PLANT WAX RECONSTRUCTIONS OF PALEOECOLOGY AND PALEOHYDROLOGY AT WORANSO-MILLE, ETHIOPIA

The Woranso-Mille locality in the Afar region of Ethiopia is an important site for the understanding of human evolution as demonstrated by the discovery of the “Burtele” foot with an opposable big toe, and the newly named hominin species Australopithecus deyiremeda. Paleoenvironmental changes are thought to be important drivers of hominin evolution, and as such we aim to reconstruct the mid-Pliocene environments inhabited by early hominins. Paleoenvironmental reconstructions at Woranso-Mille rely heavily upon isotopic analysis of soil carbonate nodules and the distributions of mammalian fauna, but not all environments form carbonates or have large assemblages of fossil mammals. As such, we have initiated a study of the geochemistry of plant waxes from terrestrial sediments that will enable us to increase the spatial and temporal resolution of paleoenvironmental reconstructions at Woranso-Mille in the Pliocene. We measure δ¹³C and δD of n-alkanoic acids from 26 samples that include lacustrine clays, alluvial and lacustrine silts, and paleosols. Among the paleosols sampled (n = 19), 9 of them also have δ¹³C and δ¹⁸O values of soil carbonates from the same units. Initial results for 10 out of the 27 leaf wax samples show that C₂₈ n-alkanoic acid concentrations range from 0.6 to 218 ng/g of sediment extracted, demonstrating the preservation of plant waxes and the feasibility of compound-specific carbon and hydrogen isotopic measurements. We find that δ¹³C of C₂₈ n-alkanoic acid values range from -36 ‰ to -23 ‰ (relative to VPDB) and δD values range from -183 ‰ to -121 ‰ (relative to VSMOW). Paleosol samples (n = 5) typically have a less negative δ¹³C of C₂₈ n-alkanoic acid (range of -30 to -23 ‰) compared to silts (n = 5, range of -36 to -26 ‰). This may represent lower δ¹³C values indicative of C3 vegetation adjacent to rivers and lakes. Regardless, the large range in carbon isotopic values suggests that environmental composition ranged from C3 dominated to C4 dominated over small spatial (2 km²) and temporal scales. This study will compare carbon and hydrogen isotope values from leaf waxes with carbon and oxygen isotopes of soil carbonates and provide better constraints on the vegetation types and water stresses that were present during the mid-Pliocene at Woranso-Mille.