Stable isotopic evidence for Plio-Pleistocene hominin paleoenvironments of the Koobi Fora formation, Turkana Basin, northern Kenya

Stable isotopic evidence from pedogenic carbonates from the Koobi Fora region of northern Kenya shows a trend of spreading grassland environments and increased evaporation of soil water from 2.0-1.8 Ma. At that time, environments display the full range of C3 and C4 flora and high variability in evaporation relative to precipitation, but maintain mean values. The diversity of habitats across subregions responded differentially to the global aridity trend and was largely controlled by the path and character of the proto-Omo River and precursors of Lake Turkana. The timing of floral change coincides with that of mammalian faunal change in the region. The distribution of lithic technologies and cutmarked bone after 1.8 Ma suggests hominins preferred more closed and wet habitats at Koobi Fora, but were ranging into grassland habitats. Isotopic partitioning of the basin is also evident with strontium of soil carbonates. Overall the pattern suggests changing values with the form of water on the landscape; however, once the fluvial system in emplaced, strontium isotopes are well-mixed across subregions, hindering usefulness for distinguishing mobility.; In the modern savanna ecosystem, oxygen isotopes in water from rivers, surface ponds, dam, and waterholes illustrate high variability within one season. Relative to those waters, soil carbonates accurately record evaporative processes on the order of 1-2 per mil. Isotopic representation of floral communities recorded in pedogenic carbonates from soils is biased toward the drier end of the photosynthetic spectrum and did not correspond to visual floral composition. Carbon and oxygen isotopes in enamel from Burchell's zebra did not covary as predicted with seasonal changes in grass and water sources. The variability necessary for the observed pattern of zebra enamel ratios is present in one season, within a small geographic area (100 km2). Strontium ratios predicted from lithology do not reflect the mixing and attenuation of the two signatures by water, vegetation, and zebra enamel. Ratios alone are not especially helpful for determining zebra mobility across substrates; however, high and low variability in strontium ratios differentiated the zebra constantly residing on one geologic substrate (fenced) from those ranging across two substrates.