Magnetic grain-size analyses of Holocene sediments in the North Atlantic and Norwegian Sea : palaeoceanographic applications

Abstract: High-resolution magnetic hysteresis measurements were carried out on three sediment cores, one from the North Atlantic and two from the Norwegian Sea. These measurements can be used to reconstruct changes in magnetic grain-size. Physical grain-size has been interpreted as a proxy for bottom current intensity (McCave et al. 1995) and this study shows that the hysteresis measurements can contribute as a proxy for near bottom currents in this region during the Holocene, but only when the magnetic measurements are combined with other proxy records. The magnetic grain-size record of one core from the Norwegian Sea has been compared with sea surface temperature reconstructions produced by Calvo et al. (2002) and Birks & Koç (2002). The dominant magnetic mineral in the sediment is low titanium content magnetite (titanomagnetite) and the grain-size variation is discussed as a potential of near-bottom current flow intensity during the Holocene. A core from the Vøring plateau (MD95-2011) shows that the magnetic grain-size has a significant positive correlation to SST during the Holocene. Maximum sediment grain-size was reached between 9000 to 6000 cal BP, which would imply that the THC was most active in the Norwegian Sea during the Holocene thermal maximum, as registered by SST and terrestrial reconstructions. The core LO09-14 on the Reykjanes Ridge in the North Atlantic shows the same behaviour during the first part of the Holocene, but after 7500 cal BP there are indications of a shift in the oceanographic conditions and at 3700 cal BP the magnetic mineral sediment source changed and the magnetic properties cannot be used to reconstruct near-bottom current flow intensity. Core M23258-2 from south of Svalbard does not show the same behaviour at all and the variable magnetic properties of the core appear to be dominated by ice-rafted-debris.