GEOCHEMICAL ANALYSIS OF TUFFS AT THE ARDIPITHECUS RAMIDUS SITE AT ARAMIS, ETHIOPIA

Three tuff layers were sampled from the Lower Aramis Member at Aramis, Ethiopia, where the in-situ skeleton of Ardipithecus ramidus (4.4 Ma old) was excavated. These tephra layers that bracketed the basal hominin fossil are intercalated with floodplain mudstones and sandstones. From older to younger, these tuffs include the 1+ m thick Gaala Tuff Complex (GATC), an unnamed tuff that is 20 cm thick, and the 10 cm thick Daam Aatu Basaltic Tuff (DABT). According to previous publications, the GATC and DABT have statistically indistinguishable ⁴⁰Ar/³⁹Ar dates of 4.4 Ma.

This on-going study uses electron microprobe, SEM microscopy, Raman microscopy, and thin-section petrography to investigate geochemical signatures of these tuff layers. Results of the electron microprobe analysis are compiled to make a number of bivariate plots like iron (III) oxide against aluminum oxide, calcium oxide, and titanium oxide. SiO₂, Na₂O and K₂O contents are also assessed, particularly to characterize the unnamed tuff. These analyses suggest that the unnamed tuff is predominantly trachytic in character. Three tuff layers are largely indistinguishable from each other in the bivariate plots (Fe₂O₃ vs Al₂O₃, Fe₂O₃ vs CaO, and Fe₂O₃ vs TiO₂). Whereas TiO₂ vs Cl plot is found effective in distinguishing these tephra layers. Geochemical fingerprints of these tuffs can be useful in correlating these event beds with tephra layers in other hominin fossil sites in East Africa.