Chantal Leroyer

Pollen-based quantitative reconstructions of Holocene regional vegetation cover (plant-functional types and land-cover types) in Europe suitable for climate modelling

We present quantitative reconstructions of regional vegetation cover in north-western Europe, western Europe north of the Alps, and eastern Europe for five time windows in the Holocene [around 6k, 3k, 0.5k, 0.2k, and 0.05k calendar years before present (bp)] at a 1° × 1° spatial scale with the objective of producing vegetation descriptions suitable for climate modelling. The REVEALS model was applied on 636 pollen records from lakes and bogs to reconstruct the past cover of 25 plant taxa grouped into 10 plant-functional types and three land-cover types [evergreen trees, summer-green (deciduous) trees, and open land]. The model corrects for some of the biases in pollen percentages by using pollen productivity estimates and fall speeds of pollen, and by applying simple but robust models of pollen dispersal and deposition. The emerging patterns of tree migration and deforestation between 6k bp and modern time in the REVEALS estimates agree with our general understanding of the vegetation history of Europe based on pollen percentages. However, the degree of anthropogenic deforestation (i.e. cover of cultivated and grazing land) at 3k, 0.5k, and 0.2k bp is significantly higher than deduced from pollen percentages. This is also the case at 6k in some parts of Europe, in particular Britain and Ireland. Furthermore, the relationship between summer-green and evergreen trees, and between individual tree taxa, differs significantly when expressed as pollen percentages or as REVEALS estimates of tree cover. For instance, when Pinus is dominant over Picea as pollen percentages, Picea is dominant over Pinus as REVEALS estimates. These differences play a major role in the reconstruction of European landscapes and for the study of land cover-climate interactions, biodiversity and human resources.

River system evolution and environmental changes during the Lateglacial in the Paris Basin (France).

Abstract Lateglacial sequences from the Paris Basin floodplains provide a general overview of the valleys evolution related to climatic changes from the Weichselian Upper Pleniglacial to the beginning of the Holocene. Comparison between morphostratigraphical and palaeobiological data allows discussion of fluvial dynamics and related palaeoenvironmental conditions. The Upper Pleniglacial/Oldest Dryas transition remains poorly documented. It is represented by sands and silts deposited by a braided channel network. The Bolling begins with a significant incision related to a meandering system which is followed by local organic and minerogenic deposition, contemporaneous with vegetation development. The Older Dryas is evident in several sequences. The Allerod appears more stable and the forest extension is accompanied by a thin soil formation on the banks. The Younger Dryas is characterised by a strong infilling of the valleys by calcareous silts. This last sharp event precedes a second major downcutting phase which takes place in the early Postglacial and is followed by a decline in river flow and sediment supply. This evolution reflects a broad regional pattern of fluvial response which illustrates its sensitivity to climatic change. The study of various morphosedimentary contexts shows their influence on the fluvio-dynamic record and bio-proxies which complicates the detection of minor phases.

Holocene history of the River Seine, Paris, France: bio-chronostratigraphic and geomorphological evidence from the Quai-Branly

Bio-chronostratigraphic observations compiled from Quai-Branly in Paris (France) and their comparison with previous studies in the Paris Basin allow documentation of the morphodynamic evolution of the River Seine during the middle and late Holocene. This history begins in the Boreal (between 9500 and 8850 cal. BP), with the deposition of tufa, expressing a stabilized river bed. During the second part of the sub-Boreal the water-table was low. At the beginning of the sub-Atlantic (towards 2800/2700 cal. BP), alluvial dynamics increase, as is recorded elsewhere throughout the Paris Basin. At the beginning of the second part of the sub-Atlantic (around 2000 cal. BP), flood dynamics persisted at a lower intensity. Human occupation occurred from the fifth century AD. Before or around the beginning of the seventeenth century AD, a natural levee was built, indicating the progressive attachment of the Quai-Branly area to the floodplain. The hydrodynamic evolution of the River Seine observed at the Quai-Branly site is similar to that recorded in other river valley floors of the Paris Basin. This evolution appears to be in response to global climatic changes rather than to variations induced by human activities.

The expansion of Central and Northern European Neolithic populations was associated with a multi-century warm winter and wetter climate

It is still debated whether climate changes had an impact on the emergence, spread, and disappearance of early production-based (Neolithic) adaptations. To date, and despite the incorporation of various paleoclimatic proxies, there exists no spatial reconstruction of the regional impact of the North Atlantic cooling events on Central–Western European climate and environments during the early Holocene. In order to address these two issues, we estimated seasonal and annual temperature and precipitation from a marine pollen record from Trondheimsfjord (central Norway) along with 68 pollen records distributed across Central–Western Europe for the time period associated with the Linearbandkeramik (LBK) cultural tradition, 7600–6900 yr cal. BP. Two distinct vegetation-derived rapid, <100 years, climate changes, contemporaneous with reduced warm Atlantic water (AW) inflow and winter storminess in the northern North Atlantic, bracket the expansion of the LBK. The geographic expansion of LBK populations appears to coincide with winter warming by ca. 2.5°C on average, and an increase in summer and winter precipitation, while its decline is associated with decreases in winter temperature, by ~1.5°C on average, and summer rainfall. Our results confirm that LBK subsistence practices were well-adapted to wet and relatively warm winters and cool summers, which are favorable to some cultigens, such as einkorn. This is in contrast to the hypothesis that cooler and wetter climatic conditions would induce increased instability of agricultural communities leading to the decline of LBK populations.

Une inhumation sous tumulus de l’Âge du Bronze et une occupation de la fin du Néolithique à Gonesse, La Fosse aux Larrons (Val-d’Oise)

Decouvert fortuitement en 1976 suite a un decapage industriel, un tumulus a couronne de pierres comportant une inhumation centrale a fait l’objet d’une fouille de sauvetage. Situe dans la vallee du Crould au nord de Paris, les diverses etudes environnementales (geoarcheologie, malacologie, palynologie) ont permis de restituer l’evolution du paysage durant le Subboreal et l’insertion des occupations humaines successives. La stratigraphie revele une premiere implantation caracterisee par une couche archeologique partiellement conservee comportant des vestiges ceramiques, lithiques et des restes de faune. Des artefacts de meme nature se retrouvent dans la constitution du tumulus edifie ulterieurement en partie au detriment de cette occupation initiale. L’etude du monument montre en effet un creusement peripherique destine a collecter des sediments pour former un noyau central ceinture d’une couronne de pierres recouverte d’une chape de terre elle-meme surmontee d’un mince niveau organique. Au centre se trouvait un inhume de sexe masculin depourvu de mobilier, objet d’une etude anthropologique detaillee. Les datations 14C, les analyses environnementales et l’examen du mobilier concordent pour situer avec vraisemblance les restes d’habitat au debut du Neolithique final et l’edification du tumulus a la charniere de l’Âge du Bronze ancien et moyen.