Paleoclimate reconstruction using Pleistocene paleosols along the middle Atbara River in Eastern Sudan

Along the middle Atbara River, Pleistocene alluvial sediments dated to ~200 to 20 ka are exposed in eastern Sudan over a length of about 200 km with a maximum thickness of 50 m. The Atbara River today has its headwaters in the northern Ethiopian Plateau and is the last major tributary of the Nile before it flows through the Sahara. Previous studies and our fieldwork since 2018 have resulted in extensive finds of fossil terrestrial mammals - including hominins - and archaeological stone tools in these Pleistocene sediments. Intercalated within the alluvial sedimentary sequences are paleosols distinguished by various pedogenic features such as calcretes, root structures and pedogenic slickensides. These paleosols are especially interesting archives of the paleoclimate and paleoenvironment during the deposition of these sedimentary sequences. To study the paleosols, mineralogical, petrographical and geochemical investigations were carried out. Field investigations as well as mineralogical analyses show that mainly paleosols like aridisols (desert soils) and vertisols (swelling clay soils) are present. Aridisols are characterized by calcic layers and calcareous nodules and root structures (calcic horizon Bk) and contain minerals such as calcite, ferrodolomite and gypsum. Clay minerals (argillic horizon Bt) such as smectite, illite, illite/smectite, chlorite and other easily weatherable minerals (e. g., feldspars) are also common. Vertisols have a blocky appearance with peds (cracks) and pedogenic slickenside structures due to the shrinking and swelling of smectitic clays (argillic horizon Bt). Aridisols reflect arid to semiarid climates, while vertisols occur in regions with subhumid to semi-arid climates. The mineralogy (abundant plagioclase and chlorite) of both paleosols indicates basaltic source rocks, which are widespread in the study area. The δ13CVPDB isotopic values of the pedogenic carbonates show that the vegetation varied between grassland, wooded grassland and woodland/bushland/thicket/shrubland, which can occur in both aridisols and vertisols. The δ18OVPDB values mainly reflect a slightly increased evaporation/precipitation ratio and thus a semi-arid climate during the Mid-Late Pleistocene, similar to conditions today.