A PALEOSOL CARBONATE ISOTOPE RECORD FROM THE SHUNGURA FORMATION, ETHIOPIA

The Shungura Formation in the Lower Omo Valley, Ethiopia, consists of Pliocene and Pleistocene fluvial, lacustrine and deltaic sediments. Carbonate nodules are abundant in vertic paleosols that formed on the floodplain of the paleo-Omo River. Carbon and oxygen isotope values of paleosol carbonates from 34 distinct horizons between 3.02 and 1.88 Ma (Shungura Formation Members B-H) average -8.9‰ (1σ 1.2‰) and -5.6‰ (1σ 1.6‰) respectively. Low δ¹³C values of paleosol carbonates suggest that trees and shrubs flanked the paleo-Omo River, except for the latest portions of the record when paleosol carbonate δ¹³C values increase to -4.5‰. These results agree with the prevalence of woodland fossil bovid taxa in the Shungura Formation but they differ from faunal and palynological analyses that point to an increase in grasslands between 2.8 and 2.5 Ma. Additionally, at 2.8 Ma Shungura terrestrial proxies and marine sediments record a trend towards increased aridity in East Africa. The δ¹⁸O values of the Shungura paleosol carbonates show no sign of increased aridity at this time. The multiple paleoenvironmental proxies available for the Pliocene Shungura Formation work at a range of spatial scales and record different environmental features. These differences are responsible for discrepancies between the Shungura paleosol carbonate record and the faunal, pollen and marine records.