La sedimentation au Quaternaire recent dans le “Northwest Atlantic Mid-Ocean Canyon” - Apport des donnees mineralogiques et geochimiques

Abstract

A mineralogical (clay minerals and other minerals) and geochemical (Fe, Ti, P, Mn, Ba, Sr, Zn, Cu, Ni, Pb, Rb, Zr, S) study was made of four cores of Quaternary sediments from the “Northwest Atlantic Mid-Ocean Canyon”. A group of common characteristics has been noted for each climatic episode. Some more specific characteristics have also been identified. During the cold period (Würm), the materials are mainly terrigenous detrital (rich in calcite, dolomite, illite, chlorite, feldspars, poor in trace elements and marked by illite/Fe, Ti and quartz/Zr correlations) carried from the nearby continental platform by turbidity currents. The almost complete lack of other inputs can be explained by a blockage of the main circulation of North Atlantic waters masses during the glacial period. During the warm period (Riss-Würm and Holocene interglacials), the sedimentary material inputs were more complex: the terrigenous detrital deposits (similar to the Würm ones), are mixed with new materials (rich in montmorillonite, plagioclase feldspars, augite, epidote and characterized by montmorillonite and plagioclase feldsparts/trace elements correlations). They could have come from volcanic areas such as Iceland, Reykjanes Ridge or the Gibbs fracture and could have been carried by “Norwegian deepsea waters”. Biogenetic carbonate fine-grained materials brought in surface water by the Gulf Stream are also abundant. Furthermore, the Gulf Stream seems to be responsible for montmorillonitic clay minerals input from southern areas toward northern areas. Finally, the coarser-grained materials, ice-rafted by icebergs, can be important in high latitudes (especially southeast of Cape Farewell). Thus, the sedimentary evolution of “the Northwest Atlantic Mid-Ocean Canyon” during the recent Quaternary can be explained both by climatic changes and the nature of the sedimentary sources which successively contributed to the deposits. It gives information concerning the hydrological characteristics of the studied area since 120,000 years. During the glacial period, the mainly autochthonous characteristics of the deposits show that oceanic circulations were reduced; the Polar Front was probably located near 40°N which led to a blockage of the “Norwegian deep-sea water” circulation and to a displacement of the surface currents towards the south of the studied area. During the interglacial periods on the contrary, the diversification of sedimentary materials shows the establishment of circulations similar to those of the present.