Implications of planktic foraminiferal size fractions for the glacial-interglacial paleoceanography of the polar North Atlantic

Foraminiferal abundances derived from different mesh-size fractions were investigated from the polar North Atlantic for the time interval of the later part of marine oxygen isotope (MIS) 8 into early MIS 7 and the last 30 cal. ka in order to select the most representative fraction for paleoceanographic applications. The records cover the time interval with quite different glacial-interglacial climatic conditions. Whereas the interval of MIS 8-7 represents intermediate glacial-interglacial conditions, the younger core section characterizes more extreme climatic conditions consisting of the rather cold last glacial maximum (LGM) and the very warmer Holocene. The records from MIS 8 to 7 show that the larger size fractions (125-250 mum, >150 mum) reveal almost monospecific assemblages of the polar species Neogloboquadrina pachyderma (s), indicating a prevalence of cold polar-like surface water conditions during this time. By contrast, the smaller size fractions (100-150 mum, 80-150 mum) exhibit considerable increase of the subpolar species Turborotalita quinqueloba, which is representative of inflow of Atlantic water into the polar North Atlantic. In these smaller fractions, the two species show a strong negative correlation with respect to changes in glacial-interglacial conditions. Although T quinqueloba yielded sufficiently high numbers of total specimens in the two smallest fractions, which makes this species useful for interpreting temporal water mass alterations, the relative abundance changes in the 80-150 mum are twice as large as in the 100-150 mum fraction. Size fraction faunal analysis covering the last 30 cal. ka revealed occurrences of small-sized T. quinqueloba (80-150 mum) in the LGM interval, indicating seasonally open water conditions during this time. This finding demonstrates the advantage of employing small-sized fractions for paleoceanographic reconstructions of periods when relatively cold conditions prevailed in the Nordic seas.