An integrated study of the 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 lacustrineand deep-sea records, although significant insights are also provided by fluvio-lacustrine successionsof 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 PleistoceneDandiero Basin fill (Eritrean Danakil) represents the only continental succession in the northernmostsector of the African Rift Valley that provided abundant fossil vertebrates, including human remains. Thepresent study integrates already available data with new sedimentological, pedological, magnetostratigraphic,paleontological and paleoanthropological investigations of the 300 m thick Aalat section (NorthDandiero Basin). This sedimentary succession records repeated shifts from fluvial to lacustrine depositionalsettings, which occurred under the tight interaction between local tectonics and Pleistoceneclimate changes. Accumulation was associated with axial sedimentation in a NS-trending extensionalbasin, 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 developmentcharacterizes the whole succession. Sporadic soil horizons testify to carbonate dissolution, leaching andaccumulation in calcic and petrocalcic horizons (indicating an overall dry climate). The alternate withlocal to extensive iron-oxide/hydroxide segregation, promoted by water infiltration under varyingdrainage conditions and/or seasonal contrast, that record more humid conditions. Magnetostratigraphicdating and correlation indicates that this section is among the world's thickest record embracing theEarly-Middle Pleistocene transition, spanning from the Jaramillo to the base of Brunhes chron. Theterrestrial vertebrate fauna includes a typical Early to Middle Pleistocene East African mammalianassemblage for this age and is dominated by taxa characterized by strong water dependence. Theichthyofauna, 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 Aalatsuccession add valuable evidence about the patterns of variation and evolutionary dynamics in AfricanHomo erectus/ergaster near the end of the Early Pleistocene.