Site 983 lies just south of Iceland at 60°N (Fig. 1). Diatom abundance and preservation at this site fluctuate in response to changing oceanographic conditions affecting diatom productivity. Correlating diatom abundance variations with the oxygen isotope record of Site 983 reveals a great similarity between the two records (Fig. 3). In general, interglacials are related to high diatom productivity (common to abundant diatoms), whereas glacial stages are often barren of diatoms. However, diatom production was significant during glacial Stages 18, 20, and 30.
During the late Quaternary glacial periods, oceanic polar fronts and sea ice migrated equatorward, with some of the most dramatic changes occurring in the North Atlantic (McIntyre, et al., 1976; CLIMAP Project Members, 1981). Reduced surface-water productivity during glacials indicates that these were the intervals during which the Polar Front migrated southeastward, thereby covering Site 983 with sea ice. Considering the proximity of the sea-ice margin to the area at present, it is very probable that these diatom-barren glacial intervals reflect periods when the site was under heavy sea-ice cover.
Presence of significant diatom production during glacial stages 18, 20, and 30 indicates open-marine conditions over Site 983 during these times. High diatom production during glacial stages 18 and 20 is also recorded from Site 919, which suggests that the North Atlantic was free of sea ice during these glacial periods (Koç and Flower, 1998). These glacial stages are within the first 100-k.y. cycles after the Mid-Pleistocene Transition. As indicated by the benthic oxygen isotope records, they were not as severe as the late Quaternary glacials (Mix et al., 1995).
Neodenticula seminae is part of the modern diatom assemblage of the middle- and high-latitude North Pacific (Barron, 1981; Sancetta, 1982). Our results indicate that this species is limited to the interval 0.84-1.26 Ma in the North Atlantic. Baldauf (1986) had interpreted the occurrence of N. seminae in the North Atlantic as the presence of cool, low-saline, surface waters in the central North Atlantic during the early Quaternary. The interval of N. seminae in the North Atlantic straddles the transition from the dominance of 41-k.y. cycles in the climate records to the dominance of 100-k.y. cycles. It is, therefore, possible to interpret the first occurrence of N. seminae in the North Atlantic as a sign for cooling, which started at 1.26 Ma, leading to the establishment of the 100-k.y. cycles with severe glacial periods. The presence of N. seminae in the North Atlantic is, therefore, attributable to the unique conditions related to the Mid-Pleistocene Transition.