Molluscan evidence for enhanced deglacial advection of Agulhas water in the Benguela current, off southwestern Africa
Abstract Shells in surficial, unconsolidated sediments at −120 to −140 m depth on the Orange Shelf, Benguela region, off southwestern Africa, have accumulated during the post-glacial transgression. A large sample of the shells, from 53 individual sites, revealed the presence of a significant percentage of bivalves (∼14%) that do not live along the western shelf at present, but inhabit the warm-temperate Algoa biogeographic province of the Agulhas Bank region. Altogether 17 Algoa species have been found on the Orange Shelf, which today is dominated by a cool-temperate, upwelling regime. Their presence suggests warmer conditions in the Benguela during the deglacial transgression, as a result of enhanced advection close to the coast of water derived from the Agulhas Current, and concomitant suppression of upwelling. Bathymetric preferences, comparative taphonomy and radiocarbon-dating of the Algoa taxa from the Orange Shelf suggest that two periods of enhanced advection of Agulhas water were involved, 13.5-12.5 kyr B.P. and ∼10 kyr B.P. This timing corresponds with major steps away from the Ice Age World, such as warming in the Southern Ocean, North Atlantic warming and increasing production of North Atlantic Deep Water. Previous suggestions for enhanced deglacial advection of Agulhas water in the Benguela are now supported by molluscan evidence. Rounding of Africa by the Agulhas Current, at times of warming in the Southern Ocean and reduced austral summer insolation due to orbital parameters, is consistent with modelling and observations that indicate such behaviour under conditions of reduced volume transport associated with southward shifting of the zonal wind profile south of the continent and reduced trade winds over the southwest Indian Ocean.