PLANKTONIC FORAMINIFERAL VARIATIONS IN THE SOUTHWESTERN ATLANTIC SINCE THE LAST GLACIAL–INTERGLACIAL CYCLE

Relative abundances of planktonic foraminiferal species from two cores from the subtropical, southwestern Atlantic indicate changes in oceanic conditions during the last glacial–interglacial cycle. During interglacial intervals (biozone X or Marine Isotopic Stage 5 (MIS-5) and biozone Z or MIS-1), the relative abundance of the intermediate water-dwelling species of menardiform plexus was high, whereas those of deep-dwelling species Globorotalia truncatulinoides and Globorotalia inflata were relatively low, suggesting high temperature conditions (≥22°C) and/or increased upper-water stratification during this period of time. In contrast, the absence of menardiform plexus and high abundances of cold-water species (G. truncatulinoides and G. inflata) during the glacial interval (biozone Y [MIS-4, MIS-3, and MIS-2]) suggest cold-water conditions (≤22°C) and/or a reduction of upper-water stratification. Two intervals of moderate temperature and/or low salinity during the last glaciation, however, are suggested by the increase in abundance of Pulleniatina plexus. Foraminiferal fauna have suggested a difference of 1 to 2°C between the last glacial interval and the late Holocene. Millennial-scale paleoceanographic events have been identified during the last interglacial interval (MIS-5), suggesting that warm conditions and/or a stratified water column were replaced by short intervals of cooler water and reduced upper-water stratification, as indicated by changes in abundances of menardiform plexus and G. truncatulinoides. The short interval during which menardiform plexus disappeared during the Holocene suggests a temperature decrease and can be related to the widespread stadial event in the Holocene at 8.2 ka.