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 vigourous was the bottom current in the glacial North Atlantic Ocean. Distribution of sediment grain size can provide such information, if the characteristics of the source functions are known. The interest of this paper 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 derived from a sediment core in the Icelandic basin (Gardar Drift). The distribution 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 (Iceland) for the silt fraction than for the fine fraction. These data suggest that the increase in the silt fraction during glacial periods is related primarily to an increase in the bottom current strength, rather than to changes in sediment supply (i.e. ice-rafting, turbidity current). The increase in vigour of the bottom current, recorded during the isotopic stages 6, 4 and the last post glacial periods, could have been initiated by a southward shift of the zone of deep water formation in the northern part of the Icelandic basin.