Pleistocene fluctuations in the Agulhas Current retroflection based on the calcareous plankton record

Author(s): Flores, José Abel; Gersonde, Rainer; Sierro, F. Javier
Author Affiliation(s): Primary:
Universidad de Salamanca, Departamento de Geología, Salamanca, Spain
Alfred Wegener Institute for Polar and Marine Research, Federal Republic of Germany
Volume Title: Marine Micropaleontology
Source: Marine Micropaleontology, 37(1), p.1-22. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0377-8398 CODEN: MAMIDH
Note: In English. Includes appendix. 86 refs.; illus., incl. 3 tables, sketch map
Summary: Piston core (PS2487-6), recovered south of Cape Town, and sediment surface samples, recovered in the area of the Agulhas Current retroflection, were used to construct paleoceanographic scenarios for the late Quaternary in a region with an important role in global water mass transfer. Coccolithophore (calcareous nannofossil) and planktonic foraminifera assemblages and oxygen isotope data were collected. Stratigraphic control is based on calibration of the δ18O stratigraphic signals with calcareous nannofossil events that are thought to be synchronous over a broad range of latitudes. Study of the surface sediments permits the characterisation of the Agulhas Current, Subtropical Convergence and Subantarctic coccolithophore assemblages. The Agulhas Current assemblage has relatively high proportions of Florisphaera profunda, Gephyrocapsa oceanica and Umbilicosphaera spp. These species are absent or present in low proportions in subantarctic waters. The abundance of coccolithophores during isotope stages 1, 5 and 7 is characteristic of relatively warm, stratified surface waters, with a deep nutricline and chlorophyll maximum, which strongly suggests that the area was under the influence of the Agulhas Current retroflection. The incursion of Globigerinoides ruber, abundant today in the Agulhas Current, also supports this interpretation. Conversely, during glacial stages 2-4 and 6, a strong reduction in warm and stratified water indicators can be observed, together with an increase in cold-eutrophic species. The interval from isotope stages 8 to 12 displays an assemblage dominated by Gephyrocapsa caribbeanica, the highest values being seen in the so-called Mid-Brunhes event, accompanied by a clear reduction in subtropical Holocene species. During the glacial interval from isotope stages 10-12, G. caribbeanica dominated the assemblage. The ecological interpretation of this species is controversial due to a clear evolutionary overprint. Isotope stage 12 is here interpreted as having been the coldest one in the period studied. It is characterised by a strong increase in Neogloboquadrina pachyderma (dextral and sinistral) and a remarkable decrease in tropical and subtropical planktonic foraminifera. From isotope stage 13 to 18, a hiatus is interpreted. Below this hiatus the stratigraphic resolution is poorer, although the glacial-interglacial cyclicity is well defined in the Agulhas Current Retroflection area (Core PS2487-6) for the last 25 isotope stages. The glacial-interglacial cyclicity is thought to be due to a fluctuation in the Subtropical Convergence Zone, probably linked to the eastward and westward displacement of the Agulhas Current retroflection. In any case, the core studied was always in a subtropical environment, under the influence of the Agulhas 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. ruber and Globigerinoides sacculifer within the planktonic foraminifera, clearly respond to interglacial pulses, reaching maximum values during short periods close to the major deglaciations.
Year of Publication: 1999
Research Program: DSDP Deep Sea Drilling Project
ODP Ocean Drilling Program
Key Words: 24 Surficial Geology, Quaternary Geology; Agulhas Bank; Agulhas Current; Algae; Assemblages; Biostratigraphy; Calcareous composition; Calcidiscus; Calcidiscus leptoporus; Cenozoic; Coccolithophoraceae; Cores; Currents; DSDP Site 360; Deep Sea Drilling Project; Florisphaera profunda; Fluctuations; Foraminifera; Gephyrocapsa; Gephyrocapsa oceanica; Globigerinacea; Globigerinidae; Globigerinoides; Indian Ocean; Invertebrata; Isotope ratios; Isotopes; Leg 114; Leg 40; Microfossils; Neogloboquadrina; Neogloboquadrina pachyderma; O-18/O-16; ODP Site 704; Ocean Drilling Program; Ocean currents; Oxygen; Paleo-oceanography; Planktonic taxa; Plantae; Pleistocene; Protista; Quaternary; Rotaliina; Stable isotopes
Coordinates: S355046 S355044 E0180548 E0180547
S465246 S465245 E0072515 E0072515
Record ID: 1999048256
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands