HR: 0800h
AN: U21A-0707    [Abstracts]
TI: Discriminating Between Tectonic and Climatic Controls on Early Hominin Paleoenvironments From the Koobi Fora Region, Northeastern Turkana Basin, Kenya: Part II
AU: * Quinn, R L
EM: rlquinn@eden.rutgers.edu
AU: Lepre, C J
EM: clepre@eden.rutgers.edu
AB: Global climate is often elected as a catalyst for environmental change and used to characterize selective pressures acting on Plio-Pleistocene African hominins. Vrba's Habitat Theory (1992) and Pott's Variability Selection (1998) credit mammalian evolutionary pattern and process to global climate regulated by orbital forcing. Feibel (1999: 276) argues the need for a middle ground, tethering the "global-scale climatic phenomena" to "environmental change, habitat shift, and biotic evolution" and offers the basin as a scale for analysis. Feibel suggests that all basins are not created equal, and will respond to climate change with different sensitivities and thresholds. As such, interpretations of climate proxies must account for differences in basin size, climatic regime(s), topography, geology, and water availability when drawing relationships to global phenomena. Here we examine pedogenic carbonate isotopes (d13C, d18O) from the Plio-Pleistocene Koobi Fora Region to elucidate the differential influences of climate, tectonics, and deposition on ecological factors of early hominin evolution in the northeastern Turkana Basin of Kenya. One of the richest Plio-Pleistocene fossil localities in Africa, Koobi Fora has served as a setting for hominin evolution between 4.0 and 1.0 Ma. Numerous paleosols, stratigraphically controlled by tuffaceous marker beds, are preserved in the Plio-Pleistocene sediments of the Koobi Fora Formation. Cerling and others (1988) and Wynn (2000) conducted isotopic studies of pedogenic carbonates from the Plio-Pleistocene Omo Group deposits of the Turkana Basin. With these data Wynn (2004) demonstrates stepwise d13C shifts over the last 4.0 Ma, with marked events at 2.5 and 1.8 Ma, and interprets increased aridity on a basin scale due to comparable records on the east and west side of present Lake Turkana. In this study, we increased the sample size of the current database and conducted widespread sampling of synchronous lateral horizons in the Koobi Fora Formation emphasizing 2.0 to 1.5 Ma. With attention to paleohydrology, we partitioned the basin by proximity to the ancestral Omo River system, which was the main control on basin-wide hydrology and deposition during the Plio-Pleistocene (e.g. Feibel et al., 1991). We interpret the high variability of our isotopic results from synchronous units, spanning the C3-C4 range, as indicative of ecotonal variability complemented by the paleogeographic reconstructions. Change in mean isotopic ratios across members, interpreted as increased aridity (Wynn, 2004), occurred during a period of depositional reorganization due to a major tectonic event and a diachronic decrease in basinal accommodation space. This reorganization is recorded in the Koobi Fora Formation as an up-section progression from deep-lake, delta, shallow-lake, to river and floodplain depositional facies. We conclude that the tectonic regime of the Plio-Pleistocene Turkana Basin is equally important as global climate change for the distribution of vegetation on the hominin landscape in the Koobi Fora Region. The preservation of ecotonal gradients is highly dependent on the ability of environmental perturbations to emplace a dynamic depositional geography at the basin-wide, regional, and landscape scale.
DE: 9305 Africa
DE: 3344 Paleoclimatology
DE: 4267 Paleoceanography
SC: Union [U]
MN: 2004 AGU Fall Meeting