Late-glacial and postglacial fluctuations of the tree limits in the Mediterranean Pyrenees : the use of pollen ratios

International audienceThe use of pollen ratios to estimate the fluctuations of the late-glacial and holocene tree-limits and the timber line is discussed. An attempt is made with fossil data from some sites from the eastern part of the Pyrenees. Comparisons with the records of Pinus stomata indicating local occurence of Pinus show a good correlation between the two complementary approaches and emphasize the value of the pollen ratio. However, in all cases, the components of the ratio have to be discussed. For reconstructing the timber line, local floristic studies are necessary so that the most appropriate pollen indicators can be chosen

Archives lacustres de l'évolution du climat et des activités humaines récentes dans les Pyrénées ariégeoises au cours de l'Holocène (étang majeur, vallée du Haut-Vicdessos, Pyrénées, France)

International audienceDans le cadre de l'Observatoire Hommes Milieux Haut-Vicdessos, des archives lacustres sont utilisées pour distin-guer les impacts de l'Homme ou du climat dans les Pyrénées (Ariège, 42°N). Associée à la cartographie acoustique de l'Étang Majeur, les analyses des sols et des sédi-ments lacustres mettent en évidence une sédimentation Tardiglaciaire riche en ti-tane, qui contraste avec une sédimentation Holocène de type dy résultant de l'érosion diffuse des tourbes présentes en amont du lac. L'enregistrement indique des périodes plus humides, datées en 1200, 1950, 3400 et 4550 cal BP et associées aux apports d'un canyon drainant les zones d'altitude. Depuis 1907, le niveau d'eau du lac est ré-gulé par deux barrages hydroélectriques. Il en résulte un marnage de 10 m affec-tant jusqu'à 37 % du bassin. Ceci a pour conséquences de remobiliser le matériel issu des berges, et d'augmenter la produc-tivité algaire et les taux d'accumulation

Type regions F-zg and F-r, the northen slope of the Pyrenees

From the Atlantic Ocean to the Mediterranean Sea, the Pyrenees form the westernmost mountain chain of E-W orientation in the south of western Europe. We can distinguish two regions : an Atlantic zone and a Pyrenean zone. The Pyrenean chain shows in each of these zones a great diversity of local climatic characteristics. This paper attempts to illustrate the methodological approach we need to reach a palaeoecological synthesis. Five or six studied sites in different vegetation belts of the Chaîne des Hurtières are presented

Climate and vegetation changes during the Lateglacial and early-middle Holocene at Lake Ledro (southern Alps, Italy)

International audienceAdding to the on-going debate regarding vegetation recolonisation (more particularly the timing) in Europe and climate change since the Lateglacial, this study investigates a long sediment core (LL081) from Lake Ledro (652ma.s.l., southern Alps, Italy). Environmental changes were reconstructed using multiproxy analysis (pollen-based vegetation and climate reconstruction, lake levels, magnetic susceptibility and X-ray fluorescence (XRF) measurements) recorded climate and land-use changes during the Lateglacial and early-middle Holocene. The well-dated and high-resolution pollen record of Lake Ledro is compared with vegetation records from the southern and northern Alps to trace the history of tree species distribution. An altitudedependent progressive time delay of the first continuous occurrence of Abies (fir) and of the Larix (larch) development has been observed since the Lateglacial in the southern Alps. This pattern suggests that the mid-altitude Lake Ledro area was not a refuge and that trees originated from lowlands or hilly areas (e.g. Euganean Hills) in northern Italy. Preboreal oscillations (ca. 11 000 cal BP), Boreal oscillations (ca. 10 200, 9300 cal BP) and the 8.2 kyr cold event suggest a centennial-scale climate forcing in the studied area. Picea (spruce) expansion occurred preferentially around 10 200 and 8200 cal BP in the south-eastern Alps, and therefore reflects the long-lasting cumulative effects of successive boreal and the 8.2 kyr cold event. The extension of Abies is contemporaneous with the 8.2 kyr event, but its development in the southern Alps benefits from the wettest interval 8200-7300 cal BP evidenced in high lake levels, flood activity and pollen-based climate reconstructions. Since ca. 7500 cal BP, a weak signal of pollen-based anthropogenic activities suggest weak human impact. The period between ca. 5700 and ca. 4100 cal BP is considered as a transition period to colder and wetter conditions (particularly during summers) that favoured a dense beech (Fagus) forest development which in return caused a distinctive yew (Taxus) decline.We conclude that climate was the dominant factor controlling vegetation changes and erosion processes during the early and middle Holocene (up to ca. 4100 cal BP)

Recent climatic and anthropogenic imprints on lacustrine systems in the Pyrenean Mountains inferred from minerogenic and organic clastic supply (Vicdessos valley, Pyrenees, France)

High-resolution seismic profiling has been combined with geochemical analyses of both watershed samples and five lacustrine cores retrieved from two natural lacustrine basins of glacial origin: Lake Majeur and Lake Sigriou (1630 m a.s.l. and 1995 m a.s.l., respectively, Eastern French Pyrenees). Identifying specific minerogenic and organic markers of autochthonous and allochthonous supply, data allow documenting past climatic and anthropogenic pressures. Over the past century, the lacustrine sediment of Lake Majeur has been essentially composed of algae, drastically contrasting with the natural sedimentary infill of the basin, mainly resulting from soil erosion from the mid–late Holocene. Since ad 1907, the Lake Majeur has been used for hydroelectricity production. Human-induced lake-level regulations, affecting up to 37% of the lacustrine surface, have increased by fourfold the accumulation rate of the lake and favoured water enrichment. Rubidium abundance within the lacustrine sediments of the two lakes reflects the mid–late Holocene palaeohydrology. After dam construction in ad 1907, greater quantities of rubidium found in Lake Majeur sedimentary infills indicate drier climatic periods, such as from ad 1975 to ad 1982, during which water reservoirs were particularly in demand. Inversely, before the dam was built, rubidium fluctuations were correlated with wetter conditions and hydrological events were recorded as sandy layers deposited by canyon reactivation, synchronous with European climatic deterioration phases. We notably document that the Mediaeval Climate Anomaly was interrupted by some humid periods dated c. ad 940, ad 1080, ad 1100 and ad 1250. We also date the onset of the ‘Little Ice Age’ c. ad 1360 and identify that this period was wetter after c. ad 1500

The subfossil tree deposits from the Garonne Valley and their implications on Holocene alluvial plain dynamics

International audienceSubfossil tree trunks deposits are common in large rivers, but their status as a source for dating alluvial sequences and palaeoenvironmental studies is still discussed. Particularly their origin and the process(es) of deposition as well as a possible remobilization were pointed as a limit to their use to document river alluvial changes. In this work we report the discovery of the largest subfossil trunks deposits in the Garonne valley. These new data are compared to the previous ones. A set of 17 tree trunks and more than 300 smaller wood fragments were collected. The xylologic study shows the prevalence of Quercus and a single occurrence of Ulmus. These two hardwood species are commonly associated with riparian forest. The 14C dating carried out on seven trunks and a single branch of Quercus on the outermost identified growth rings, indicates age ranging from 8400-8000 cal. BP for the oldest fragment (bough) to 4300-4000 cal. BP for the most recent tree trunk. Radiocarbon ages of the trunks are aggregated into two main periods: 5300-5600 cal. BP (four trunks) and 4300-4000 cal. BP (three trunks). The radiocarbon (charcoal) dating of the top of the alluvial sequence overlaying the trunks gives an age between 1965-1820 and 1570-1810 cal. BP, i.e. between the 2nd and the 5th c. AD. In addition, the discovery of two unpublished subfossil tree trunks deposits in Finhan are reported (six trunks). At the light of these results, we discuss previously proposed models for the Garonne floodplain building

Histoire des forêts du versant nord des Pyrénées au cours des 30000 dernières années

Les recherches playnologiques réalisées dans le versant nord des Pyrénées ainsi que les données anthracologiques disponibles et les études de macrorestes végétaux permettent de décrire les étapes de la mise en place des principales essences forestières au cours des 30000 dernières années. Dans les Pyrénées centrales, avant 30000 BP, l'épicéa était bien développé. Les données anthracologiques permettent de penser qu'il a disparu des Pyrénées vers 18000-17000 BP. Entre 11000 BP et 10000 BP, l'ultime phase de dégradation climatique de la fin du dernier épisode glaciaire, le Dryas récent, a davantage affecté la couverture végétale des Pyrénées méditerranéennes que celle de la moitié nord occidentale. Au Postglaciaire, le sapin se développe précocement dans la partie orientale de la chaîne et beaucoup plus tard dans la partie occidentale. L'homme agit très tôt sur le milieu montagnard, dès 6200 BP dans les Pyrénées méditerranéennes. Son ection s'accentue particulièrement vers 4000 BP, au Moyen Age, puis au 16è-17è et 18 è siècles lors du développement des forges, déterminant d'importantes modifications phytogéographiques, notamment une extension du hêtre aux dépens du sapin

Late-Holocene climatic variability south of the Alps as recorded by lake-level fluctuations at Lake Ledro, Trentino, Italy

International audienceA lake-level record for the late Holocene at Lake Ledro (Trentino, northeastern Italy) is presented. It is based on the sediment and pollen analysis of a 1.75 m high stratigraphic section observed on the southern shore (site Ledro I) and a 3.2 m long sediment core taken from a littoral mire on the southeastern shore (site Ledro II). The chronology is derived from 15 radiocarbon dates and pollen stratigraphy. The late-Holocene composite record established from these two sediment sequences gives evidence of centennial-scale fluctuations with highstands at c. 3400, 2600, 1700, 1200 and 400 cal. BP, in agreement with various palaeohydro-logical records established in central and northern Italy, as well as north of the Alps. In addition, high lake-level conditions at c. 2000 cal. BP may be the equivalent of stronger river discharge observed at the same time in Central Italy's rivers. In agreement with the lake-level record of Accesa (Tuscany), the Ledro record also suggests a relatively complex palaeohydrological pattern for the period around 4000 cal. BP. On a millennial scale, sediment hiatuses observed in the lower part of the Ledro I sediment sequence indicate that, except for a high-stand occurring just after 7500 cal. BP, lower lake levels generally prevailed rather before c. 4000 cal. BP than afterwards. Finally, the lake-level data obtained at Lake Ledro indicate that the relative continuity of settlements in humid areas of northern Italy during the Bronze Age (in contrast to their general abandonment north of the Alps between c. 3450 and 3150 cal. BP), does not reflect different regional patterns of climatic and palaeohy-drological conditions. In contrast, the rise in lake level dated to c. 3400 cal. BP at Ledro appears to coincide with a worldwide climate reversal, observed in both the hemispheres, while palaeoenvironmental and archaeological data collected at Lake Ledro may suggest, as a working hypothesis, a relative emancipation of proto-historic societies from climatic conditions

De la naturalité des zones humides. Origine, artificialisation, restauration des zones humides du Sud-Ouest (France)

International audienc

Les changements dans la végétation à l'Holocène au Lac du Ledro (Trentino, Italy).

Présentation oraleDans le cadre du projet ANR LAMA coordonné par M. Magny et N. Combourieu-Nebout, nous présentons les analyses palynologiques de la carotte profonde prélevée au Lac du Ledro (Trentino, Italie). Ce lac, avec un bassin versant de 48 km2, se trouve au sud des Alpes à 650 m d'altitude. La carotte a été repérée dans un secteur non perturbé par sismique-réflexion. Le modèle d'âge établi à partir des 13 dates 14C valide l'enregistrement pollinique à une résolution temporelle inférieure au siècle (63 ans). Les données palynologiques disponibles couvrent maintenant tout l'Holocène. Elles montrent l'apparition successive du cortège de végétation connu sur le versant sud des Alpes. Au cours de la première moitié de l'Holocène, des changements climatiques brutaux sont enregistrés par la végétation, la géophysique et la chimie élémentaire et correspondent à des fluctuations du 18O dans la calotte de glace groenlandaise (GISP2). Les refroidissements de l'Oscillation « Préboréal » (PBO) et de l'évènement 8200 cal BP sont respectivement marqués par l'arrêt de l'expansion forestière du début de l'Holocène et l'intégration du sapin dans les écosystèmes locaux. Dans la seconde moitié de l'Holocène, deux phases de recul marquées dans le taux de pollen arboréen conjointement aux occurrences continues de pollen issu de l'activité humaine (céréales et taxons indicateurs d'anthropisation) sont observées et confirmées par l'accroissement de l'érosion des sols (susceptibilité magnétique). Elles enregistrent l'installation, en deux étapes successives, d'occupations humaines à l'âge du Bronze moyen puis à l'âge du Fer. Ces deux phases sont séparées par une phase de déprise agricole et de recolonisation forestière qui n'est pas expliquée par une oscillation climatique (GISP2). Des évènements de crue sont révélés par les figures sédimentaires et les fortes valeurs en Zirconium. Le modèle d'âge et les données polliniques sont rediscutés après avoir enlevé ces évènements instantanés de la séquence sédimentaire