Anomalous occurrences of Neogloboquadrina pachyderma (left) in a 420-ky upwelling record from Walvis Ridge (SE Atlantic)

Core T89-40, eastern Walvis Ridge between the subtropical gyre and Benguela coastal upwelling system, contains three types of levels of abundant left- coiled Neogloboquadrina pachyderma, a cold, eutrophic species, next to subt ropical species. Type A peaks (362, 110 and 53-43 ky BP) are accompanied wi th high percentages of other eutrophic species. They are attributed to inte nsified upwelling in the Northern Benguela region. Type B peaks (129 and 92 ky BP) are accompanied by moderate (<48%) contributions of other eutrophic species and increased numbers of subtropical species. These suggest intens ified upwelling in the Northern Benguela cells and may reflect increased se asonal contrasts between the winter upwelling and the subtropical summer co nditions. The highest C-peaks, up to 38%, are associated with strongly reduced percen tages of other eutrophic species and with abundant subtropical species (Mar ine Isotopic Stage 11.3 (401 ky) and 9.3 (326 ky)). The subtropical species preceeded the C-peaks by ca sky. We argue that the C-peaks were not produc ed by local reproduction but expatriated from the Northern Benguela upwelli ng cells. Here more nutrient-rich waters may have produced a mono-specific Neogloboquadrina pachyderma (left) fauna during strong polewards shifts of the frontal systems in the South Atlantic, which could have been transporte d 700 km offshore to the core location, unadmixed with eutrophic species fr om the surrounding waters. We propose meandering shelf-edge jets, strong co ntour jets, as a mechanism for the transport. The timing of the C-peaks and associated subtropical peaks agrees with the known precessional cyclicity of the SE Atlantic front movements and zonality of the trade winds, which s upports the shelf-edge jet hypothesis. (C) 2000 Elsevier Science B.V. All r ights reserved.