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Babette Böckel, Karl-Heinz Baumann:
Late Quaternary coccolith assemblages in the SE Atlantic: implications for surface-water conditions and carbonate accumulation
(Poster)

Coccoliths retrieved from two sediment cores from the continental slope offshore Cape Town (GeoB 3603) and central Namibia (GeoB 1710) have been examined for changes in distribution and abundance. Furthermore, biometric measurements are carried out in order not only to determine the carbonate contribution from coccoliths but also to study the degree of species-level variation and microevolution.

Located in the SE Atlantic, the study area offers an ideal opportunity to monitor variations in warm- and cold-water transfer to the Benguela Current system. Core GeoB 3603 underlies the northern branch of the Agulhas Current, which is one of the world's major western-boundary currents responsible for a substantial leakage of tropical and subtropical water from the S Indian Ocean into the S Atlantic. The northward advection of Agulhas filaments even influences the Benguela upwelling region, where core GeoB 1710 is located. Vertical and lateral shifts in the current's movements, and in the upwelling intensity, especially during glacial intervals, would affect inter- and intra-ocean heat-transfer and, thus, the thermohaline circulation.

Analysis of the two sediment cores provides information on variations in surface-water circulation and productivity in the SE Atlantic during the past 250ky. Both cores exhibit highest coccolith abundances during Interglacials 7 and 5. In either core, the coccolith assemblages are dominated by G. ericsonii, G. muellerae and E. huxleyi. Subtropical and tropical species play a minor role. The interglacial nannofloral assemblage consists of G. ericsonii, E. huxleyi, C. leptoporus, F. profunda and G. oceanica, indicating higher productivity and warmer surface-water conditions. In glacial times, G. muellerae is the dominant species, hinting towards colder surface-waters. F. profunda, a species adapted to the lower photic zone and exhibiting a sensitivity towards lower temperatures, shows fairly continuous abundances in core GeoB 3603 throughout the studied interval. This can be interpreted as a clue to incessant warm-water influx via the Agulhas Current.

While the carbonate record of core GeoB 3603-2 shows fairly steady values (60-76.9wt-%), the coccolith carbonate curve exhibits strong variations (4.9-42.7wt-%), reflecting the glacial-interglacial cyclicity, thus corresponding relatively well to the alkenone temperature record. Deviations from the alkenone temperatures during glacials can be explained by the dominance of the cold-water species, G. muellerae, which then accounts for a substantial amount of coccolith carbonate.

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 [Division of Micropalaeontology] [Department of Geosciences] [Bremen University]

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