The Hadar region of Ethiopia is today a hot, dry, desert zone that lies adjacent to the higher, cooler, and more moist Ethiopian Plateau. The Hadar formation, accumulated between 3.4 and 2.3 million years ago, harbors much evidence of the history of the tectonic boundary between these regions and has yielded abundant fossils of Pliocene age, including Australopithecus afarensis. The lower 80% of this formation, and the source of A. afarensis fossils, was deposited in a sedimentary basin characterized by an environment much wetter and cooler than that seen today, similar to the modern plateau. The upper portion of the formation reflects the dryer, warmer conditions of today, and is distinct from the preceding periods by a recently discovered geological disconformity. Geological strata above and below the disconformity contain carbonates from which a climatic stratigraphy can be measured using stable isotopes of carbon and oxygen. Carbon isotopes in paleosols provide a measure of the relative abundance of wet-adapted versus dry-adapted plants in earlier time periods, while oxygen isotopes in the same strata may be used to infer paleo-temperature, -elevation, and relative precipitation. Comparison of the isotopic signals from the stratified paleosols in the Hadar formation to contemporary standards in the region will permit a more precise and detailed reconstruction of these ancient environments. Jointly supported by the SBER and INT divisions, this project will provide a better understanding of 1) the environmental context of A. afarensis, 2) the nature of the physiographic, climatic, and environmental transitions at the disconformity, and 3) their relationship to biological change and speciation at Hadar.
