PLIO-PLEISTOCENE ENVIRONMENTS OF GONA, ETHIOPIA: THE ISOTOPIC RECORD OF PEDOGENIC CARBONATE AND FOSSIL TEETH

We use the stable carbon and oxygen isotopic composition of pedogenic carbonate and fossil teeth to reconstruct Plio-Pleistocene environmental change at Gona, in the lower Awash region of Ethiopia. Paleo-vertisols rich in soil carbonate are well exposed in all of our study sections. An increase in d¹³C values of pedogenic carbonates over the last 5 Ma provides insight to early hominid environments which were dominantly woodlands and grassy woodlands in the early Pliocene and became more open by the late Pliocene. d¹⁸O values also increase through time, from average values of –8.9‰ (PDB) in the early Pliocene, to –6.3 ‰ in the late Pliocene and Pleistocene. The wide range (-3.3 to –13.3‰) of d¹⁸O values in paleovertisol carbonates from all stratigraphic levels likely reflects strong seasonality and periods of intense evaporation throughout the record. Lower average d¹⁸O values of pedogenic carbonate in the early Pliocene deposits indicate an environment that was probably warmer and wetter than today. This shift in d¹⁸O values has also been documented by other studies of Plio-Pleistocene pedogenic carbonates in East Africa. Enamel from early Pliocene hippo teeth from our study area have relatively low d¹⁸O values, suggesting that the shift in d¹⁸O values in paleosols is produced in part by a regional shift in d¹⁸O values of meteoric water.