Ocean-surface and wind dynamics in the Atlantic Ocean off Northwest Africa during the last 140 000 years

A combined micropaleontological analysis of core CAMEL-1, from the oligotrophic Sierra Leone Rise area, has allowed reconstruction of the paleoclimatic and paleoceanographic history of the region for the last 140 kyr. The ratio (N) between the Reticulofenestrids (coccolithophore indicators of relatively high nutrient contents) versus Florisphaera profunda (a Lower Photic Zone coccolithophore) allowed us to monitor changes in the nutricline depth. These results were compared with those obtained for marine diatom and planktic foraminifera assemblages. Thus, a shallow nutricline/thermocline (and high productivity) during stages 6, 5d, 5b, 4 and 2 is proposed. This situation can be correlated with maximum input of biosiliceous wind-transported particles (fresh-water diatoms and phytoliths) in sea sediments. The shallow nutricline is correlated with an intensification in Atlantic divergence and/or a North Equatorial Current intensification, when the NE trades were enhanced for glacial and stadials (cold) periods. A clear precessional component is observed in the surface water dynamics during the last climatic cycle, minima in the N ratio coinciding with minimum insolation during winter in the Boreal hemisphere. During MIS 5, 4 and 2 dry conditions were dominant on the northern African continent, corresponding to a more intense NE trade wind circulation. The increased phytolith concentrations during MIS 6 are consistent with a more intense seasonality on the African continent. Cold planktic foraminifera assemblages show an eccentricity component linked to northern Ice-sheet dynamics. This situation is enhanced in isotope stages 6 and 3, due to an intensification in the North Equatorial Current. The eccentricity component is also observed both in the N ratio (coccolithophore-related) and in the total planktic foraminifera.