Variations in coccolithophore productivity off South Africa over the last 500 kyr

Abstract

Abstract We present a 500-kyr productivity reconstruction of the southwest Indian Ocean (MD96–2077; Natal Valley) and the eastern South Atlantic (Sites 1266; Walvis Ridge) from coccolithophore assemblages and coccolith geochemistry. The study sites are situated in two different hydrographic regimes: one located in the upper boundary of the Agulhas Current (AC) and the other in the open ocean branch of the Benguela Current (BC). We aim to decipher how marine phytoplankton communities responded to variations in water-column characteristics over multiple glacial/interglacial cycles. Gephyrocapsa species (G. caribbeanica, G. ericsonii) dominates the assemblage in the Walvis Ridge whereas Florisphaera profunda is more prominent and co-dominates with Gephyrocapsa species (G. oceanica, G. ericsonii) in the Natal Valley. Higher abundance of F. profunda in MD96–2077 than at Site 1266 suggests that Natal Valley has a deeper nutricline/thermocline and lower productivity than in the Walvis Ridge over the last ~500 kyrs. Asynchrony in the productivity changes is suggested to be related to different driving mechanisms between the eastern and the western sides off South Africa. The equatorward STF migration during glacial periods has potentially reduced the intensity of the AC and promoted surface water productivity in the Natal Valley. By contrast, productivity in the Walvis Ridge shows opposite patterns, showing enhanced productivity during interglacial periods, which we attribute to a localized mixing at the confluence zone between the eastward flowing South Atlantic Current and the Benguela Oceanic Current and/or an increased Agulhas leakage in combination with stronger westerlies, thereby promoting upwelling of nutrients to the surface.