45227-AC8
Geology and Paleontology of Coastal Angola
This grant supported fieldwork on fossil vertebrates and their environmental context in Angola. It is part of a jointly funded project with the National Geographic Society. We collected samples for isotopic analysis from measured stratigraphic sections and excavated fossil vertebrates. In addition, two undergraduate students and two graduate students sampled and are currently analyzing sediment cores from the Congo Basin drilled in the 1960's. These cores are each 2000 meters in length and include rocks of Paleozoic through Mesozoic age. The students are analyzing stable isotopes and clay minerals to document climate change in the Congo Basin, which can be used to test hypotheses generated from Angolan fieldwork. Our study underscores the importance of understanding continental positions and atmospheric circulation in predicting upwelling and organic production. Organic material produced at upwellings is captured in organic-rich marine petroleum source rocks. The reason the Congo Basin cores mentioned above are important is because they provide a continuous record of climate as Africa drifted northward. We predict that as the Congo Basin drifted out of arid latitudes into lower, more humid latitudes, the climate became more mesic. The desert of the Skeleton Coast, however, remained constant in latitude as Africa drifted north. Thus, the Congo and the locality of Iembe would have been in the desert zone during the Cretaceous, and the Orange River area, now desert, would have been temperate. The climate history of the current Skeleton Coast should be shown to be decoupled from the climate history of the Congo Basin as Africa drifted north. Tooth enamel from Angola was included in an initial study of mosasaur paleoecology based on stable carbon isotopes. Mosasaurs are marine lizards that grew to large size. They were the Mesozoic equivalents of toothed whales. A simple model in which only diet and trophic level affect the d13C of mosasaur tooth enamel would predict values for all taxa within a few per mil, with larger mosasaurs having enriched carbon isotope values due to feeding at higher trophic levels. The opposite trend is observed, however, as our data range from –1.2‰ for the small-bodied Coniasaurus to –14.8‰ for Prognathodon. There is a rough correlation between body size and d13C value among the mosasaur taxa tested here, with large-bodied genera having more 13C-depleted values. This suggests that others factors such as respiratory physiology and background values of foraging habitat affected mosasaur d13C values, as they affect modern marine squamates and sea mammals. The Cretaceous tropical Atlantic Ocean was the setting for an initially tectonically controlled late Aptian (113 Ma) shallow water (≤300m) connection between the northern and southern portions of the Atlantic, followed by a deep-water connection by the Turonian. Ocean currents changed with deepening of the South Atlantic and progressive widening of the Equatorial Atlantic Gateway. This gateway was used by late Turonian mosasaurs and sea turtles as evidenced by the mosasaur Angolasaurus and a new turtle taxon close to Sandownia, both found at Iembe and derived from northern clades. The presence of a sauropod in late Turonian sediments, also from Iembe, suggests that this animal was tolerant of warm, arid conditions as the desert elephants of Namibia are today. Further, it suggests that the waning terrestrial dispersal route between South America and Africa was situated in a region where high temperature, low rainfall, and sparse vegetation would be expected to restrict the movement of more mesic and ecologically sensitive species. Productive areas of coastal upwelling led to the deposition offshore of organic-rich sediments varying in position along the African coast with time, culminating in the Benguela Upwelling that commenced in the Miocene. There is therefore another hypothesis to explain middle Cretaceous organic rich sediments and petroleum source rocks along the west coast of Africa other than global oceanic anoxic events.
