Geochemical analysis and physical characterization of fossilized soils during periods of early human activity and migration: a study of Gona, Ethiopia

Abstract

Environmental change is often inferred to have driven dispersals of Anatomically Modern Humans (AMH) out of Africa, yet the precise landscape context of these migrations remains unclear. Furthermore, fluvial-based archives of paleoenvironment during periods of dispersal are scarce. Gona, an area in northeastern Ethiopia with one of the most continuous records of East African Paleolithic and Neolithic archaeology, contains abundant Middle to Late Pleistocene fluvial deposits interbedded with tephra. This study examines the physical and chemical changes of 11 fossilized soils (paleosols) extracted from Gona’s paleoanthropological sites that range in age from the Middle to the Late Pleistocene (~380-11ka). The paleosols from Gona provide valuable insight into the landscapes in which our earliest direct ancestors interacted, as they are a dynamic biogeochemical archive of weathering, which is related to the surrounding environment at the time of formation. We focus on paleo-Vertisols, or paleosols with vertic features that formed in distal floodplain settings to provide a control on landscape position. Colorimeter results show that the paleosols become darker from the Middle to Late Pleistocene and Early Holocene. We also see an increase in organic carbon (OC) content with notable peaks at 77.5, 50, and 11 ka. CALMAG-based estimates of mean annual precipitation (MAP) range from 405 to 527 mm/yr (+/- 108), with the exception of notable increases at 77.5 (688 mm/yr +/- 108) and 11ka (656 mm/yr +/- 108). These increases in OC and MAP coincide with previously documented episodes of wetter climates - MIS-5a and the African Humid Period. The paleo-Vertisols at Gona record evidence of wetter paleoclimates during certain periods of AMH migration and suggest that this terrestrial fluvial record is a unique source for paleoenvironmental data. This localized terrestrial dataset complements additional regional-scale paleoenvironmental records when interpreting the forcing and responses of Out-of-Africa migrations.