Carbon and oxygen isotopic signature of tooth enamel as a proxy for the Pliocene paleoenvironment and paleoclimate from Mt. Galili Formation, Ethiopia

Analyses of the isotope ratios of naturally existing materials have increasingly been used to reconstruct paleoenvironmental and paleoclimatic conditions. In this study carbon (C) and oxygen (O) isotopic composition of bioapatite from fossil tooth enamel is used to reconstruct Pliocene environmental and climatic changes at Galili, in the Awash valley of Ethiopia. Enamel samples were obtained from 15 different mammalian taxa. The samples were collected from multiple localities at different stratigraphic levels spanning between >5.29 Ma and ~3.6 Ma. The wide range of samples helps to develop both spatial and temporal framework of mammalian dietary adaptation, habitat distribution, and climate change and variability within the Galili Formation. The mammalian dietary adaptations at Galili indicate the presence of a variety of vegetation, which includes gallery forest, closed woodland, open woodland and grasslands with a heterogeneous habitat structure on the landscape. However, the temporal change indicates a general tendency towards more open habitat (i.e., grassland to wooded grassland). It is also worth noting the significance of C4 grasses on the paleolandscape by Late Miocene or early Pliocene at the latest as it could have an important implication on hominin morphological adaptation (e.g., bipedalism). The oxygen isotopic composition of tooth enamel indicates a variable temporal climatic condition, where the middle Pliocene (~4.38 Ma - ~3.6 Ma) was wetter than the early Pliocene (>5.29 Ma - ~4.38 Ma). Similarly, estimated δ18O of meteoric water based on the most 18O-depleted hippo tooth enamel, displays range of values between -1.8% (VSMOW) and -5.24% (VSMOW), which elucidate changes in the isotopic composition of source water and hence precipitation. In general, by comparing the result of this study with data from younger localities in the region, it is evident that there is a major shift in the isotopic composition of water at ~ 3.0 Ma, which was accompanied by a 6.0% increase from middle Pliocene to the present, while a 3.8% shift from early to middle Pliocene. These changes during the Pliocene in the region may in part be attributed to a regional decrease in the amount of precipitation, rift formation, shift in the position of the Inter Tropical Convergence Zone (ITCZ) and changes in the moisture source superimposed on global climate changes.