MAGNETIC-SUSCEPTIBILITY VARIATIONS IN UPPER PLEISTOCENE DEEP-SEA SEDIMENTS OF THE NE ATLANTIC - IMPLICATIONS FOR ICE RAFTING AND PALEOCIRCULATION AT THE LAST GLACIAL MAXIMUM

Magnetic susceptibility (MS) variations are used to intercorrelate 17 Upper Pleistocene sediment cores taken from the NE Atlantic, between 4 0 degrees and 60 degrees N. The MS-based correlation depends on region ally consistent patterns of variation in the deposition of ice-rafted detritus (IRD) in response to Pleistocene glaciations, and especially to high-frequency ice-rafting episodes referred to in recent studies a s ''Heinrich events.'' The sedimentological and rock-magnetic basis fo r the apparent relationship between the MS signal and IRD content of N E Atlantic sediments is examined by (1) comparing the MS profiles of s elected cores with their records of coarse fraction (>150 mu m) lithic fragment abundance and Neogloboquadrina pachyderma (sin) percentages, and (2) normalizing MS by expressing it both on a carbonate-free basi s, and as a quotient with anhysteretic remanent magnetization (a param eter sensitive to magnetic mineral grain size variations). These compa risons show that variations in bulk-sediment MS are only partly driven by simple carbonate dilution (+/- productivity and dissolution) effec ts. Changes in both the concentration and grain size of magnetic miner als within the lithogenic noncarbonate fraction also impose a signific ant influence on bulk MS values. In particular, horizons rich in IRD a re associated with significant increases in the relative proportion of coarse grained (multidomain) ferrimagnetic particles in the sediment. This is because ice-rafting, in contrast to most other mechanisms cap able of transporting detrital magnetic minerals to pelagic environment s, has a high potential for delivering large ferrimagnetic grains as c omponents of sand-sized, polycrystalline lithic fragments. This fundam ental linkage between the IRD content and MS signal of NE Atlantic sed iments is used to reconstruct the patterns of variation in IRD deposit ion and, by inference, surface currents of the last glacial maximum (L GM, similar to 18-19 ka) relative to the present-day NE Atlantic, usin g the time-slice mapping approach developed by the CLIMAP project grou p. Our LGM/Holocene MS ratio map, based on sample pairs from over 80 d eep-sea cores, confirms that there was a weak, cyclonic gyre north of the polar front in the LGM North Atlantic. The gyre comprised a sluggi sh warm current in the NE Atlantic flowing north between latitudes 47 degrees and 62 degrees N, partly fed by subtropical waters from south of the polar front, and carrying large numbers of icebergs derived fro m several sources, most of which melted between Latitudes 45 degrees a nd 52 degrees N. The warm current probably continued its flow into the Iceland Basin, where it fed into a south-flowing current which transp orted melting icebergs from Iceland and Scandinavia along the western flank of the Reykjanes Ridge.