The magnetic fraction: A tracer of deep water circulation in the North Atlantic

摘要

We report on a mineral magnetic study of a series of Holocene marine sedimentary sequences distributed from northern to southern Bjorn and Gardar drifts (56°N to 61°N) and in the Charlie–Gibbs fracture zone (about 53°N). All the cores are located at water depths bathed by the Iceland–Scotland Overflow Water (ISOW), a precursor of the North Atlantic Ocean. Deep Water formed in the Nordic Seas and flowing into northeast North Atlantic after passing over the volcanic sills between Iceland and Scotland. The magnetic fraction is composed of low Ti-content magnetites with a grain size distribution in the pseudo-single domain range. These grains originate from a unique source in the North through the erosion by the bottom waters of the Icelandic basaltic province. The magnetic grain size remains fairly constant in each core but, on average, progressively fines towards the south and downstream. Likewise the magnetic concentration and mean magnetic flux decrease towards the south. The spatial evolution of the magnetic concentration and of the magnetic grain size suggests a transport of these magnetites by bottom currents associated with the overflow water downstream from the source with progressive deposition of this magnetic fraction along the path of the ISOW. This study provides evidence that characterization of the magnetic parameters of sediments deposited in such an oceanographic context of a unique source upstream and a well-defined water mass may be used to detect and trace deep oceanic circulation changes.