Paleosols and interpreted landscapes associated with Pliocene hominins from Woranso-Mille, Ethiopia

ABSTRACT The Woranso-Mille paleontological research area in the Afar region of Ethiopia has yielded fossils of several species of Pliocene hominins, including at least two species of Australopithecus in the ca 3.5–3.2-million-year-old strata of the Burtele (BRT), Leado Dido’a (LDD), Nefuraytu (NFR), and Kuserale Dora (KSA) collection areas that are the focus of this study. In these collection areas, A. deyiremeda is found at BRT, whereas A. afarensis is found at LDD and NFR. A significant part of the strata includes successions of paleosols that provide critical information about the landscapes on which these hominins and other species walked, foraged, and dwelled. Nine primary pedotypes, interpreted as various Entisols, Inceptisols, and Vertisols, occurred in association with fluvial sandstones and floodplain pond deposits. The environments in which these paleosols formed include levee, proximal floodplain, clastic wetland, seasonal wetland, and distal floodplain in a seasonally dry climate. Many paleosols show overprinting indicating localized changes in soil forming conditions, including sediment input and moisture levels, over 101 to 102 years. Climate proxies from the paleosols suggest a seasonally dry environment and generally well-drained soils; however, pedogenic features also indicate areas that are permanently to seasonally wet to waterlogged. Across the different collection areas, we observe a greater number of paleosols with lower levels of pedogenic development and evidence of locally high moisture content in the older paleosol successions (BRT/LDD) followed by an increase in the number of paleosols with high levels of development in the younger sections (NFR/KSA). These shifts in soil-forming processes reflect changes in parent material, sedimentation rate, landscape stability, soil moisture, and soil drainage that relate to localized changes in fluvial style, likely the product of autogenic processes. Our observations of rhizoliths in these paleosols reflect landscape-scale changes and suggest a shift from predominantly patchy, early successional vegetation and early shrublands at BRT, early shrublands to mixed woodlands at LDD, to more laterally expansive shrublands and grasslands at NFR and KSA. In sum, most of the paleosols at BRT suggest the presence of a mosaic of poorly developed soils on an unstable landscape with largely patchy, fast-growing weedy vegetation. In contrast, paleosols at LDD, NFR, and KSA reflect a generally more stable but still heterogenous landscape, characterized by expansive grasslands and shrublands growing on well-developed soils. The differences in the paleosols in the collection areas suggest potential differences in habitat preferences by A. deyiremeda and A. afaraensis.