Paleohydrological Changes in Jura (France), and Climatic Oscillations around the North Atlantic from Allerød to Preboreal

Regional palaeohydroiogical changes recorded in the Jura lakes, France, have led to a tentative reconstruction of a fine-scale pattern of climatic changes from Allerød to the early Holocene. The Younger Dryas (YD) can be subdivided into three parts: after a first wet phase, this cool period was characterized by increasing dryness; a short rise in lake level developed during its last part. Moreover, the YD was preceded and followed by two short rises in lake-level, which interrupted the lake-level lowerings developing during the Allerød and the Preboreal. Climatic oscillations reconstructed in Jura appear to be in phase with glacier and tree-limit movements in the Alps, with fluctuations in oxygen-isotope records from the Swiss lakes and the Greenland ice sheet, and with climatic oscillations recorded in the Norwegian Sea. Other correlations between (1) these climatic oscillations, (2) 14C plateaux recorded in Swiss lake sequences, and (3) fluctuations in the residual ∆14C, support a correlation between regional palaeohydroiogical changes defined in Jura and broad-scale climatic oscillations; they also support the existence of a link between climatic changes in Western and Central Europe and oceanic circulation. Moreover, as working hypothesis, these correlations suggest high-precision timing for the climatic and environmental changes occurring during the early Holocene.Les changements paléohydrologiques enregistrés dans les remplissages des lacs du Jura permettent de proposer une reconstitution détaillée des oscillations climatiques de I'Allerød aux débuts de l'Holocène. Trois phases sont distinguées à l'intérieur du Dryas récent : une phase à bilan hydrique déficitaire encadrée par deux phases humides. D'autre part, deux courtes phases de hausse du niveau des lacs surviennent avant et après le Dryas récent, interrompant momentanément la baisse des plans d'eau qui se développe pendant I'Allerød et le Préboréal. Ces oscillations climatiques reconstituées dans le Jura apparaissent en phase avec les mouvements des glaciers et de la limite supérieure de la forêt dans les Alpes, avec les stratigraphies isotopiques établies dans les lacs suisses et au Groenland ainsi qu'avec des oscillations climatiques enregistrées en mer de Norvège. D'autres corrélations entre ces oscillations climatiques, les plateaux du radiocarbone repérés dans les lacs suisses et les fluctuations du residual ∆14C résiduel indiquent l'existence d'une corrélation entre les changements paléohydrologiques reconnus dans le Jura et des oscillations plus globales du climat. Elles renforcent aussi l'idée d'un lien entre la circulation océanique et les changements climatiques affectant alors l'Europe occidentale et centrale. Enfin, elles permettent de proposer des références chronologiques de haute précision pour les changements climatiques et environnementaux survenant au début de l'Holocène.In den Juraseen festgestellte régionale palaohydrologische Veranderungen erlaubten eine vorlàufige detaillierte Rekonstruktion der klimatischen Wechsel vom Allerod bis zum frùhen Holozàn. Die jùngere Dryaszeit kann in drei Phasen unterteilt werden: nach einer ersten feuchten Phase, charakterisierte sich die darauffolgende kalte Période durch zunehmende Trockenheit; wàhrend der letzten Phase entwickelte sich eine kurze Anhebung des Seenniveaus. AuBerdem gab es zwei kurze Anhebungen des Seenniveaus, eine vor und eine nach der Dryaszeit, welche die Senkungen des Seenniveaus, die wàhrend des Allerod und dem frùhen Holozàn auftraten, unterbrachen. Die im Jura rekonstruierten klimatischen Schwankungen scheinen mit den Gletscherund Baumgrenze-Bewegungen in den Alpen zeitlich ùbereinzustimmen, wie auch mit den Fluktuationen in den Sauerstoffisotopenbelegen von Schweizer Seen und der Gronland-Eisdecke, und mit den im norwegischen Meer festgestellten Klimaschwankungen. Andere Korrelationen zwischen (1) diesen Klimaschwankungen, (2) C Plateaux, die in Schweizer Seen-Sequenzen festgestellt wurden und (3) Fluktuationen im residual ∆14C Rùckstand, stùtzen eine Korrelation zwischen den im Jura festgestellten regionalen palàohydrologischen Veranderungen und globaleren Klimaschwankungen; sie stùtzen auch die Annahme einer Beziehung zwischen den Klimawechseln in Westund Zentraleuropa und der Ozean-strômung

Du climat à l'homme : Dynamique holocène de l'environnement dans le jura et les alpes Actes du colloque GDR JURALP organisé à Aix-en-Provence les 15 et 16 novembre 2007

L'éditeur autorise la diffusion de ce numéro dans une archive ouverte.Ce volume constitue les actes de la table ronde qui s'est tenue les 15 et 16 novembre 2007 à la Maison Méditerranéenne des Sciences de l'Homme d'Aix-en-Provence, et qui a été organisée dans le cadre du GDR 2992 du CNRS, JurAlpes, sur le thème " Dynamique holocène de l'environnement dans le Jura et les Alpes : du climat à l'homme ". Les dix huit contributions rassemblées dans cet ouvrage couvrent un large éventail de champs disciplinaires et de méthodes d'étude. Après les deux premiers chapitres dédiés successivement aux reconstitutions des fluctuations des glaciers des Alpes occidentales depuis 5000 ans (P. Deline et M. Le Roy) et à l'usage des spéléothèmes en tant que proxy de la variabilité climatique annuelle (Y. Perrette), les cinq contributions suivantes s'appuient sur l'analyse de séquences sédimentaires lacustres (E. Chapron et al. ; M. Magny et al. ; L. Millet et al. ; F. Arnaud et al. ; A. Leroux et al.). Les lacs étudiés sont des lacs d'altitude (lac de Joux dans le Jura, lacs d'Anterne, de Bramant, de Blanc Huez et de Dessus-Verney dans les Alpes), ou des grands lacs de vallées (lacs du Bourget et de Constance, ou Bodensee). Suivent quatre chapitres qui s'intéressent aux vecteurs fluviatiles, de la vallée du Rhône (J.-P. Bravard et al. ; J.-F. Berger et al.), au bassin de la Durance (C. Miramont et al.) jusqu'au delta du Rhône (C. Vella et al.). Viennent ensuite trois contributions qui sont plus particulièrement centrées sur l'histoire du couvert végétal et de la pédogénèse dans les zones d'altitude des Alpes en s'appuyant sur des données anthracologiques (B. Talon), dendrochronologiques (J.-L. Edouard et A. Thomas) et pédo-sédimentaires (B. Moulin et P.-J. Rey). Enfin, les quatre dernières contributions portent sur l'histoire de l'anthropisation des Alpes (P.J. Rey et al. ; P. Bintz et al. ; A. Marguet et al. ; F. Mocci et al.) et du Jura (E. Gauthier et H. Richard)

Terrestrial climate variability and seasonality changes in the Mediterranean region between 15 000 and 4000 years BP deduced from marine pollen records

Pollen-based climate reconstructions were performed on two high-resolution pollen marines cores from the Alboran and Aegean Seas in order to unravel the climatic variability in the coastal settings of the Mediterranean region between 15 000 and 4000 years BP (the Lateglacial, and early to mid-Holocene). The quantitative climate reconstructions for the Alboran and Aegean Sea records focus mainly on the reconstruction of the seasonality changes (temperatures and precipitation), a crucial parameter in the Mediterranean region. This study is based on a multi-method approach comprising 3 methods: the Modern Analogues Technique (MAT), the recent Non-Metric Multidimensional Scaling/Generalized Additive Model method (NMDS/GAM) and Partial Least Squares regression (PLS). The climate signal inferred from this comparative approach confirms that cold and dry conditions prevailed in the Mediterranean region during the Oldest and Younger Dryas periods, while temperate conditions prevailed during the Bølling/Allerød and the Holocene. Our records suggest a West/East gradient of decreasing precipitation across the Mediterranean region during the cooler Late-glacial and early Holocene periods, similar to present-day conditions. Winter precipitation was highest during warm intervals and lowest during cooling phases. Several short-lived cool intervals (i.e. Older Dryas, another oscillation after this one (GI-1c2), Gerzensee/Preboreal Oscillations, 8.2 ka event, Bond events) connected to the North Atlantic climate system are documented in the Alboran and Aegean Sea records indicating that the climate oscillations associated with the successive steps of the deglaciation in the North Atlantic area occurred in both the western and eastern Mediterranean regions. This observation confirms the presence of strong climatic linkages between the North Atlantic and Mediterranean regions

The significance of climate fluctuations for lake level changes and shifts in subsistence economy during the late Neolithic (4300-2400 b.c. ) in central Europe

In the last decades, data on the economy and environment of the Neolithic period of lake dwellings (4300-2400 b.c.) in central Europe has increased considerably and also palaeoecological data on lake level fluctuations has been thoroughly elaborated. Lake shores were mainly settled during warm and rather dry climate periods which caused a fall in the lake levels. Nevertheless, there were strong and partly very short-term shifts in the economy during the lake-dwelling period. These can be recognised only because the settlement layers can be very precisely dated by dendrochronology. In this article we discuss in an interdisciplinary way the possible interrelations between climatic and economic changes. To explain the latter, we assume crop failures as the main reason, which caused intensified hunting and gathering. There are three different possibilities which might explain this: cold and wet summers, severe droughts during spring and summer, or local over-exploitation of soils in densely settled area

Le lac du Bourget (Savoie, France) à l'Âge du Bronze : végétation, impacts anthropiques et climat

Lake Bourget (Savoie, France) during the Bronze Age: vegetation, anthropogenic impacts and climate Pollen analyses performed on three archaeological sites located on the east shore of Lake Bourget (Savoie, France) provide a detailed record of human impacts and vegetation history during the Bronze Age. The end of the Neolithic period and the entire early Bronze Age are characterized in pollen diagrams by a discreet settlement around Lake Bourget and low human impact on the forested landscape, in spite of climatic improvement. The middle Bronze Age, contemporaneous of a climatic reversal, is related to the rarefaction of anthropogenic indicators. A new human impact occurs at the beginning of the late Bronze Age. A brief high lake level, correlated to the abandonment of agropastoral activities, appears during the more intensive occupation at the end of the late Bronze Age. The beginning of the Iron Age is related to a new abandonment of the shore. In the last part of this paper, pollen analyses are correlated to the results of lake levels recorded and molecular markers preserved in a deep lacustrine core.L'étude de trois analyses polliniques effectuées sur les sites archéologiques de Chindrieux (Châtillon) et de Tresserve (Le Saut de la Pucelle), sur la rive est du lac du Bourget (Savoie, France) permet de reconstituer l'histoire de la végétation et des impacts anthropiques au cours de la période de l'Âge du Bronze. La fin du Néolithique et le Bronze ancien sont caractérisés par une occupation discrète des alentours du Bourget et donc par un faible impact anthropique sur l'environnement très forestier, en dépit d'un climat plutôt favorable. La période du Bronze moyen, contemporaine d'une péjoration climatique, correspond à la quasi-disparition de tout impact anthropique. Le début du Bronze final voit le retour toujours discret des activités humaines, activités qui s'amplifient soudainement au cours de la fin du Bronze final. On observe alors une tripartition des événements : une courte phase de transgression lacustre et de déprise agricole scinde la fin du Bronze final. Le début de l'Âge du Fer correspond à une nouvelle déprise agricole. Dans une dernière partie, les résultats des analyses paléoclimatiques et chimiques obtenues sur un forage profond au milieu du lac sont intégrés aux résultats polliniques

Impact of Holocene climate variability on lacustrine records and human settlements in South Greenland

International audienceDue to its sensitivity to climate changes, south Greenland is a particularly suitable area to study past global climate changes and their influence on locale Human settlements. A paleohydrological investigation was therefore carried out on two river-fed lakes: Lake Qallimiut and Little Kangerluluup, both located close to the Labrador Sea in the historic 5 farming center of Greenland. Two sediment cores (QAL-2011 and LKG-2011), spanning the last four millennia, were retrieved and showed similar thin laminae, described by high magnetic susceptibility and density, high titanium and TOC / TN atomic ratio, and coarse grain size. They are also characterized either by inverse grading followed by normal grading or by normal grading only and a prevalence of red amorphous particles 10 and lignocellulosic fragments, typical of flood deposits. Flood events showed similar trend in both records: they mainly occurred during cooler and wetter periods characterized by weaker Greenlandic paleo-temperatures, substantial glacier advances, and a high precipitation on the Greenlandic Ice Sheet and North Atlantic ice-rafting events. They can therefore be interpreted as a result of ice and 15 snow-melting episodes. They occurred especially during rapid climate changes (RCC) such as the Middle to Late Holocene transition around 2250 BC, the Sub-boreal/Sub-atlantic transition around 700 BC and the Little Ice Age (LIA) between AD 1300 and AD 1900, separated by cycles of 1500 years and driven by solar forcing. These global RCC revealed by QAL-2011 and LKG-2011 flood events may have influenced Human 20 settlements in south Greenland, especially the paleo-Eskimo cultures and the Norse settlement, and have been mainly responsible for their demise

Mass-movement and flood-induced deposits in Lake Ledro, southern Alps, Italy: implications for Holocene palaeohydrology and natural hazards

International audienceHigh-resolution seismic profiles and sediment cores from Lake Ledro combined with soil and riverbed samples from the lake's catchment area are used to assess the recurrence of natural hazards (earthquakes and flood events) in the southern Italian Alps during the Holocene. Two welldeveloped deltas and a flat central basin are identified on seismic profiles in Lake Ledro. Lake sediments have been finely laminated in the basin since 9000 cal. yr BP and frequently interrupted by two types of sedimentary events (SEs): lightcoloured massive layers and dark-coloured graded beds. Optical analysis (quantitative organic petrography) of the organic matter present in soil, riverbed and lacustrine samples together with lake sediment bulk density and grainsize analysis illustrate that light-coloured layers consist of a mixture of lacustrine sediments and mainly contain algal particles similar to the ones observed in background sediments. Light-coloured layers thicker than 1.5 cm in the main basin of Lake Ledro are synchronous to numerous coeval mass-wasting deposits remoulding the slopes of the basin. They are interpreted as subaquatic mass-movements triggered by historical and pre-historical regional earthquakes dated to AD2005, AD1891, AD1045 and 1260, 2545, 2595, 3350, 3815, 4740, 7190, 9185 and 11 495 cal. yr BP. Darkcoloured SEs develop high-amplitude reflections in front of the deltas and in the deep central basin. These beds are mainly made of terrestrial organic matter (soils and lignocellulosic debris) and are interpreted as resulting from intense hyperpycnal flood event. Mapping and quantifying the amount of soil material accumulated in the Holocene hyperpycnal flood deposits of the sequence allow estimating that the equivalent soil thickness eroded over the catchment area reached up to 5mm during the largest Holocene flood events. Such significant soil erosion is interpreted as resulting from the combination of heavy rainfall and snowmelt. The recurrence of flash flood events during the Holocene was, however, not high enough to affect pedogenesis processes and highlight several wet regional periods during the Holocene. The Holocene period is divided into four phases of environmental evolution. Over the first half of the Holocene, a progressive stabilization of the soils present through the catchment of Lake Ledro was associated with a progressive reforestation of the area and only interrupted during the wet 8.2 event when the soil destabilization was particularly important. Lower soil erosion was recorded during the mid-Holocene climatic optimum (8000-4200 cal. yr BP) and associated with higher algal production. Between 4200 and 3100 cal. yr BP, both wetter climate and human activities within the drainage basin drastically increased soil erosion rates. Finally, from 3100 cal. yr BP to the present-day, data suggest increasing and changing human land use

Changements climatiques et variations d’assemblages de thécamoebiens au cours de la transition Tardiglaciaire - Holocène au lac du Lautrey (Jura, France)

L’analyse des assemblages de thécamoebiens (Protistes, Rhizopodes) au cours de la transition Tardiglaciaire-Holocène au lac du Lautrey (Jura) offre l’opportunité de tester la réponse de ces micro-organismes lacustres aux changements du climat et de l’environnement. Les données obtenues lors d’une précédente étude multi-paramètres à haute résolution sur la même séquence sédimentaire ont servi de base de référence pour les observations. Les premiers résultats montrent que les événements climatiques majeurs successifs qui caractérisent la transition Tardiglaciaire-Holocène (c’est-à-dire le Dryas ancien, l’Interstade Bølling-Allerød, le Dryas récent, le Préboréal) correspondent à des changements dans la composition des assemblages de thécamoebiens (espèces dominantes et structure des assemblages) ainsi que du taux d’accumulation des thèques. Les refroidissements climatiques mineurs comme le Dryas moyen ou l’oscillation de Gerzensee sembleraient être enregistrés également, notamment par des diminutions du taux d’accumulation, excepté pour l’oscillation du Préboréal qui n’a pas été mise en évidence alors qu’elle apparaît bien caractérisée par d’autres indicateurs. Ce travail montre l’intérêt de l’utilisation des thécamoebiens pour les reconstitutions paléoclimatiques. Cependant, une meilleure compréhension des relations entre le climat et la composition des assemblages de thécamoebiens nécessite encore 1) une meilleure résolution sur cette séquence du lac du Lautrey ainsi que la mise en œuvre, sur d’autres sites, d’études similaires à haute résolution basées sur une approche pluridisciplinaire, et 2) la mise en place d’analogues modernes pour mieux cerner la signification paléoclimatique de ces micro-organismes.The establishment of a testate amoeba (Protists, Rhizopoda) record from a Lateglacial-early Holocene sediment sequence at Lake Lautrey (Jura Mountains, eastern France) offers the opportunity to test the response of lacustrine testate amoebae to climate changes using a comparison with data obtained from a previous high-resolution multi-proxy study from the same sediment sequence. The results show that the successive major climate events that characterized the Lateglacial-Holocene transition (i.e. Oldest Dryas, Bølling-Allerød Interstadial, Younger Dryas, Preboreal) are recorded as clear changes in the composition of testate amoeba assemblages (dominant species and assemblages structure), as well as in the accumulation rate of the number of tests. Minor short-lived events, such as Older Dryas or Gerzensee oscillation, seemed to be recorded too by decreases in the accumulation rate, except for the Preboreal oscillation, which cannot be recognized in the testate amoeba records of Lake Lautrey, although it was well registered by other proxies. This work supports the interest of lacustrine testate amoebae for palaeoclimatic reconstructions. However, a better understanding of the relation between climate and lacustrine testate amoebae requires (1) a better resolution at Lake Lautrey and further high-resolution studies based on a multiproxy approach, and (2) the establishment of modern analogues (present-day ecology of species) in order to have a better knowledge of the palaeoclimatic signification of those organisms

Orbital changes, variation in solar activity and increased anthropogenic activities: controls on the Holocene flood frequency in the Lake Ledro area, Northern Italy

International audienceTwo lacustrine sediment cores from Lake Ledro in northern Italy were studied to produce chronologies of floodevents for the past 10 000 yr. For this purpose, we have developed an automatic method that objectively identifies the sedimentary imprint of river floods in the downstream lake basin. The method was based on colour data extracted from processed core photographs, and the count data were analysed to capture the flood signal. Flood frequency and reconstructed sedimentary dynamics were compared with lake-level changes and pollen inferred vegetation dynamics. The results suggest a record marked by low flood frequency during the early and middle Holocene (10 000–4500 cal BP). Only modest increases during short intervals are recorded at ca. 8000, 7500, and 7100 cal BP. After 4500–4000 cal BP, the record shows a shift toward increased flood frequency. With the exception of two short intervals around 2900–2500 and 1800–1400 cal BP, which show a slightly reduced number of floods, the trend of increasing flood frequency prevailed until the 20th century, reaching a maximum between the 16th and the 19th centuries. Brief-flood frequency increases recorded during the early and middle Holocene can be attributed to climatic oscillations. On a centennial time scale, major changes in flood frequency, such as those observed after ca. 4500/4000 and 500 cal BP, can be attributed to large-scale climatic changes such as the Neo-glacial and Little Ice Age, which are under orbital and possibly solar control. However, in the Bronze Age and during the Middle Ages and modern times, forest clearing and land use probably partially control the flood activity