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Des études paléoenvironnementales antérieures ont montré que des changements majeurs de la végétation et de l’environnement se sont produits en Afrique centrale à partir de l’Holocène moyen (ex. Maley & Brenac 1998). Plusieurs d’entre elles mettent en évidence une origine humaine et supposent que les grandes migrations de population, les innovations techniques (par exemple, la technologie de la fonte du fer) et/ou de nouveaux choix dans les pratiques agricoles, conduisant à la déforestation et au défrichement, sont les moteurs de ces changements. Cependant, à ce stade, l’absence de reconstitution démographique ne permet pas de soutenir pleinement ces hypothèses. Notre étude utilise une base de données archéologiques géoréférencées pour déduire la dynamique des populations et l’évolution des pratiques culturelles en Afrique centrale occidentale au cours des 5000 dernières années. Cette base de données comprend 1139 dates calibrées au 14C provenant de 425 sites – localisés dans le sud du Cameroun, au Gabon, en République du Congo, en Guinée équatoriale et dans la partie occidentale de la République démocratique du Congo –, remontant à un maximum de 5000 ans cal. BP. La modélisation des données indique une possible croissance de la population entre ~2500 et ~1500 ans cal. BP, coïncidant avec l’apparition à l’échelle régionale de techniques et de pratiques spécifiques. L’augmentation concomitante des fosses dépotoirs, des vestiges d’utilisation de palmier à huile Elaeis guineesis, l’apparition de rares restes de millet Pennisetum glaucum et la montée en puissance des vestiges de métallurgie du fer ont eu lieu pendant la seconde moitié du Néolithique, à partir d’environ 2800 ans cal. BP. Dans les régions côtières, la croissance de la population concerne le Néolithique et le début de l’âge du fer (2500-2000 ans cal. BP et 2000-1500 ans cal. BP), tandis que dans l’Hinterland cette croissance semble légèrement plus tardive (2400 et 1300 ans cal. BP). Il n’est pas possible d’identifier un phénomène commun de diffusion à partir d’un seul centre. Les innovations techniques et les nouvelles pratiques semblent plutôt s’être répandues à travers un large réseau d’interactions culturelles qui a favorisé la formation des sociétés d’Afrique centrale occidentale au cours du troisième millénaire avant notre ère. Palaeocological studies show that major vegetation and environmental changes occurred in Central Africa from the mid-Holocene (e.g. Maley & Brenac 1998). Several suggest a human origin and assume that large population migration, technical innovations (e.g. iron-smelting technology) and/or change in agricultural practice, leading to deforestation and land clearance, are the drivers of these changes. However, at this stage, the lack of demographic reconstruction does not fully support these hypotheses. Here, a georeferenced archaeological database is used to infer population dynamics and the evolution of cultural practices in Western Central Africa over the last 5000 years. This database includes 1139 14C calibrated dates from 425 sites throughout southern Cameroon, Gabon, the Republic of Congo, Equatorial Guinea and the western part of the Democratic Republic of Congo, dating back a maximum of 5000 cal. yr BP. Data modelling indicate possible population growth from 2500 to 1500 cal. yr BP, coinciding with the occurrence at a regional scale of specific techniques and practices. The concomitant increase of refuse pits, palm oil Elaeis guineesis and iron metallurgy (plus rare remains of millet Pennisetum glaucum) took place during the second half of the Neolithic, beginning around 2800 cal. yr BP. In the coastal regions, the population growth concerns the Neolithic and the Early Iron Age (2500–2000 cal. yr BP and 2000–1500 cal. yr BP), while in the Hinterland population growth seems slightly later (2400 and 1300 cal. yr BP). It is not possible to identify a common diffusion phenomenon from a single homeland. Rather, technical innovations and new practices appear to have spread through a wide network of cultural interactions, which fostered the formation of Western Central African societies during the third millennium.

International audience; Abstract. Our understanding of climate and vegetation changes throughout the Holocene is hampered by representativeness in sedimentary archives. Potential biases such as production and preservation of the markers are identified by comparing these proxies with modern environments. It is important to conduct multi-proxy studies and robust calibrations on each terrestrial biome. These calibrations use large databases dominated by forest samples. Therefore, including data from steppe and desert–steppe sites becomes necessary to better calibrate arid environments. The Mongolian Plateau, ranging from the Baikal area to the Gobi desert, is especially characterized by low annual precipitation and continental annual air temperature. The characterization of the climate system of this area is crucial for the understanding of Holocene monsoon oscillations. This study focuses on the calibration of proxy–climate relationships for pollen and glycerol dialkyl glycerol tetraethers (GDGTs) by comparing large Eurasian calibrations with a set of 49 new surface samples (moss polster, soil and mud from temporary dry ponds). These calibrations are then cross-validated by an independent dataset of top-core samples and applied to four Late Holocene paleosequences (two brGDGT and two pollen records) surrounding the Mongolian Plateau: in the Altai mountains, the Baikal area and the Qaidam basin, to test the accuracy of local and global calibrations. We show that (1) preserved pollen assemblages are clearly imprinted on the extremities of the ecosystem range but mitigated and unclear on the ecotones; (2) for both proxies, inferred relationships depend on the geographical range covered by the calibration database as well as on the nature of samples; (3) even if local calibrations suffer from reduced amplitude of climatic parameters due to local homogeneity, they better reflect actual climate than the global ones by reducing the limits for saturation impact; (4) a bias in climatic reconstructions is induced by the over-parameterization of the models by the addition of artificial correlation; and (5) paleoclimate values reconstructed here are consistent with Mongolia–China Late Holocene climate trends and validate the application of local calibrations for both pollen and GDGTs (closest fit to actual values and realistic paleoclimate amplitude). We encourage the application of this surface calibration method to reconstruct paleoclimate and especially consolidate our understanding of the Holocene climate and environment variations in arid central Asia.

International audience; Two commonly used proxies based on the distribution of glycerol dialkyl glycerol tetraethers (GDGTs) are the TEX86 (TetraEther indeX of 86 carbon atoms) paleothermometer for sea surface temperature reconstructions and the BIT (Branched Isoprenoid Tetraether) index for reconstructing soil organic matter input to the ocean. An initial round-robin study of two sediment extracts, in which 15 laboratories participated, showed relatively consistent TEX86 values (reproducibility +/- 3-4 degrees C when translated to temperature) but a large spread in BIT measurements (reproducibility +/- 0.41 on a scale of 0-1). Here we report results of a second round-robin study with 35 laboratories in which three sediments, one sediment extract, and two mixtures of pure, isolated GDGTs were analyzed. The results for TEX86 and BIT index showed improvement compared to the previous round-robin study. The reproducibility, indicating interlaboratory variation, of TEX86 values ranged from 1.3 to 3.0 degrees C when translated to temperature. These results are similar to those of other temperature proxies used in paleoceanography. Comparison of the results obtained from one of the three sediments showed that TEX86 and BIT indices are not significantly affected by interlaboratory differences in sediment extraction techniques. BIT values of the sediments and extracts were at the extremes of the index with values close to 0 or 1, and showed good reproducibility (ranging from 0.013 to 0.042). However, the measured BIT values for the two GDGT mixtures, with known molar ratios of crenarchaeol and branched GDGTs, had intermediate BIT values and showed poor reproducibility and a large overestimation of the true (i.e., molar-based) BIT index. The latter is likely due to, among other factors, the higher mass spectrometric response of branched GDGTs compared to crenarchaeol, which also varies among mass spectrometers. Correction for this different mass spectrometric response showed a considerable improvement in the reproducibility of BIT index measurements among laboratories, as well as a substantially improved estimation of molar-based BIT values. This suggests that standard mixtures should be used in order to obtain consistent, and molar-based, BIT values.

Climatic models simulate abrupt oscillations that are associated, in the North Atlantic, with Dansgaard-Oeschger and Heinrich events. However, the geographic extension of temperature anomalies is largely uncontrolled due to the scarcity of quantitative records of sufficient time resolution on the European continent. Here, we propose, based on a recently developed temperature proxy (TEX86), a reconstruction of millennial-scale temperature variations in a Black Sea sediment archive for the last 40,000 years. Prior to any paleoclimatological interpretations the effects of potential bias, such as seasonality and depth of maximum export production on temperature reconstructions, are considered for the Black Sea. Based on previous work, a tentative method for temperature corrections, taking into account varying terrigenous inputs, is further proposed. Reconstructed temperatures for Black Sea core MD042790 were remarkably stable during the last glacial. However, significant shifts toward lower temperatures of 2°C occurred during Heinrich events 2 and 3. The deglaciation displayed a temperature increase of 10°C consistent with neighboring European reconstructions. A Younger Dryas cooling of approximately 5–6°C was clearly expressed in the reconstruction. In notable contrast to observations from nearby archives, Heinrich events imprinted our glacial temperature record consistent with a strong reorganization of oceanic circulation and a large spreading of the temperature anomaly from the North Atlantic toward the southeast. Furthermore, in contrast to high-latitude records, our Black Sea record lacks the signatures of Dansgaard-Oeschger interstadials, suggesting a decreasing temperature gradient away from the North Atlantic.

The fourth International Polar Year (IPY) officially came to an end on 14 and 15 May 2009 with a symposium organized jointly by the Collège de France and OPECST (the French parliamentary committee for the evaluation of scientific and technological choices). Many scientists and political actors, invited by Professor Édouard Bard and Senator Christian Gaudin, emphasized the success of the IPY, including the creation of at least 200 international multidisciplinary research projects, entirely dev...