The persistence of salt-affected paleosols at Gona, Ethiopia: a sedimentary archive o Middle to Late Pleistocene soil salinity within a corridor of early human migration

Abstract

The origin of Homo sapiens and their subsequent dispersal out of Africa during the Mid-to-Late Pleistocene are hallmark events in the evolution of humans and are recorded as snapshots within terrestrial sedimentary deposits in East Africa. This study uses paleosols to reconstruct paleoenvironments of those events at Gona, Ethiopia - an area with one of the most continuous records of East African Paleolithic and Neolithic archaeology. A preliminary survey of Gona paleosols from the past 250,000 years shows a wide range of paleosol types that resemble modern-day Entisols, Inceptisols and Vertisols. Approximate paleosol ages were estimated using nearby OSL, 14 C, U-series, 40 Ar/ 39 Ar and volcanic glass chemistry. The pH and EC of these paleosols were measured as they are weakly compacted and show few signs of diagenesis. Mean pH values are slightly alkaline (7.8 ±0.63) and mean EC values (9.8 ±7.9) are saline, suggesting salt-affected soil development. The high standard deviation of EC results from low-EC sodic soils found in some units. These pH and EC findings are consistent with bulk geochemical-based pedotransfer functions on paleo-Vertisols, which indicate the presence of saline and sodic conditions. Much like modern-day tributary and trunk-channel floodplains, these paleosols likely hosted halophytes like Tamarix, Vachellia and salt-tolerant grasses, flora which are commonly found at present-day Gona. Notably, some late Pleistocene salt-affected paleosols that coincide with Marine Isotope Stage 5 and the African Humid Period show signs of prolonged soil saturation and nearby standing water. High evapotranspiration due to Gona’s semi-arid climate likely resulted in low infiltration and subsurface drainage of water, insufficient to transport salt out of the system, resulting in abundant saline soil formation at the site. The high salinity of Mid-to-Late Pleistocene Gona paleosols, likely due to source material, climate, and drainage, limits the use of many bulk geochemical proxies that were developed using mostly normal, non-saline soils. Despite this, the results of this study shed light on the climate and environment of our ancestors at the pedon scale.