Paleosol carbonates from the Omo Group: Isotopic records of local and regional environmental change in East Africa

Abstract

Pliocene and Pleistocene sedimentary rocks from the Omo–Turkana Basin in East Africa are well known for fossil and archeological evidence of human evolution and provide a unique opportunity to study four million years of environmental change in a rift basin. This study uses carbon and oxygen isotope ratios of pedogenic carbonates to examine environmental variability across ~ 5000 km 2 within the Omo–Turkana Basin. An expanded isotopic dataset, including the first isotopic data on pedogenic carbonates from the Shungura Formation and new data from the Nachukui and Koobi Fora formations, is compared to published isotopic records from both the Omo–Turkana Basin and the lower Awash Basin, Ethiopia. Regionally, the carbon isotope record indicates a steady increase of C 4 vegetation in floodplain environments for the past 4 million years. The oxygen isotopic record indicates that the isotopic composition of rainfall was depleted in 18O relative to today's waters and that both basins likely received more rainfall in the Pliocene than today. A shift to higher δ 18O values in paleosol carbonate after 2 Ma in the Omo–Turkana Basin but not in the lower Awash Basin suggests that the ecology and hydrology in these two rift basins were influenced by different climatic regimes. In addition to regional trends, pedogenic carbonates sampled from different parts of the basin show that the distribution of C 4 vegetation and soil water δ 18O values varied with proximity to the axial river system, and specifically that C 3 vegetation was more dominant in soils of the Shungura Formation compared with coeval sediments downstream in the Nachukui and Koobi Fora formations. This large isotopic dataset from pedogenic carbonates provides the opportunity to examine how terrestrial systems responded to global climate change during the last 4 million years, from both local and regional perspectives. The isotopic data indicate that local basin and climate dynamics strongly influenced the impact of large-scale environmental change in East African rift basins.