Changements dans les circulations atmosphérique et océanique à la latitude des Canaries et du Maroc entre les stades isotopiques 2 et 1 [ Change in atmospheric and oceanic circulations at the latitude of the Canaries and Morocco between isotopic stages 2 and 1]

Résumé En Fr

During the Last Glacial Maximum (stade 2) geological and palynogical studies on deep sea drillings between North 24° and 36° had previously inferred to the northern shift of the tropical high pressures, of the marine trade-wind and associated up-welling and of a desert broadly spreaded on the Moghreb. But, according to the field discrimination between surficial mobile sands related to present trade-winds and lower weakly hardened sands related to paleowesterlies, recent inland studies in Fuerteventura (eastern Canary Islands) and south-western Morocco show that the Upper Pleistocene dry paleoclimate was geographically restricted to the Moroccan coasts and off-shore islands, in relation with a cold marine current supplied by ice melting waters which precisely stopped the rain formation in spite of prevailing westerlies. In Fuerteventura, a detailed field work and a critical study of numerous isotopic data relates to the repeatedly emplacement of hardened dune sands deposited between interlayered grey to brown thin paleosoils during the Stages 2 and 3. Good lithostratigraphical and isotopic correlations with these Canarian observations occur from the field data studied in various Moroccan coastal regions (Tiznit, Safi.Rabat). Finally, a chronology supported by more than very coherent 30 l4C datations infers a relation between the sand dunes spreading and intervening maxima of icebergs in Northern Atlantic; and between the interlayered paleosoils and intercalated wanning periods. At the end of the Lateglacial period, simultaneous climatic and environmental changes occur in eastern Canary Islands and coastal Morocco: a sand remobilization then an erosive crisis between 14000 and 10000 y. BP are consequential to important changes leading to interglacial conditions in the Holocene. In the both countries, morphological observations on mobile sands, supported by mineralogical and isotopic data, show that the trade-wind and associated up-welling replaced, in the Holocene, the westerlies and ice melting current; moreover major Saharan dust deposition causes upper light to brown silty deposits into the Canary Islands.

Les études sur carottes océaniques entre 24 et 36°N avaient conclu à une remontée vers le Nord du système des hautes pressions, de l'alizé maritime, de l'upwelling et d'un désert largement étendu sur le Maghreb autour du dernier maximun glaciaire (stade 2). De récentes études sur le continent et les îles montrent que le climat sec était limité aux côtes marocaines et aux îles, en relation avec un courant froid d'eau de fonte glaciaire, qui précisément bloquait la formation des pluies malgré la prédominance des vents d'Ouest Une chronologie fondée sur plus de 50 dates 14C très cohérentes montre une liaison entre l'extension des dunes et les pics de fréquence des icebergs dans l'Atlantique nord et entre les phases de réchauffement et les paléosols. Entre 14000 et 10000 BP, cette situation a brusquement fait place aux conditions interglaciaires et l'alizé et son upwelling associé ont remplacé les vents d'Ouest et les courants de fonte glaciaire.