Comparison of Independant Proxies in the Reconstruction of Deepwater Circulation and Productivity Changes in the Southeast Atlantic

F. X. Gingele Baltic Sea Research Institute, Seestrasse 15, 18119 Rostock-Warnemuende, Germany

G. Schmiedl Department of Geosciences, University of Tuebingen, 72074 Tuebingen, Germany

Independant proxies were assessed in two Late Quaternary sediment cores from the eastern South Atlantic Ocean to compare qualitative and quantitative productivity and deepwater changes during the last 400 ka. Both core sites, situated at approximately 3000 m water depth are presently within the North Atlantic Deepwater (NADW), but represent different productivity regimes.

Late Quaternary fluctuations of productivity are indicated by all proxies at site GeoB 1710 from the Southwest African continental slope. Influenced by the lateral extension of the coastal upwelling a higher productivity is recorded during cold stages 2-3 and 6. The benthic foraminiferal assemblage dominated by E. exigua is replaced by high-productivity faunas. The geochemical proxies TOC and barium indicate a factor 2 increase in surface productivity.

A constant low productivity throughout the last 400 ka is documented by the persistence of E. exigua at site GeoB 1214 from the outer Walvis Ridge and corroborated by very low TOC-contents. However, barium shows a slight increase during glacial stages 2-4, 6, 8, 10 and 12, which we interpret as a rise in surface productivity of up to 50%. This may be due to the higher abundance of nutrient-rich eddies, which reach the Walvis Ridge from the extended area of coastal upwelling.

Late Quaternary fluctuations of North Atlantic Deepwater (NADW) are indicated by the characteristic Fontbotia wuellerstorfi-fauna at the Walvis Ridge and Bulimina alazanensis at the Southwest African continental slope. The propagation of NADW is exclusively observed during interglacials with maxima in stages 1, 9 and 11. These maxima are remarkably consistent with kaolinite/chlorite peaks in the clay mineral record. Kaolinite, a product of intensive chemical weathering is injected into the NADW at low latitudes. Entrained in this water mass, it is transported south by lateral advection. Chlorite, which is formed under cold weathering regimes is a characteristic mineral of water masses of southern origin and dominates during glacial stages.