Evolution of the West African Mangrove During the Late Quaternary: A Review

The review of pollen data on mangrove pollen deposition in modern and late Quaternary sediments of West Africa points to two distinct signals linked to the sedimentary environment concerned. Along the littoral and on the slope of the continental shelf, mangrove peat deposits recording more than 40% of Rhizophora percentages reflect the postglacial sea-level rise and give evidence for the associated paleogeographical modifications (e.g. during the Nouakchottian transgression). Deep oceanic records show that the mangrove was present along the West African coasts during the Late Glacial Maximum reflecting local conditions of fresh water input and sea surface temperatures not as low as previously suggested. Mangrove developed after 12 500 BP as far north as 21°N; its maximum extension was recorded ca. 9500 BP reflecting the enhanced monsoon circulation over West Africa.La revue des données concernant la sédimentation pollinique des taxons de mangrove dans les sédiments actuels et quaternaires récents d'Afrique occidentale met en évidence la différence du signal enregistré selon qu'il s'agit de sédiments littoraux ou de sédiments marins profonds. Le long du littoral et sur les pentes du plateau continental, les tourbes de mangrove contenant des pourcentages de pollen de Rhizophora supérieurs à 40 % de la somme pollinique totale enregistrent très clairement les différentes étapes de la remontée postglaciaire du niveau marin et donnent la mesure des modifications paléogéographiques associées (comme par exemple lors de la transgression du Nouakchottien, il y a 5500 ans BP). Les sédiments marins profonds indiquent que la mangrove était présente, bien que réduite, le long des côtes ouest africaines lors du dernier maximum glaciaire indiquant la permanence de conditions locales d'apport d'eau douce et des températures de surface de l'océan proche moins basses que précédemment indiquées. La mangrove se développe dès 12 500 BP et s'étend probablement jusqu'à 21°N; son extension maximale a été atteinte il y a 9500 ans BP en réponse à l'augmentation des pluies de mousson atlantique au-dessus de l'Afrique nord-occidentale.Die Ùberprùfung von Pollen-Daten von Mangrove-Pollenablagerungen in gegenwârtigen und Spât-Quartâr-Sedimenten Westafrikas weist zwei unterschiedliche Signale auf, je nach der betreffenden Sediment-Umgebung. Entlang der Kùste und an den Hângen des Kontinentalplateaus spiegeln Mangrove-Torf-Ablagerungen, die ùber 40 % Rhizophera-Anteile enthalten, die postglaziale Anhebung des Meeresspiegels und belegen die damit verbundenen paleogeographischen Verânderungen (z.B. wàhrend der Nouakchottian-Transgression). Tiefseebelege zeigen. dass die Mangrove entlang der westafrikanischen Kusten im spàten glazialen Maximum vorhanden war, und dass die lokalen Bedingungen fur Susswasserzufuhr und Meeresoberflâchentemperaturen nicht so niedrig waren, wie zuvor angenommen. Die Mangrove entwickelte sich nach 12 500 v.u.Z. und breitete sich nôrdlich bis 21°N aus; ihre maximale Ausbreitung ist um etwa 9500 v.u.Z. belegt und spiegelt die verstârkte Monsun-Zirkulation ùber West-Afrika

Vegetation Controls on Weathering Intensity During the Last Deglacial Transition in Southeast Africa

Tropical climate is rapidly changing, but the effects of these changes on the geosphere are unknown, despite a likelihood of climatically-induced changes on weathering and erosion. The lack of long, continuous paleo-records prevents an examination of terrestrial responses to climate change with sufficient detail to answer questions about how systems behaved in the past and may alter in the future. We use high-resolution records of pollen, clay mineralogy, and particle size from a drill core from Lake Malawi, southeast Africa, to examine atmosphere-biosphere-geosphere interactions during the last deglaciation (~ 18-9 ka), a period of dramatic temperature and hydrologic changes. The results demonstrate that climatic controls on Lake Malawi vegetation are critically important to weathering processes and erosion patterns during the deglaciation. At 18 ka, afromontane forests dominated but were progressively replaced by tropical seasonal forest, as summer rainfall increased. Despite indication of decreased rainfall, drought-intolerant forest persisted through the Younger Dryas (YD) resulting from a shorter dry season. Following the YD, an intensified summer monsoon and increased rainfall seasonality were coeval with forest decline and expansion of drought-tolerant miombo woodland. Clay minerals closely track the vegetation record, with high ratios of kaolinite to smectite (K/S) indicating heavy leaching when forest predominates, despite variable rainfall. In the early Holocene, when rainfall and temperature increased (effective moisture remained low), open woodlands expansion resulted in decreased K/S, suggesting a reduction in chemical weathering intensity. Terrigenous sediment mass accumulation rates also increased, suggesting critical linkages among open vegetation and erosion during intervals of enhanced summer rainfall. This study shows a strong, direct influence of vegetation composition on weathering intensity in the tropics. As climate change will likely impact this interplay between the biosphere and geosphere, tropical landscape change could lead to deleterious effects on soil and water quality in regions with little infrastructure for mitigation

Palaeogeographical reconstructions of Lake Maliq (Korça Basin, Albania) between 14,000 BP and 2000 BP

International audienceSince the early 1990s, excavations of a protohistoric lakeside settlement in the Korça basin carried out by a French–Albanian archaeological team have induced geomorphological and palynological studies about the sedimentary records of Lake Maliq. These studies allow us to distinguish a series of centennial-scale high and low lake level events between 4200 and 4000 cal BP (2899–2637 BC/2843–2416 BC) and 2600 cal BP (822–671 BC), probably due to large-scale climate changes (in the Mediterranean basin). In addition, the sediment sequence also gives evidence of a millennial-scale trend of lake level rise. It appears to be an interplay between lake level rises and falls against tectonic subsidence of the basin allowing accommodation space for sediment deposition. The variations of the lake's level and the lake's surface area influenced the development and the abandonment of the nearby lakeside settlements (like the tell of Sovjan). In order to prepare an archaeological survey around the now dried up lake, we made a 3D model of the Holocene deposit from the lake including these lake level results, geomorphological mapping, excavation data, numerous core logs, AMS 14 C dating and SRTM DEM data. The GIS model allowed us to propose four palaeogeographical reconstructions of the extension of Lake Maliq: around 14,000 BP, during the Mesolithic (around 9000 BP – 8781–8542 BC), the Early/Middle Bronze Age transition (around 3800 BP – 2310–2042 BC) and the Iron Age (2600 BP – 822–671 BC). A map of the thickness of the sediments above potential archaeological layers is also proposed

Changes in the West African landscape at the end of the African Humid Period

Existing pollen datasets from northern Africa stored in the African Pollen Database were used to assess changes in landscape physiognomy at the end of the African Humid Period (AHP) from 5000 cal yr BP to the present using arboreal pollen percentages. The thirty-six sites available were used to map changes in arboreal cover at a sub-continental scale. Based on their location in present-day forested and non-forested areas and their relatively higher temporal resolution eight of them were selected to examine the timing and amplitude of the vegetation response in more detail, and particularly in the Sahel. In spite of low pollen production and dispersal of many tropical plants, which lead to the under representation of most of the trees relative to their abundance in the landscape, we were able to distinguish the geographical pattern and timing of vegetation changes. The landscape response to the end of the AHP was far from homogeneous particularly in the Sahel where a clear east-west gradient of changing tree cover is indicated with the central Sahel being notably species poor. In areas where forests were well developed during the AHP, i.e. in the south and west, the establishment of the modern landscape was abrupt with a threshold crossed between 3300 and 2500 cal yr BP according to local conditions. Elsewhere in northern Africa the switch from tree (C3) to grass (C4) dominated landscapes occurred more gradually during the same period. This review shows that the timing of the ecosystem response at the end of the AHP was remarkably synchronous throughout northern Africa

IV. Paléoclimats de l’Arabie du Sud

Depuis environ 2 millions et demi d’années, le climat de la terre subit une alternance de périodes glaciaires et interglaciaires, qui suit des cycles déterminés par les changements de l’orbite terrestre. Aux latitudes tropicales, les périodes glaciaires correspondent principalement à des phases sèches, où domine l’activité éolienne responsable d’un important transport de poussières à l’océan. À l’inverse, les phases interglaciaires sont des périodes humides caractérisées par la domination de ..

Sahara and Sahel vulnerability to climate changes, lessons from the past

International audienceSince the Sahelian drought in the 1970s, climate variability in north tropical Africa has been the subject of intensive research focusing on the functioning of the Atlantic monsoon system as well as on past variations in rainfall from historical and natural archives. More severe and longer droughts have occurred in the tropics during the past millennium, including a drought that occurred 200-300 years ago during which the level of Lake Bosumtwi (Ghana) dropped by almost four times as much as it did during the drought of the 1970s. Climate crises of even greater amplitude and developing over only a few years have been recorded during the Holocene leading for dramatic fall of lake levels near the Equator and the complete drying of fresh water bodies in the Sahel and the Sahara. In particular, an “abrupt” climate change has been recorded off the Mauritanian coast at the end of the African Humid Period (AHP) 5500 years ago illustrating the onset of the modern climate regime. Was this change abrupt or gradual, and amplified or not through vegetation change and feedbacks to the atmosphere is still the subject of debate.In the frame of the French project “SAHELP”, we have used paleohydrological and palynological data between 10 and 28°N in the Sahara and Sahel in order to understand the response of the hydrological system and the vegetation cover to rainfall fluctuations from the onset of the AHP to its end and to catch the regional pattern of moisture availability through time. Special attention have been paid on climate indices (dry spell, wind direction and intensity of events) to characterized the different phases of the climate deterioration. Our work is based on selected “case studies” in different hydrological environment: the mega Lake Chad and Lake Yoa (Chad), Lake In Atei (Algeria) and the Niayes interdunes depressions (Senegal)

Vegetation at the Time of the African Humid Period

International audienceAn orbitally induced increase in summer insolation during the last glacial-interglacial transition enhanced the thermal contrast between land and sea, with land masses heating up compared to the adjacent ocean surface. In North Africa, warmer land surfaces created a low-pressure zone, driving the northward penetration of monsoonal rains originating from the Atlantic Ocean. As a consequence, regions today among the driest of the world were covered by permanent and deep freshwater lakes, some of them being exceptionally large, such as the “Mega” Lake Chad, which covered some 400 000 square kilometers. A dense network of rivers developed.What were the consequences of this climate change on plant distribution and biodiversity? Pollen grains that accumulated over time in lake sediments are useful tools to reconstruct past vegetation assemblages since they are extremely resistant to decay and are produced in great quantities. In addition, their morphological character allows the determination of most plant families and genera.In response to the postglacial humidity increase, tropical taxa that survived as strongly reduced populations during the last glacial period spread widely, shifting latitudes or elevations, expanding population size, or both. In the Saharan desert, pollen of tropical trees (e.g., Celtis) were found in sites located at up to 25°N in southern Libya. In the Equatorial mountains, trees (e.g., Olea and Podocarpus) migrated to higher elevations to form the present-day Afro-montane forests. Patterns of migration were individualistic, with the entire range of some taxa displaced to higher latitudes or shifted from one elevation belt to another. New combinations of climate/environmental conditions allowed the cooccurrences of taxa growing today in separate regions. Such migrational processes and species-overlapping ranges led to a tremendous increase in biodiversity, particularly in the Saharan desert, where more humid-adapted taxa expanded along water courses, lakes, and wetlands, whereas xerophytic populations persisted in drier areas.At the end of the Holocene era, some 2,500 to 4,500 years ago, the majority of sites in tropical Africa recorded a shift to drier conditions, with many lakes and wetlands drying out. The vegetation response to this shift was the overall disruption of the forests and the wide expansion of open landscapes (wooded grasslands, grasslands, and steppes). This environmental crisis created favorable conditions for further plant exploitation and cereal cultivation in the Congo Basin