GRAIN-SIZE AND SR-ND ISOTOPES AS TRACER OF PALEO-BOTTOM CURRENT STRENGTH, NORTHEAST ATLANTIC-OCEAN

A crucial question in modern paleoceanographic research is how vigouro us was the bottom current in the glacial North Atlantic Ocean. Distrib ution of sediment grain size can provide such information, if the char acteristics of the source functions are known. The interest of this pa per is to use the Sr-Nd isotopic composition of lithic particles as a tracer of origin, in order to determine possible changes in the source function, over the last 185 kyr. Grain-size distribution data are der ived from a sediment core in the Icelandic basin (Gardar Drift). The d istribution pattern reveals a well sorted silt size (modal size 20 mu m) in the glacial deposits. Sr and Nd isotopic composition measured on separate size fractions indicates a more constant local origin (Icela nd) for the silt fraction than for the fine fraction. These data sugge st that the increase in the silt fraction during glacial periods is re lated primarily to an increase in the bottom current strength, rather than to changes in sediment supply (i.e. ice-rafting, turbidity curren t). The increase in vigour of the bottom current, recorded during the isotopic stages 6, 4 and the last post glacial periods, could have bee n initiated by a southward shift of the zone of deep water formation i n the northern part of the Icelandic basin.