Magnetomineralogical Characterization of Heinrich Events Preserved on Turbiditic Sediments From the Galicia Bank (NW Iberian Margin)

Abyssal marine environments combine the ideal conditions for recording the climatic instability occurred during the last glacial period. However, on a more dynamic zone, such as the Galicia Bank slope, the turbiditic activity may compromise the fidelity of a given record. In this context, we will show how very detailed magnetic measurements make possible the detection and characterization of distinct climatic events in these environments, such as the Heinrich events H1 to H3 described in this paper. The study area is located on the south-western flank of the Galicia Bank, at depths ranging from 3363 to 4171 meters. It is influenced by the Lower North Atlantic Deep Water (LNADW) and Antarctica Bottom Water (AABW) deep currents that flow northwards as a result of thermohaline equilibrium. Three gravity cores of around 3 m long and representative of the main sedimentary environments in the area (fault scarp; turbidite lobe and interlobe depression) were selected for detail magnetic analysis. Measurements at room (magnetic susceptibility, ARM, hysteresis) and low temperature (susceptibility and SIRM) revealed the occurrence of magnetic-enriched laminae within the magnitude range documented for Heinrich events recorded in sediment of the abyssal plain bordering the Iberian continental margin. The formation of loops in the ARM-100 vs. susceptibility plots highlighted the typical increase in magnetic grain size that characterizes Heinrich sediments at latitudes inside the Rudimann Belt. Recognition of IRD layers in very detrital-diluted horizons was possible after the evaluation of magnetic domain state. Low temperature measurements were also useful to clarify on the mineralogy and grain size of this ice rafted particles, which proved to be stoichiometric magnetite. This was supported by the occurrence of a clear Verwey transition at 120K. The significant loss of total remanence after a complete SIRM-300K cycle in these horizons indicated the presence of larger grain sizes, when compared to other samples studied along the cores.