Pleistocene fluctuations in the Agulhas Current Retroflection based on thecalcareous plankton record

Piston core (PS2487-6), recovered south of Cape Town, and sediment surface samples, recovered in the area of the Agulhas Current retroflection, were u sed to construct paleoceanographic scenarios for the late Quaternary in a r egion with an important role in global water mass transfer. Coccolithophore (calcareous nannofossil) and planktonic foraminifera assemblages and oxyge n isotope data were collected. Stratigraphic control is based on calibratio n of the delta(18)O stratigraphic signals with calcareous nannofossil event s that are thought to be synchronous over a broad range of latitudes. Study of the surface sediments permits the characterisation of the Agulhas Curre nt, Subtropical Convergence and Subantarctic coccolithophore assemblages. T he Agulhas Current assemblage has relatively high proportions of Florisphae ra profunda, Gephyrocapsa oceanica and Umbilicosphaera spp. These species a re absent or present in low proportions in subantarctic waters. The abundan ce of coccolithophores during isotope stages 1, 5 and 7 is characteristic o f relatively warm, stratified surface waters, with a deep nutricline and ch lorophyll maximum, which strongly suggests that the area was under the infl uence of the Agulhas Current retroflection. The incursion of Globigerinoide s ruber, abundant today in the Agulhas Current, also supports this interpre tation. Conversely, during glacial stages 2-4 and 6, a strong reduction in warm and stratified water indicators can be observed, together with an incr ease in cold-eutrophic species. The interval from isotope stages 8 to 12 displays an assemblage dominated b y Gephyrocapsa caribbeanica, the highest values being seen in the so-called Mid-Brunhes event, accompanied by a clear reduction in subtropical Holocen e species. During the glacial interval from isotope stages 10-12, G. caribb eanica dominated the assemblage. The ecological interpretation of this spec ies is controversial due to a clear evolutionary overprint. Isotope stage 1 2 is here interpreted as having been the coldest one in the period studied. It is characterised by a strong increase in Neogloboquadrina pachyderma (d extral and sinistral) and a remarkable decrease in tropical and subtropical planktonic foraminifera. From isotope stage 13 to 18, a hiatus is interpre ted. Below this hiatus the stratigraphic resolution is poorer, although the glacial-interglacial cyclicity is well defined in the Agulhas Current Retr oflection area (Core PS2487-6) for the last 25 isotope stages. The glacial- interglacial cyclicity is thought to be due to a fluctuation in the Subtrop ical Convergence Zone, probably linked to the eastward and westward displac ement of the Agulhas Current retroflection. In any case, the core studied w as always in a subtropical environment, under the influence of the A,Agulha s Current, which was enhanced during interglacial periods. For the whole of the interval studied, increases in Calcidiscus leptoporus, Umbilicosphaera spp., and Syracosphaera spp. among the calcareous nannofossils, and in G. I-uber and Globigerinoides sacculifer within the planktonic foraminifera, c learly respond to interglacial pulses, reaching maximum values during shea periods close to the major deglaciations. (C) 1999 Elsevier Science B.V. Al l rights reserved.