Sub Millennial-Scale Climate Variability off the Western Iberian Margin During the Penultimate Glacial Period (MIS 6)

High-resolution palaeoclimate records of planktonic foraminiferal fauna, stable isotopes and ice-rafted debris counts obtained from the Portuguese margin (40°N, 9°W) exhibit large fluctuations during the penultimate glacial period (MIS 6). This study focuses on four main points: 1) the distinction between cooling episodes related to Heinrich-type ice-rafting events and those related to increased regional upwelling, 2) the magnitude and apparent similarity in the frequency of climate change associated with ice-rafting events to those described during the last glaciation, 3) the effect on the ventilation of North Atlantic Deep water during the most important cold events, and 4) the variability of the Portuguese benthic delta ¹⁸O and the Vostok methane record seemingly in phase. Sea-surface temperature off Portugal in Stage 6 was in general warmer than during the last glacial, pointing towards a weaker southward influence of polar water masses. Ice-rafting occurred mainly in mid-MIS 6 (between 163 and 143 kyrs) as a group of poorly differentiated, short duration quasi-continuous events, mainly marked by the high abundance of sinistral N. pachyderma. Differences exist in IRD composition relative to the last glacial, with a reduced Canadian-derived detrital carbonate component, combined with an important contribution of volcanic particles originating from northern Britain and Icelandic ice-sheets. The lower magnitude and higher frequency of these events suggests that the warmer temperatures would have induced iceberg waning closer to the source areas. Centennial periodicity similar to that observed in Stage 3 (700 years) is also a persistent feature of the penultimate glacial, although the group of mechanisms behind this variation is not fully understood. Solar radiation changes may exert some control, and it is reasonable to surmise that a diverse combination of forcing factors and reaction times of the different components of the complex ocean-atmosphere-ice-sheet system would have produced a record with the same cyclicity in the deep-sea sediments.