Isotopic records of Plio-Pleistocene climate and environments in eastern Africa.

摘要

Tropical responses to climate change in the Pliocene and Pleistocene epochs provide perspective on the effects of future climate perturbations. Marine records show that African environments have become more arid and grassy in the last four million years. Although terrestrial records in eastern Africa document these trends, the mode and timing of this response is not uniform between rift basins. This dissertation builds additional records of terrestrial climate and environmental change from two basins in eastern Africa, using carbon and oxygen isotopic data from pedogenic carbonates and fossil teeth.;Carbon isotopic data from extant mammals in eastern Africa document distinctions among animals living in grasslands, montane forests, and closed-canopy forests. Using the framework established from extant herbivores, carbon isotopic data of fossil teeth from Gona, Ethiopia indicate that the hominid Ardipithecus lived in habitats with abundant C4 grass, not closed-canopy forest, during the late Miocene and early Pliocene. Carbon isotope results from pedogenic carbonates and fossil teeth in the Turkana and Awash Basins in Kenya and Ethiopia confirm the prevalence of C4 grasses by the early Pliocene and demonstrate heterogeneity in the distribution of C 3 and C4 plants within and between basins.;Oxygen isotopic data from pedogenic carbonates and fossil teeth are used as indicators of both aridity and the 18O composition of waters. A survey of extant herbivores in eastern Africa reveals that the oxygen isotopic separation between non-obligate and obligate drinkers varies with aridity. When this principle is applied to the fossil record at Turkana, oxygen isotopic data from herbivores indicate that there were oscillations between mesic and xeric conditions during the Pliocene. The oxygen isotopic record of pedogenic carbonates also indicates varying degrees of aridity during the Pliocene and Pleistocene in both the Awash and Turkana Basins. A shift in oxygen isotope values of pedogenic carbonates and fossil hippopotamid teeth at Turkana documents an increase in the 18O content of waters during the early Pleistocene in Kenya, but not in Ethiopia. Contrasts between the isotopic composition of waters in Kenya and Ethiopia, in the past and today, suggest a climatic and isotopic divide in the region.