Alternating marine-terrestrial transgression since the last deglaciation recorded in sediments from the Palaeo-Agulhas Plain offshore southern South Africa

Palaeoceanographic conditions along the southern coast of South Africa are impacted by atmospheric and oceanographic interactions between the Atlantic and Indian Oceans, although there have been rather few studies of how these conditions have varied since the last deglaciation under the influence of a major marine-terrestrial transgression. In order to better understand palaeoceanographic and palaeoenvironmental changes and their driving mechanisms offshore the southern coast of South Africa, organic-walled dinoflagellate cysts (dinocysts) have been investigated in marine sediment core GeoB20628-1 along with published multiproxy data including pollen and spores, microcharcoal, foraminiferal organic linings and crustacean appendages. The results reveal markedly distinct changes in the depositional environment over the last 14 ka driven by the interplay of marine transgression, fluvial input, upwelling intensity and their interactions with regional vegetation and climate changes. Statistical analysis indicates the sequential occurrence of three distinct zones dominated by different dinocyst groups. High frequencies of dinocysts produced by autotrophic taxa (Group 1), such as Operculodinium centrocarpum, Tuberculodinium vancampoae and Spiniferites spp. signals the existence of a lagoonal environment during a low sea level stand during the last deglaciation. During the subsequent transition to the Holocene, maximum values of heterotrophic taxa (Group 2), including Brigantedinium spp., Echinidinium spp., Protoperidinium monospinum, are consistent with input of nutrients associated with strong river discharge. Group 2 was then replaced in the Holocene by other heterotrophic taxa (Group 3). Selenopemphix quanta, Echinidinium aculeatum and Echinidinium transparantum, indicate characteristically cool and nutrient-rich waters with active upwelling under the influence of strengthened tropical easterlies.