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National audience; Les forages IODP 302, réalisés au centre de l’Océan Arctique près de la ride deLomonosov, bénéficient d’un cadre bio-chrono-stratigraphique de la fin du Paléocène àl’Eocène moyen- supérieur (Backman et al., 2006). Malgré la multiplicité des analyses(sédimentologie, géochimie, micropaléontologie) menées sur ces archivessédimentaires, une seule étude concerne le pollen, concentrée sur la base du forageM0004 (PETM – ETM2) et proposant des valeurs paléoclimatiques biaisées par lafaiblesse des identifications botaniques des grains de pollen (Willard et al., 2019).Dans notre étude soutenue financièrement par IODP-France, nous avons analysé lecontenu pollinique des forages M0004 et M0002. L’abondance des pollens, leurexcellent état de préservation ainsi que la diversité végétale élevée (plus de 100 taxonsidentifiés) témoignent du voisinage très proche de terres émergées de la margesibérienne. L’identification botanique rigoureuse du pollen permet des reconstitutionsrobustes de la végétation et du paléoclimat. L’environnement était forestier, dominéalternativement par Glyptostrobus ou Castanopsis-Lithocarpus. Le diagrammepollinique est directement corrélable avec la courbe de δ18O de référence (Cramer et al.,2009). Les maxima thermiques sont exprimés par de forts pourcentages de plantessubtropicales avec la présence parfois importante de plantes tropicales. L’élément demangrove, Avicennia (que nous avions signalé dans l’Eocène inférieur de l’îlesibérienne de Faddeevsky et du Delta de Mackenzie, Suan et al., 2017 ; Salpin et al.,2019), signe chaque maximum thermique, du PETM au MECO, avec des valeurs de latempérature moyenne annuelle élevées, comprises entre 18 et 22°C, des températuresdu mois le plus chaud pouvant atteindre une moyenne de 27°C et du mois le plus froidne descendant pas au-dessous de 10°C. Les précipitations annuelles allaient de 1200 à1400 mm. Cette étude révèle en outre l’impact qu’une longue période très chaude peutavoir sur les plus hautes latitudes du globe et sur l’expansion de la végétationthermophile.

International audience

New pollen data from a core at Lagoa do Caçó, Maranhão state, Brazil (2°58′S 43°25′W; 120 m elevation), show higher frequencies of Podocarpus at the end of the Pleistocene than today. The increase in Podocarpus, which follows the successive increase of various pioneer species such as Didymopanax, Melastomataceae/Combretaceae, and Cecropia, implies a progressive late-glacial increase of moist and cool climatic conditions. A comparable increase in Podocarpus is found in other lowland records in Amazonia. A review of published pollen data from Amazonia suggests that the moisture source was from the southeast. By contrast, present-day moisture comes from the tropical Atlantic and from the Amazon basin, with its convective precipitation. The likely cause for the southeastern moisture source between ca. 15,000 and 14,500 cal yr B.P. was enhanced polar (Antarctic) advection that reached low latitudes and maintained year-round the meteorological equator in its austral-winter position at northern latitudes or reduced drastically its southward summer displacement. This hypothesis is consistent with marine and ice core records.

Climatic changes during the last climatic cycle have been studied using three sediment cores from the Konya plain, a now dry, closed and semi-arid lacustrine basin at 1000 m altitude in central Anatolia, Turkey. The reconstruction of regional climatic characteristics and evolution is based on mineral, diatom, pollen and molluscan content of sediments. Correlations are made between cores using 14C and U-Th ages, stratigraphy and drought levels shown by changes in mineral contents. Environmental responses to local and regional climatic changes are traced by trends in authigenic carbonates, evaporites, detrital mineral content and by diatom-inferred salinity levels. Our data have shown that, during the period covering the end of the previous Glacial (Termination II) and the last Interglacial (between c. 150 and 117 ka), peat and freshwater shallow lakes expanded. From 117 to 66 ka, the plain was occupied by lakes, the salinity and existence of which varied in time and space. Specific events are marked by mineralogic and stratigraphic signals at (i) c. 101 ka and (ii) 66 ka. From 66 ka to 30 ka, desiccation of the lake is marked by a hiatus. At c. 27 ka, milder climatic conditions led to the extension of freshwater marshes and lakes in the central depressions while palaeosols developed on the emerged parts of the plain. From 25 to 20·5 ka 14C cal., the sediments of a freshwater to brackish lake are present in one core only. From 20·5 ka 14C cal. onwards, strong evaporitic conditions occurred, the lake edges being transformed into playas. Upper parts of the sequences registered other lacustrine short phases, both before the Younger Dryas and during the Holocene. Copyright © 1999 John Wiley & Sons, Ltd.

Pollen analysis of two cores from the Lake Sinnda, located in one of the driest areas of the southern Congo, reveals a history of vegetation and climate in this region during the past 5000 yr. A major change centered around 3000-2500 yr B.P. is indicated by an abrupt decrease in forest pollen and by a corresponding increase in grassland pollen. Concurrent drying up of the lake shows that climate, in particular aridity, was the major cause of this change. This paleoclimatic reconstruction conforms with evidence for drier conditions in other parts of western equatorial Africa, such as the development of isolated enclosed savannas and of heliophilous forested formations. The aridity is recorded more fully at Lake Sinnda than at the previous studied ones. It probably lasted longer, from 4200 to 1300 yr B.P., and was more progressive than previously inferred. The aridity predates agriculture marked by pollen of the oil palm at Lake Sinnda. (Résumé d'auteur)

Environmental conditions of the lowland tropical forests during the last glacial maximum (LGM) between ca 20,000 and 18,000 14C yr B.P., are reevaluated in terms of dating control and lithology analyzed in seven pollen records from South America. The reevaluation shows that probably in none of the published records are LGM sediments present or abundant. This conclusion is based on the occurrence of abrupt lithologic changes coupled with changes in sedimentation rate interpolated from radiocarbon dates. These findings suggest that the LGM was represented probably by a hiatus of several thousand years, indicative of drier climates than before or after.

Located at the transition between the Saharan and Sahelian zones, at the center of one of the largest endorheic basins, Lake Chad is ideally located to record regional environmental changes that occurred in the past. However, until now, no Holocene archive was directly cored in this lake. In this paper, we present pollen data from the first sedimentary sequence collected in Lake Chad (13° N; 14° E; Sahel region). Dated between ca. 6700 and ca. 5000 cal yr BP, this record is continuous and encompasses part of the termination of the African Humid Period (AHP). Vegetation reconstructions are based on standard analyses of pollen diagrams and are strengthened by quantitative approaches. Potential biomes are reconstructed using the biomization method and mean annual precipitation (Pann) is estimated using the modern analogues technique. Results show that, between ca. 6700 and ca. 6050 cal yr BP, a vegetation close to humid woodland or humid savanna, including elements currently found further southward, thrived in the vicinity of the Mega-Lake Chad in place of the modern dry woodland, steppe and desert vegetation. At the same time, montane forest populations extended further southward on the Adamawa Plateau. The high abundance of lowland humid pollen taxa, particularly of Uapaca, is interpreted as the result of a northward migration of the corresponding plants during the AHP. This preferential zonal occurrence of these taxa in Lake Chad Basin (LCB) (rather than extrazonal) is driven by more humid local and regional climate conditions at this latitude, as shown by mean Pann estimated values of ca. 800 (−400/+700) mm during this period. However, we cannot rule out that an increase of the Chari–Logone inputs into the Mega-Lake Chad might have also contributed to control the abundance of these taxa. Changes in the structure and floristic composition of the vegetation towards more open and drier formations occurred after ca. 6050 cal yr BP, following a decrease in mean Pann estimates to approximately 600 (−230/+600) mm. But, the constant significant presence of lowland humid taxa until ca. 5000 cal yr BP, contemporaneous with a slight increase in steppic taxa, demonstrates that at this date, the modern vegetation was not yet established in the vicinity of Lake Chad. Our data indicate that between ca. 6700 and ca. 5000 cal yr BP vegetation and climate changes must have occurred progressively, but that century-scale climate variability was superimposed on this long-term mid-Holocene drying trend as observed around ca. 6300 cal yr BP, where pollen data indicate more humid conditions.

A detailed palynological analysis of two cores recovered from a swamp in the Southern Bateke Plateaux (Congo), provides information on the botanical history of this region during the last 24,000 yr B.P. Rich and well-diversified pollen counts exhibit changes in the vegetational communities related to hydrological and climatic fluctuations. Around 24,000 yr B.P., the occurrence of hydromorphous forest elements gives evidence of a humid phase. Between 24,000 and around 13,000 yr B.P., swampy herbaceous communities expanded, indicating slightly drier climatic conditions. From approximately 13,000 yr B.P. onward, the beginning of a humid episode is reflected by the development of mesophilous forests. The decrease of these forests is documented since 3000 yr B.P. Grasslands are found to extend locally whereas Elaies guineensis (oil palm) spreads out, indicating both climatic change but also anthropogenic activities.

We investigated past climate variability and the zonal short and long-range transport of air masses in tropical South America using chemical, isotopic and palynological signals from a 42 m-long ice core recovered in 2003 from the saddle of the Nevado Coropuna, southern Peru (72°39´ W; 15°32´ S; 6080 m a.s.l.). We found that precipitation at this site depends mainly on the easterly circulation of air masses originated from the tropical Atlantic Ocean. Nevertheless, sporadic Pacific air masses arrivals, and strong cold waves coming from southern South America reach this altitude site. In spite of post-depositional effects, we were able to identify two strong ENSO (El Niño-Southern Oscillation) event signatures (1982–1983 and 1992) and the eruptive activity of the nearby Sabancaya volcano (1994).

For decades, palynologists working in tropical South America are using the genus Podocarpus as a climate indicator although without referring to any modern data concerning its distribution and limiting factors. With the aim to characterize the modern and past distribution of the southern conifer Podocarpus in Brazil and to obtain new information on the distribution of the Atlantic rainforest during the Quaternary, we examined herbarium data to locate the populations of three Brazilian endemic Podocarpus species: P. sellowii, P. lambertii, and P. brasiliensis, and extracted DNA from fresh leaves from 26 populations. Our conclusions are drawn in the light of the combination of these three disciplines: botany, palynology, and genetics. We find that the modern distribution of endemic Podocarpus populations shows that they are widely dispersed in eastern Brazil, from north to south and reveals that the expansion of Podocarpus recorded in single Amazonian pollen records may have come from either western or eastern populations. Genetic analysis enabled us to delimit regional expansion: between 5 degrees and 15 degrees S grouping northern and central populations of P.sellowii expanded c. 16,000 years ago; between 15 degrees and 23 degrees S populations of either P. lambertii or sellowii expanded at different times since at least the last glaciation; and between 23 degrees and 30 degrees S, P. lambertii appeared during the recent expansion of the Araucaria forest. The combination of botany, pollen, and molecular analysis proved to be a rapid tool for inferring distribution borders for sparse populations and their regional evolution within tropical ecosystems. Today the refugia of rainforest communities we identified are crucial hotspots to allow the Atlantic forest to survive under unfavourable climatic conditions and, as such, offer the only possible opportunity for this type of forest to expand in the event of a future climate change.

Suite aux résultats de l’étude préliminaire réalisée sur l’affleurement de travertin d’Irdi (Errachidia) (Boudad et al., 2003), un carottage a été fait en novembre 2005 sur ce site afin de déterminer l’épaisseur des formations travertineuses. L’étude a été ensuite étendue à d’autres affleurements travertineux marocains en ayant recours à des méthodes d’analyse complémentaires (palynologie, paléomagnétisme) pour identifier les différentes phases de concrétionnement et pour mieux cerner le cadre paléoclimatique de ces formations. Des encroûtements travertineux actuels qui se forment aujourd’hui à Irdi dans l’oued Ziz ont été analysés afin demieux appréhender les processus physico-chimiques qui président à la cristallisation de ces structures ; en particulier, la mesure du rapport d’activité 234U/238U montre que l’eau responsable de la formation des travertins fossiles avait la même origine que celle d’aujourd’hui. Plusieurs phases de concrétionnement, qui se répartissent du Pléistocène supérieur au Pléistocène moyen, ont été identifiées. Ces phases devront être intégrées dans l’évolution climatique du Maroc et corrélées aux stades glaciaires et interglaciaires déterminés en milieu marin. After a preliminary study of the travertine outcrop of Irdi (Errachidia province), this site was drilled and the survey was extended to other exposures in Morocco. Complementary methods of investigation were used (palynology, paleomagnetism) together with U-series analyses in order to identify the successive deposition phases and to precise the associated paleoclimatic framework. For a better understanding of the travertine formation, we have studied the travertine formations which are presently forming at Irdi. The respective 234U/238U activity ratios of the present water and fossil travertines are similar, i.e. physical and chemical conditions of travertinization were more or less similar today as in the past. Several crystallization phases were identified from Upper Pleistocene to Middle Pleistocene. These phases must be integrated into the climatic evolution of Morocco and correlated to glacial and interglacial stages defined in marine environment.

An unusual molecular fossil (onocerane I) has been detected in Quaternary lacustrine sediments (Lagoa do Caco, NE Brazil) for the first time. This component was initially thought to attest to the former presence of ferns or club mosses. From consideration of possible plant precursors of onocerane-related molecules recorded in the literature and comparison with palynological results and palaeoclimatic data, we here provide evidence that club mosses and ferns cannot account as sources of onocerane I in this setting. Onocerane I is abundant in the lipid extracts of sediments deposited during the two driest periods recorded in Northern Brazil (Last Glacial Maximum and Younger Dryas). This component is therefore suspected to be diagnostic of the development of plants adapted to dry or semi-arid conditions in this region.

L'histoire de la vegetation et des paleoenvironnements de l'Afrique centrale a beaucoup progresse depuis une quinzaine d'annees grâce en particulier a l'etude pollinique de sequences sedimentaires, essentiellement lacustres, prelevees dans une dizaine de sites (ces sites sont positionnes sur la figure 1). Seuls deux sites ont fourni des enregistrements qui remontent jusque vers 25128 000 ans (C/l4) BP: le lac Ngamakala, situe au cœur des savanes des Plateaux Bateke, vers le sud du Congo, et le lac Barombi Mbo dans les forets de l'Ouest Cameroun. Toutefois, de par sa position, seul le second site permet veritablement une reconstitution du milieu forestier. Les huit autres sites ont fourni des enregistrements qui couvrent des parties plus ou moins longues de l'Holocene, les plus anciens ne remontant pas au-dela de 7000 BP.

Analysis of diatom and pollen contents on organic sediments from a swamp forest on the coastal plain of Sâo Paulo State (Southeastern Brazil) allowed palaeoenvironmental reconstruction of the region during the Late Holocene. From at least 4900 to 3470 cal yr BP the site was a lagoon surrounded by a relatively open forest. At ca. 3470 cal yr BP, after the retreat of the sea, a swamp forest occupied the area; it remains until the present day. The forest was locally altered between ca. 1300 and 675 cal yr BP when, due to a rise in groundwater, a fresh water lake was formed. The climate was similar to the present during the whole Late Holocene, except for three slightly more humid episodes: 3470-2750, around 1950, and 1300-675 cal yr BP. Our results show that sea level was higher than the present zero level from 4900 to 3470 cal yr BP, contradicting a widely accepted relative sea level trend that suggests that an important oscillation brought the sea level down below the present zero between ca 4560 and 4150 cal yr BP. Conversely, these results tend to confirm more recent studies that propose a continuously declining relative sea level trend. In this region there are numerous "sambaquis", shell mounds built by fisher-gatherer populations that lived near the seaside. Some of them are 50 km away from the coast. They are certainly linked to the presence of a lagoon. Despite an important human occupation, no anthropogenic impact on vegetation could be recorded during the Late Holocene. Comparison of our data with those from 21 sites situated in southeastern, southern and central Brazil shows that the more humid episodes recorded at Cananéia-Iguape might be correlated with El Nino-like events. Trends in climatic evolution dunng the Late Holocene are opposite at north and south of 20° S latitude., L'analyse palynologique et diatomologique des sédiments organiques d'une forêt marécageuse de la plaine côtière de l'État de Sâo Paulo, dans le sud-est du Brésil, a permis de reconstituer l'évolution du milieu régional au cours de la deuxième moitié de l'Holocène. Depuis au moins 4900 jusqu'à 3470 ans cal BP, la région d'étude était occupée par une lagune entourée par une forêt relativement claire. Vers 3470 ans cal BP, la mer s'est retirée, une forêt marécageuse s'est installée et s'est maintenue jusqu'à l'époque actuelle. Elle a été localement altérée entre environ 1300 et 675 ans cal BP, suite à la remontée de la nappe phréatique et à la formation d'un lac d'eau douce. Le climat est resté semblable au climat actuel pendant tout l'Holocène supérieur, à l'exception de trois épisodes légèrement plus humides vers 3470-2750, 1950 et 1300-675 ans cal BP. Les diatomées rencontrées entre 4900 et 3470 ans cal BP appartiennent au milieu lagunaire ou marin, ce qui signifie que le niveau de la mer est resté, en permanence, au moins 2 m au-dessus du niveau actuel pendant toute cette période. Ceci est en contradiction avec l'idée, largement diffusée, selon laquelle une importante oscillation négative aurait amené le niveau de la mer au-dessous du zéro actuel entre 4560 et 4150 ans cal BP. Par contre, ces résultats tendent à confirmer les études plus récentes qui proposent que le niveau a baissé de façon régulière depuis 5800 ans cal BP. La région d'étude renferme de nombreux "sambaquis", des amas coquilhers construits par des populations de Pêcheurs- Cueilleurs qui habitaient à proximité de la mer. Certains de ces sites sont localisés à plus de 50 km du littoral actuel. Cet éloignement s'explique par la présence d'une lagune, source importante de nourriture, jusqu'à 3470 ans cal BP. Malgré une importante présence humaine, aucune altération de la végétation n'a pu être attribuée à une action anthropique. La comparaison de nos données avec celles de 21 sites brésiliens répartis entre 15° et 32° de latitude sud, ont montré que les épisodes d'augmentation de l'humidité enregistrés à Cananéia-Iguape se retrouvent dans les sites localisés au sud du parallèle 20° S. À l'inverse, pour les sites localisés au nord de cette latitude, les mêmes périodes ont enregistré des épisodes légèrement plus secs. Cette inversion des tendances climatiques entre ces deux ensembles régionaux est attribuée à des événements de type "El Nino"., Ybert Jean Pierre. Évolution de l'environnement dans la zone littorale sud et sud-est du Brésil à l'Holocène supérieur ; variation du climat et du niveau relatif de la mer ; leur influence sur l'occupation humaine. In: Quaternaire, vol. 13, n°3-4, 2002. Événements rapides, instabilités, changements culturels au Quaternaire. pp. 237-245.

Analysis of organic sediments from a swamp forest on the coastal plain of São Paulo State (southeastern Brazil) has been carried out. Diatom and pollen contents from the same samples were analysed. This study allowed reconstruction of the regional paleoenvironment during the Late Holocene. From at least 4400 to 3250 yr BP the site was a lagoon surrounded by a relatively open forest. At ca. 3250 yr BP, after the retreat of the sea, a swamp forest occupied the area. This swamp forest remains until the present day. It was locally altered during a short period between ca. 1400 and 775 yr BP when, due to a rise in groundwater, a fresh water lake or marsh was formed. The climate was similar to the present during the whole Late Holocene, except for three slightly more humid episodes: 3250-2600, around 2000, and 1400-775 yr BP. No human impact on the vegetation was recorded during the Late Holocene. Our results show that sea level was higher than the present zero level from 4400 to 3250 yr BP, contradicting a widely accepted relative sea level trend that suggests that at least two important negative sea level oscillations occurred during the Holocene (4100-3800 and 3000-2700 yr BP). Conversely, these results tend to confirm more recent studies that propose a continuously declining relative sea level trend. Comparison of our data with those from 24 sites situated in southeastern, southern and central Brazil show that the more humid episodes recorded at Cananéia-Iguape might be correlated with El Niño like events. Trends in climatic evolution during the Late Holocene are opposite at north and south of 20ºS latitude.

Two cores (CSS 2 and CSS 10) were collected from two lakes situated in the Carajas area (6°35′S, 49°30′W) that show different stages of silting up. The results of geochemical and petrographic organic analyses, backed up by 14 C radiometric dating, lead to a reconstruction of the variations in the hydrological regime of these lakes over the last 30,000 years. Evidence is revealed for a period of drying up, characterized by a sedimentary hiatus between 22,000 and 13,000 14 C years BP, followed by a period of refilling between 13,000 and 11,000 14 C years BP. (15,400–12,900 cal years BP). The latter interval is primarily characterized by a mainly clastic sedimentary input rich in quartz, kaolinite and iron hydroxides, leading on to the authigenic formation of siderite. This type of sedimentation implies the erosion of the small catchment basin during periods of transition between arid and humid climatic regimes. From 8000 to 4000 14 C years BP (8900–4500 cal years BP), as well as from 2700 to 1500 14 C years BP (2780–1360 cal years BP), special hydrological conditions prevailed. They are characterized in the sedimentary record by the presence of micro-laminae containing wood charcoal interlayered with sponge spicule material. This interlamination reflects the rapid alternation of dry and humid periods. A comparison with the results obtained by pollen analysis shows that sedimentological indicators, once filtered for strictly local effects, can provide a full record of the regional-scale variations in the environment.

Sediment cores and outcrops from the Bolivian Altiplano are used to interpret late Pleistocene/Holocene paleoclimates and lake evolution based on a multi-proxy approach (ostracod content, palynology, sedimentology, and radiocarbon dating). Despite the different sensitivity of the lacustrine basins to environmental changes, interpreting the records in terms of paleohydrology, climate, and more especially timing of events is difficult. Notwithstanding these problems, Lake Titicaca in the north and Lake Pocoyu (Lake Poopo, salars of Coipasa and Uyuni) in the south reveal a similar evolution in general trends through the course of time. During the Minchin phase (until 30-26 kyr BP), the altiplano was wetter than present. The Last Glacial Maximum (ca. 26-14 kyr BP), marked by cooler, drier conditions, was followed by a return to a wetter climate, interrupted by short arid events, between ca. 14 and 10.5 kyr BP. Another cycle of aridity, between 10.5 and 8 kyr BP, took place abruptly just after the humid Tauca phase. The mid-Holocene is interpreted as climatically unstable, with an alternation of humid and dry episodes. Since 3.9 kyr BP, wetter conditions have persisted and intensified to the present. The data are interpreted in terms of changes in inter-tropical convergence zone extension (summer precipitation) and in polar air mass intensity (winter precipitation). (Résumé d'auteur)

International audience; Climatic changes during the last climatic cycle have been studied using three sediment cores from the Konya plain, a now dry, closed and semi-arid lacustrine basin at 1000 m altitude in central Anatolia, Turkey. The reconstruction of regional climatic characteristics and evolution is based on mineral, diatom, pollen and molluscan content of sediments. Correlations are made between cores using 14C and U-Th ages, stratigraphy and drought levels shown by changes in mineral contents. Environmental responses to local and regional climatic changes are traced by trends in authigenic carbonates, evaporites, detrital mineral content and by diatom-inferred salinity levels. Our data have shown that, during the period covering the end of the previous Glacial (Termination II)and the last Interglacial (between c. 150 and 117 ka), peat and freshwater shallow lakes expanded. From 117 to 66 ka, the plain was occupied by lakes, the salinity and existence of which varied in time and space. Specific events are marked by mineralogic and stratigraphic signals at (i) c. 101 ka and (ii) 66 ka. From 66 ka to 30 ka, desiccation of the lake is marked by a hiatus. At c. 27 ka, milder climtic conditions led to the extension of freshwater marshes and lakes in the central depressions while palaeosols developed on the emerged parts of the plain. From 25 to 20.5 ka 14C cal., the sediments of a freshwater to brackish lake are present in one core only. From 20.5 14C cal. onwards, strong evaporitic conditions occurred, the lake edges being transformed into playas. Upper parts of the sequences registered other lacustrine short phases, both before the Younger Dryas and during the Holocene. (Résumé d'auteur)

Les résultats polliniques majeurs obtenus dans le sud Cameroun, particulièrement au lac Barombi Mbo, permettent de retracer l'histoire de la végétation au cours du Quaternaire récent. Après la dernière grande phase aride, de 20 a 10 000 ans BP, la forêt s'est complètement réinstallée dans la région peu après le début de l'Holocène. Ensuite, l'évènement est survenu vers 3 000 BP avec une ouverture et un recul brutal du massif forestier au profit des savanes. Cette péjoration climatique a atteint son maximum entre 2500 et 2000 BP et s'est étendue à toute l'Afrique tropicale montrant qu'il s'agit d'un changement de grande ampleur. Ultérieurement, à partir d'environ 2000 BP, la forêt a commencé à coloniser à nouveau les aires perdues. Cette transgression forestière se poursuit encore au XXe siècle. Les données polliniques montrent aussi que le palmier à huile (Elaeis guineensis) a présenté une forte extension entre 3000 et 2000 BP, en association avec d'autres arbres pionniers. (Résumé d'auteur)

Le lac Njupi, au sein de la ligne volcanique du Cameroun, s'est constitué par barrage d'une dépression à 1 km à l'est du lac Nyos. Les deux sondages du lac Nyos analysés ont traversé les sédiments déposés au marges du lac. Les dépôts les plus anciens sont datés à 3400 BP, cet âge est proposé comme un minimum pour l'apparition du lac Njupi. Des études sédimentologiques, palynologiques et géochimiques d'une coupe de 2 m permettent de reconstituer l'histoire fini-holocène de cet environnement. Il s'agit d'un dépôt tourbeux (plus de 30% de carbone organique) avec une importante fraction de spicules silicieux de Spongiaires. La sédimentation a été relativement régulière, entrecoupée seulement durant les 800 ans du dépôt, de cinq lamines plus grossières caractérisées par des grains de quartz et de gros débris ligneux. Ces flux sont soulignés par des concentrations de carbohydrates ou parfois par des composés phénoliques bien conservés. Cette étude confirme et précise l'existence d'une phase aride qui s'intensifie ici entre 2500 et 2000 ans BP. Cette crise climatique qui a débuté vers 3000 ans BP est, en fait, très générale dans l'Afrique centrale Atlantique (sud Cameroun). Le lac Njupi, situé aujourd'hui dans un paysage essentiellement composé de savanes d'où sa dénomination de "Grass Fields", permet d'observer avant 2500 ans BP un paysage mixte de forêts et savane en mosaïque, puis son évolution vers une savane avec quelques îlots forestiers, couvert défini alors par 75% à 85% de pollens herbacés. La végétation montagnarde, caractérisée par #Podocarpus$ et #Olea capensis$, régresse vers 2300 ans BP, au moment où #Elaeis guineensis$ (le Palmier à Huile) initie son expansion, d'abord comme un arbre pionnier, offrant ainsi ultérieurement à l'Homme l'opportunité de sa culture. (Résumé d'auteur)

*INRA, Centre de Versailles (FRA) Diffusion du document : INRA, Centre de Versailles (FRA); National audience

L'analyse palynologique détaillée d'une carotte prélevée dans le lac Ossa, situé au coeur de la forêt littorale atlantique du SW Cameroun, apporte de nouvelles informations sur l'histoire botanique et climatique de cette région depuis 4 770 ans BP. La microflore pollinique inventoriée, riche et très diversifiée, indique la permanence de l'environnement forestier durant toute cette période. De 4 770 à 2 730 BP, les forêts Biafréenne et afromontagnarde sont bien représentées dans les spectres polliniques. Puis vers 2 730 BP (âge estimé), une modification majeure du paysage apparaît. La forêt de type primaire est remplacée par une forêt perturbée riche en arbres pionniers (#Alchornea$, #Macaranga$). Ultérieurement on note l'extension du palmier à huile (#Elaeis guineensis$) qui culmine vers 1 600 BP. (Résumé d'auteur)

L'analyse palynologique d'une carotte prélevée dans le lac Kitina, sur la bordure occidentale du Mayombe congolais, apporte des informations nouvelles sur l'évolution, depuis 5 400 ans BP, des conditions paléoclimatiques et sur les associations végétales présentes autour du lac. Si une couverture arborée a toujours été présente, des modifications importantes liées à un assèchement climatique sont enregistrées ca 2 500 BP : apparition de faciès forestiers plus mésophiles et de savanes incluses. Ce n'est que vers 490 BP que se fait le retour à un climat plus humide et que se mettent en place les associations végétales présentes actuellement. Ces modifications ont été plus tardives et moins intenses que dans d'autres régions congolaises, plus sensibles aux forçages climatiques. (Résumé d'auteur)