NEOGENE PALEOCEANOGRAPHIC SIGNALS FROM PALYNOMORPHS AND DISPERSED ORGANIC MATTER IN THE MIDDLE CAPE BASIN,ODP LEG 175

The Benguela Current in the Middle Cape Basin of the Southern Atlantic is responsible for creating one of the best-known upwelling systems in the world. This study presents the palynological interpretation of Neogene sediments in this region. Analyses of 59 samples from ODP Leg 175 (Holes 1085A, 1086A, 1087C) yielded abundant marine palynomorphs, few spores and pollen, and several types of dispersed organic matter. Dinoflagellate cysts are the dominant marine palynomorphs, and they are mainly gonyaulacoid-type cysts (G-cysts). This dominance is in sharp contrast to their distribution in other upwelling regions where protoperidinioid-type taxa (P-cysts) are dominant and could be due to biological and/or taphonomic processes. Discharge from Orange River and wind action probably contributed terrigenous material and palynofacies analysis of dispersed organic matter supports this conclusion. Cluster analysis of organic matter from the most continuous hole (1085A) yielded three palynofacies assemblages (A, B, C) defined primarily by amorphous organic matter (AOM), structured phytoclasts, black debris, and marine palynomorphs. Palynofacies assemblage A is characterized by the highest percentages of AOM, the least percentages of structured phytoclasts and black debris, and is represented also in the more condensed Holes 1086A and 1087C. We interpret this assemblage as recording periods of intensified upwelling and increase in productivity that began during the early late Miocene, because of an abundance of oceanic dinoflagellate cysts such as Nematosphaeropsis and Impagidinium. The reverse is observed for assemblage C, which has fewer oceanic taxa while assemblage B has an intermediate abundance of the oceanic taxa. Middle Miocene samples are richer in terrigenous material, including grass and chenopodiaceous pollen, suggesting possibly wetter conditions resulting in increased runoff on the adjacent landmass.