ELSEVIER Marine Geology 131 (1996) 233-249 MARIHE @EOLO@F INTEANA1"IONAL JOURNAL OF MARINE GEOLOGY, GEOCHE~fISTRY AND GEOPHYSICS Grain-size and Sr-Nd isotopes as tracer of paleo-bettom current strength, Northeast Atlantic Ocean M. Revel a, M. Cremer a, F.E. Grousset a, L. Labeyrie b a Dbpactement de G~ologie et Oc~anographie, URA CNRS 197, Universitb Bordeaux L Avenue des Facult~s, 33405 Talence Cedex, France b CFR, laboratoire mixte CNRS-CEA, domaine du CNRS 91198 Gif/Yvette, France Received 9 December 1994; revision accepted 18 January 1996 Abstract A crucial question in modern paleoceanograpbic 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 frc~m a sediment core in the Icelandic basin (Gardar Drift). The distribution pattern reveals a well sorted silt size (modal size 20 gm) 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. 1. Introduction The thermohaline circulation in the northern Atlantic Ocean is characterized by a south-flowing bottom circulation (North Atlantic Deep Water: NADW), the start of the global, heat exchanging "conveyor belt" (Broecker et al., 1990). One branch of this cold water, the Iceland-Scotland Overflow Water (ISOW), flows south over the Iceland-Faeroe sills into the eastern Atlantic (Worthington and Wright, 1970; Swift, 1984; Tsuchiya et al., 1992; Schmitz and McCartney, 1993) and transports lithic particles along the main paths of circulation, where they are deposited in large contourite drifts (McCave et al., 1980; 0025-3227/96/$15.00© 1996ElsevierScienceB.V. All rights reserved SSDI 0025-3227 (96)00005-9 Fig. 1). Because the bottom currents exert primary control on these drifts (McCave et al., 1995), they are optimal sites to record the bottom water characteristics. The grain-size characteristics of the lithic par- ticles can be used as a direct physical proxy of the vigour of deep circulation. Several authors (Ledbetter and Ellwood, 1980; Ledbetter, 1984, 1986; Blaeser and Ledbetter, 1982; McCave, 1985; Wang and McCave 1990; McCave et al., 1995) have proposed that the mean size of the non- cohesive silt fraction (10-63 gin) can be used as an indicator of relative current strength because bottom currents size-sort coarse silt during events of resuspension and then deposition. However, the