SEASONAL PATTERNS OF VERTICAL PARTICLE-FLUX IN EQUATORIAL AND COASTAL UPWELLING AREAS OF THE EASTERN ATLANTIC

Eight time-series sediment traps were deployed on moored arrays off Mauritania (Cap Blanc), in the northern and southern Guinea Basin, and off Namibia (Walvis Ridge). The highest total fluxes (66.8 and 59.1 g m-2 y-1), as well as carbonate (28.7 and 32.9 g m-2 y-1) and biogenic opal fluxes (5.4 and 8.2 g m-2 y-1) were recorded at the two coastal sites, off Mauritania and off Namibia, respectively. Intermediate rates were recorded north and low rates south of the equator. Surprisingly, carbonate-producing organisms dominated total fluxes at all sites, whereas biogenic opal was only a minor contributor. The most distinct flux maxima were in July-August both at the Cap Blanc and the northern Guinea Basin site (2-degrees-N). Carbonate fluxes also peaked during that period. In contrast, highest opal fluxes were measured there in late winter/early spring. At the Cap Blanc site, high lithogenic fluxes correlate well with fluxes of biogenic components. At both equatorial sites, prominent flux maxima were observed in boreal spring, attributed to the southernmost penetration of the ITCZ. These fluxes were characterized by a relatively high lithogenic contribution. South of the equator (2-degrees-S), sedimentation decreased rapidly to almost zero after May when the ITCZ started to migrate northwards. Seasonality was most expressed at the Walvis Ridge site, where a distinct bi-modal flux pattern occurred (June-July and October-November). In austral spring, elevated fluxes were determined for a relatively long period when wind-driven upwelling is strongest and the Namibia upwelling cell shows the greatest extension to the west approaching the trap position. Contribution of lithogenic material to total fluxes was very low at this site.