assemblages did not reveal the oldest fire phases. The network of roots and mainly the frost-thaw reduces the size of wood charcoal fragments in the soil.Attraverso I'analisi pedoantracologica e Ie datazioni assolute al 14C di quattro profili pedologici provenienti dalle Alpi francesi, vengono messi in evidenza i fenomeni di infossamento e di frammentazione dei carboni nei suoli di montagna. Le datazioni sono indispensabili ma insufficienti per comprendere Ia stratigrafia dei carboni. Le datazioni debbono essere precedute da un'analisi degli insiemi antracologici sia qualitativi (composizione tassonomica) che quantitativi allorchè i suoli sono ricchi in carbone. Il processo di stratificazione nei suoli è diverso da quello nei laghi o nelle torbiere; infatti Ie radici, Ia pedofauna ed i processi di erosione provocano una diversa vélocité di infossamento dei carboni. Quelli appartenenti alio stesso taxon hanno cosi una stratificazione detta "a scaglie". Per Ia stazione MAUR 6, situata a 1770 m d'altitudine, I'insieme delle datazioni permette di identificare gli incendi subiti dalla vegetazione, mentre Ia stazione di QUEYRAS 2, situata a 2670 m d'altitudine, per Ia ripartizione dei carboni, non sembra idonea per mettere in evidenza Ie diverse fasi d'incendio soprattutto le più antiche; il feltro delle radici, ma soprattutto I'alternanza gelo-disgelo, contribuiscono alia riduzione della taglia dei frammenti di carbone

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

Disentangling the trajectories of alpha, beta and gamma plant diversity of North American boreal ecoregions since 15,500 years

Assessment of biodiversity in a changing world is a key issue and studies on the processes and factors influencing its history at relevant time scales are needed. In this study, we analyzed temporal trends of plant diversity using fossil pollen records from the North American boreal forest-taiga biome (NABT). We selected 205 pollen records spanning the last 15,500 years. Diversity was decomposed into α and γ richness, and β diversity, using Shannon entropy indices. We investigated temporal and spatial patterns of β diversity by decomposing it into independent turnover (variation in taxonomic composition due to species replacements) and nestedness (variation due to species loss) components. The palynological diversity of the NABT biome experienced major rearrangements during the Lateglacial and early Holocene in response to major climatic shifts. The β nestedness likely reflected plant immigration processes and generally peaked before the β turnover value, which mirrors spatial and temporal community sorting related to environmental conditions and specific habitat constraints. Palynological diversity was generally maximal during the Lateglacial and the early Holocene and decreased progressively during the Holocene. These results are discussed according to macro-ecological processes, such as immigration, disturbances, and environmental fluctuations, with climate most notably as the main ecological driver at millennial scales

Forest management is driving the eastern North American boreal forest outside its natural range of variability

Fire is fundamental to the natural dynamics of the North American boreal forest. It is therefore often suggested that the impacts of anthropogenic disturbances (eg logging) on a managed landscape are attenuated if the patterns and processes created by these events resemble those of natural disturbances (eg fire). To provide forest management guidelines, we investigate the long-term variability in the mean fire interval (MFI) of a boreal landscape in eastern North America, as reconstructed from lacustrine (lake-associated) sedimentary charcoal. We translate the natural variability in MFI into a range of landscape age structures, using a simple modeling approach. Although using the array of possible forest age structures provides managers with some flexibility, an assessment of the current state of the landscape suggests that logging has already caused a shift in the age-class distribution toward a stronger representation of young stands with a concurrent decrease in old-growth stands. Logging is indeed quickly forcing the studied landscape outside of its long-term natural range of variability, implying that substantial changes in management practices are required, if we collectively decide to maintain these fundamental attributes of the boreal forest

Sediment reworking by marine benthic species from the Gullmar Fjord (Western Sweden): Importance of faunal biovolume

In order to compare and quantify sediment reworking activities by different species/functional groups of macrofauna, a laboratory experiment was carried out with species from the Gullmarsfjord (Western Sweden). Monospecific communities of Amphiura filiformis, Echinocardium cordatum, Scalibregma inflatum and Abra nitida were introduced in experimental mesocosms, with identical densities (795 ind. m−2), for 10 days. Sediment reworking was studied by quantifying downward and upward movements of fluorescent inert tracers (luminophores). Luminophores with different colour were initially deposited both at the sediment surface and within the sediments. Population biomass and biovolume were also determined. Surface tracers reworking coefficients ranged from 0.6 to 2.2 cm2 y−1 and 0.9 to 4.1 y−1, respectively for the biodiffusive-like and non-local transports. Calculated biodiffusive-like coefficient was between 1.0 and 2.3 cm2 y−1 for the deep tracers. For both tracers, the E. cordatum population presented the highest reworking coefficients. Among the morphological and/or ethological parameters that could determine overall patterns of reworking and differences between species, results have shown a direct relationship between the apparent biodiffusive mixing and the biovolume of the individuals (Db=0.35 ⁎ Biovolume). This suggests that the biovolume of macrofauna may allow a rough estimate of the biodiffusive-like reworking intensity of particles deposited on the sediment surface

Tectonic and climatic controls on the Chuquibamba landslide (western Andes, southern Peru)

The contribution of landslides to the Quaternary evolution of relief is poorly documented in arid contexts. In southern Peru and northern Chile, several massive landslides disrupt the arid western Andean front. The Chuquibamba landslide, located in southern Peru, belongs to this set of large landslides. In this area, the Incapuquio fault system captures the intermittent drainage network and localizes rotational landslides. Seismic activity is significant in this region with recurrent Mw9 subduction earthquakes; however, none of the latest seismic events have triggered a major landslide. New terrestrial cosmogenic dating of the Chuquibamba landslide provides evidence that the last major gravitational mobilization of these rotational landslide deposits occurred at ~ 102 ka, during the Ouki wet climatic event identified on the Altiplano between 120 and 98 ka. Our results suggest that wet events in the arid and fractured context of the Andean forearc induced these giant debris flows. Finally, our study highlights the role of tectonics and climate on (i) the localization of large Andean landslides in the Western Cordillera and on (ii) the long-term mass transfer to the trench along the arid Andean front

In situ Comparison of Tree-Ring Responses to Climate and Population Genetics: The Need to Control for Local Climate and Site Variables

Tree species responses to climate change will be greatly influenced by their evolutionary potential and their phenotypic plasticity. Investigating tree-rings responses to climate and population genetics at the regional scale is crucial in assessing the tree behavior to climate change. This study combined in situ dendroclimatology and population genetics over a latitudinal gradient and compared the variations between the two at the intra- and inter-population levels. This approach was applied on the northern marginal populations of Thuja occidentalis (eastern white-cedar) in the Canadian boreal forest. We aimed first to assess the radial growth variability (response functional trait) within populations across the gradient and to compare it with the genetic diversity (microsatellites). Second, we investigated the variability in the growth response to climate at the regional scale through the radial growth-climate relationships, and tested its correlation with environmental variables and population genetic structure. Model selection based on the Akaike Information Criteria revealed that the growth synchronicity between pairs of trees of a population covariates with both the genetic diversity of this population and the amount of precipitation (inverse correlations), although these variables only explained a small fraction of the observed variance. At the regional scale, variance partitioning and partial redundancy analysis indicate that the growth response to climate was greatly modulated by stand environmental variables, suggesting predominant plastic variations in growth-response to climate. Combining in situ dendroclimatology and population genetics is a promising way to investigate species' response capacity to climate change in natural stands. We stress the need to control for local climate and site conditions effects on dendroclimatic response to climate to avoid misleading conclusions regarding the associations with genetic variables

A 13000 year environmental history of Lake Colbricon Inferiore (Trentino, Italia)

Lake sediments have long been used as an archive for paleoclimatic-paleoenvironmental reconstructions, with more and more refined techniques developed in the late 40 years. This work is an attempt to reconstruct with a multiproxy paleolimnological technique the ecosystem response in Colbricon Inferiore, a small high-mountain lakes located in the Paneveggio-Pale di S. Martino Natural Park(Trento, Italy). A ca. 360cm long core was retrieved with a piston core; a gravity corer was used for the most recent sediment. Chronology was established by a combination of 210Pb and 14C isotopes. Diatom assemblages show changes in species composition that could be associated to an initial warm phase (Allerod), a following colder one (YD) and a progressive amelioration of climate until the optimum (8500-3500). During the transition period (10000-8000) carotenoids point out to an increase of cyanobacteria (Echinenone and myxoxanthophyll) associated with an increase of fire frequency and the first settlement around Lake Colbricon. Along the climatic optimum, algal pigment are rather stable and Chryptophytes (alloxanthin) are well represented pointing to a high and stable water level. Diatom assemblages are dominated by the planktonic Discostella stelligeroides. At ca. 3500 yrs. B.P., a new phase begins with a decrease of D. stelligeroides and an increase of the small benthic fragilariaceae (Staurosira and Pseudostaurosira species) typical of cold-water conditions. At this level, there is a general increase of carotenoids and a shift in the ratio CD:TC. This suggest, rather than an increase in productivity, a change in the preservation conditions associated with the worsening of climate. After 2000 cal yr BP the lake seems quite stable: diatom assemblages are dominated by Staurosira and Pasudostaurosira species. Algal pigment are also quite stable apart myxoxanthophyll that increase in the more recent phase

Variability in fire frequency and forest composition in Canada's Southeastern Boreal Forest: A challenge for sustainable forest management

Because some consequences of fire resemble the effects of industrial forest harvesting, forest management is often considered as a disturbance having effects similar to those of natural disturbances. Although the analogy between forest management and fire disturbance in boreal ecosystems has some merit, it is important to recognize that it has limitations. First, normal forest rotations truncate the natural forest stand age distribution and eliminate over-mature forests from the landscape. Second, in the boreal mixedwoods, natural forest dynamics following fire may involve a gradual replacement of stands of intolerant broadleaf species by mixedwood and then softwood stands, whereas current silvicultural practices promote successive rotations of similarly composed stands. Third, the large fluctuations observed in fire frequency during the Holocene limit the use of a single fire cycle to characterize natural fire regimes. Short fire cycles generally described for boreal ecosystems do not appear to be universal; rather, shifts between short and long fire cycles have been observed. These shifts imply important changes in forest composition at the landscape and regional levels. All of these factors create a natural variability in forest composition that should be maintained by forest managers concerned with the conservation of biodiversity. One avenue is to develop silvicultural techniques that maintain a spectrum of forest compositions over the landscape