Does global warming favour the occurrence of extreme floods in European Alps? First evidences from a NW Alps proglacial lake sediment record

Flood hazard is expected to increase in the context of global warming. However, long time-series of climate and gauge data at high-elevation are too sparse to assess reliably the rate of recurrence of such events in mountain areas. Here paleolimnological techniques were used to assess the evolution of frequency and magnitude of flash flood events in the North-western European Alps since the Little Ice Age (LIA). The aim was to document a possible effect of the post-19(th) century global warming on torrential floods frequency and magnitude. Altogether 56 flood deposits were detected from grain size and geochemical measurements performed on gravity cores taken in the proglacial Lake Blanc (2170 m a.s.l., Belledonne Massif, NW French Alps). The age model relies on radiometric dating (Cs-137 and Am-241), historic lead contamination and the correlation of major flood- and earthquake-triggered deposits, with recognized occurrences in historical written archives. The resulting flood calendar spans the last ca 270 years (AD 1740-AD 2007). The magnitude of flood events was inferred from the accumulated sediment mass per flood event and compared with reconstructed or homogenized datasets of precipitation, temperature and glacier variations. Whereas the decennial flood frequency seems to be independent of seasonal precipitation, a relationship with summer temperature fluctuations can be observed at decadal timescales. Most of the extreme flood events took place since the beginning of the 20(th) century with the strongest occurring in 2005. Our record thus suggests climate warming is favouring the occurrence of high magnitude torrential flood events in high-altitude catchments

The precursors of chocolate aroma.

The subject of this research was an experimental study of the development of chocolate aroma during the commercial processing of cocoa beans. The components of unroasted aroma cocoa beans have been fractionated, and the various groups of substances obtained tested for the development of aroma when roasted. The results obtained provide further evidence of the involvement of simple amino acids, reducing sugars and flavonoids in the formation of the primary aroma of chocolate products, and a mechanism for the formation and mixing of these substances during cocoa fermentation is described. Model mixtures of these compounds have been examined in order to assess the extent of the contribution of individual compounds to the production of the aroma. The reaction of these model mixtures have also been examined under the conditions obtaining during commercial bean roasting, and combined gas chromstography-mase spectrometry has been used to compare their volatile reaction products with these present in roasted cocoa beans. The results of these studies indicate the importance of the natural environment of the precursors in the control of the extent of subsequent reactions, and the consequences of the 'dry state' reaction conditions in this respect. Various types of compound formed in these reactions are described and their possible importance in the recognition of chocolate aroma is discussed. The experimental results are discussed in terms of literature surveys of the current state of knowledge of both chocolate aroma development, and the aroma potential of amino acid-reducing sugar reactions

Chemical analysis of pottery demonstrates prehistoric origin for high-altitude alpine dairying

The European high Alps are internationally renowned for their dairy produce, which are of huge cultural and economic significance to the region. Although the recent history of alpine dairying has been well studied, virtually nothing is known regarding the origins of this practice. This is due to poor preservation of high altitude archaeological sites and the ephemeral nature of transhumance economic practices. Archaeologists have suggested that stone structures that appear around 3,000 years ago are associated with more intense seasonal occupation of the high Alps and perhaps the establishment of new economic strategies. Here, we report on organic residue analysis of small fragments of pottery sherds that are occasionally preserved both at these sites and earlier prehistoric rock-shelters. Based mainly on isotopic criteria, dairy lipids could only be identified on ceramics from the stone structures, which date to the Iron Age (ca. 3,000 - 2,500 BP), providing the earliest evidence of this practice in the high Alps. Dairy production in such a marginal environment implies a high degree of risk even by today’s standards. We postulate that this practice was driven by population increase and climate deterioration that put pressure on lowland agropastoral systems and the establishment of more extensive trade networks, leading to greater demand for highly nutritious and transportable dairy products

Lake sedimentary dna research on past terrestrial and aquatic biodiversity: Overview and recommendations

The use of lake sedimentary DNA to track the long-term changes in both terrestrial and aquatic biota is a rapidly advancing field in paleoecological research. Although largely applied nowadays, knowledge gaps remain in this field and there is therefore still research to be conducted to ensure the reliability of the sedimentary DNA signal. Building on the most recent literature and seven original case studies, we synthesize the state-of-the-art analytical procedures for effective sampling, extraction, amplification, quantification and/or generation of DNA inventories from sedimentary ancient DNA (sedaDNA) via high-throughput sequencing technologies. We provide recommendations based on current knowledge and best practises.</jats:p

Sediment source fingerprinting: benchmarking recent outputs, remaining challenges and emerging themes

Abstract: Purpose: This review of sediment source fingerprinting assesses the current state-of-the-art, remaining challenges and emerging themes. It combines inputs from international scientists either with track records in the approach or with expertise relevant to progressing the science. Methods: Web of Science and Google Scholar were used to review published papers spanning the period 2013–2019, inclusive, to confirm publication trends in quantities of papers by study area country and the types of tracers used. The most recent (2018–2019, inclusive) papers were also benchmarked using a methodological decision-tree published in 2017. Scope: Areas requiring further research and international consensus on methodological detail are reviewed, and these comprise spatial variability in tracers and corresponding sampling implications for end-members, temporal variability in tracers and sampling implications for end-members and target sediment, tracer conservation and knowledge-based pre-selection, the physico-chemical basis for source discrimination and dissemination of fingerprinting results to stakeholders. Emerging themes are also discussed: novel tracers, concentration-dependence for biomarkers, combining sediment fingerprinting and age-dating, applications to sediment-bound pollutants, incorporation of supportive spatial information to augment discrimination and modelling, aeolian sediment source fingerprinting, integration with process-based models and development of open-access software tools for data processing. Conclusions: The popularity of sediment source fingerprinting continues on an upward trend globally, but with this growth comes issues surrounding lack of standardisation and procedural diversity. Nonetheless, the last 2 years have also evidenced growing uptake of critical requirements for robust applications and this review is intended to signpost investigators, both old and new, towards these benchmarks and remaining research challenges for, and emerging options for different applications of, the fingerprinting approach

A 4D sedimentological approach to reconstructing the flood frequency and intensity of the Rhône River (Lake Bourget, NW European Alps)

A high-resolution sedimentological study of Lake Bourget was conducted to reconstruct the flood frequency and intensity (or magnitude) in the area over the last 350 years. Particular emphasis was placed on investigating the spatio-temporal distribution of flood deposits in this large lake basin. The thicknesses of deposits resulting from 30 flood events of the Rhoˆne River were collected over a set of 24 short sediment cores. Deposit thicknesses were compared with instrumental data for the Rhoˆne River discharge for the period from 1853 to 2010. The results show that flood frequency and intensity cannot be reliably reconstructed from a single core because of the inhomogeneous flood-deposit geometry in such a large lake. From all documented flood-deposit thicknesses, volumes of sediment brought into the lake during each flood event were computed through a Kriging procedure and compared with the historical instrumental data. The results show, in this study, that reconstructed sediment volumes are well correlated to maximal flood discharges. This significant correlation suggests that the increase of embankment and dam settlements on the Rhoˆne River during the last 150 years has not significantly affected the transport of the smallest sediment fraction during major flood events. Hence, assessment of the flood-sediment volumes deposited in the large Lake Bourget is the only way to reliably reconstruct the flood frequency and intensity

De la préhistoire aux temps modernes dans les alpages d’Anterne et Pormenaz. Une approche de l’occupation de la moyenne montagne entre les Fiz et les Aiguilles Rouges. Résultats du programme de sondages

International audienc

Evolution spatio temporelle de l'oxygénation de l'hypolimnion sous contrôle du niveau trophique et des crues rhodaniennes dans le lac du Bourget depuis 150 ans

National audienceBeaucoup de lacs en Europe ont développé un faciès laminé dans leurs sédiments ces dernières décennies (lamines claires d’été correspondant à de la calcite bio-précipitée et lamines sombres d’hiver riches en matière organique) en raison du manque d’oxygène biodisponible dans l’hypolimnion. Trois paramètres peuvent être à l’origine de la dégradation des conditions d’oxygénation dans l’hypolimnion : l’élévation du niveau trophique du lac, le retournement incomplet de la colonne d’eau et l’absence de courants de densité provenant des crues. Dans le lac périalpin du Bourget (Savoie), l’analyse géochimique des sédiments nous a permis de reconstituer les conditions d’oxygénation de l’hypolimnion des 150 dernières années. Dans le but d’étudier les variations spatio-temporelles de l’oxygénation et les processus impliqués dans la régulation de l’O2 nous avons réalisé un transect de 10 carottes dans l’axe des apports des crues Rhodaniennes et un deuxième, transversal, constitué de 4 carottes. Dans le bassin Nord l’intensité de l’anoxie a été évaluée spatialement et temporellement en fonction de la présence plus ou moins marquée des lamines saisonnières, de la préservation de la matière organique mesurée en spectrocolorimétrie, et du rapport Mn/Fe. Ces résultats ont été comparés aux flux de phosphore dans les sédiments ainsi qu’aux mesures des concentrations en Ptotal, Portho et oxygène de la colonne d’eau à partir des données INRA-CISALBa (Jacquet et al. 2008). Les résultats permettent de modéliser l’étendue des dépôts rhodaniens à différentes époques, ainsi que l’étendue des sédiments hypoxiques à partir des années 1930-40. Le retour d’un faciès non laminé sur deux carottes disposées dans l’alignement des crues du Rhône, entre 1976 et 1986, indique une période de réoxygénation du fond. Cette période de temps sans lamine correspond à une période plus intense de détritisme du Rhône (Giguet-Covex et al. 2010), ceci nous laisse supposer que le processus responsable localement de l’oxygénation du fond pourrait être attribué aux crues, favorisant ainsi le retour de la vie benthique. Une étude en cours sur l’oxygénation du fond du lac à partir de l’analyse des assemblages de chironomes devrait permettre d’affiner ces résultats