An integrated study of the Homo-bearing Aalat stratigraphic section (Eritrea). An expanded continental record at the Early–Middle Pleistocene transition

The Early to Middle Pleistocene continental transition in East Africa is widely documented from lacustrine and deep-sea records, although significant insights are also provided by fluvio-lacustrine successions of the central and southern African Rift Valley, such as the at Olduvai Gorge succession (Tanzania), the Bouri Formation (Ethiopia) and the Olorgesailie Formation (Kenya). The Early to Middle Pleistocene Dandiero Basin fill (Eritrean Danakil) represents the only continental succession in the northernmost sector of the African Rift Valley that provided abundant fossil vertebrates, including human remains. The present study integrates already available data with new sedimentological, pedological, magnetostratigraphic, paleontological and paleoanthropological investigations of the 300 m thick Aalat section (North Dandiero Basin). This sedimentary succession records repeated shifts from fluvial to lacustrine depositional settings, which occurred under the tight interaction between local tectonics and Pleistocene climate changes. Accumulation was associated with axial sedimentation in a NS-trending extensional basin, with an overall tectono-sedimentary setting comparable with that of the coeval Bouri Formation (Ethiopia). Because of the high rates of sedimentation, a poor to moderate degree of soil development characterizes the whole succession. Sporadic soil horizons testify to carbonate dissolution, leaching and accumulation in calcic and petrocalcic horizons (indicating an overall dry climate). The alternate with local to extensive iron-oxide/hydroxide segregation, promoted by water infiltration under varying drainage conditions and/or seasonal contrast, that record more humid conditions. Magnetostratigraphic dating and correlation indicates that this section is among the world's thickest record embracing the Early-Middle Pleistocene transition, spanning from the Jaramillo to the base of Brunhes chron. The terrestrial vertebrate fauna includes a typical Early to Middle Pleistocene East African mammalian assemblage for this age and is dominated by taxa characterized by strong water dependence. The ichthyofauna, with its abundant Clariidae, is also consistent with the shallow water, fluvio-lacustrine paleobiotopes. The cranial, dental and postcranial human remains from the lower part of the Aalat succession add valuable evidence about the patterns of variation and evolutionary dynamics in African Homo erectus/ergaster near the end of the Early Pleistocene.