Absolute dating of an Early Paleolithic site in Western Africa based on the radioactive decay of in situ-produced 10Be and 26Al

Abstract

Along the Angolan coast, the Early Paleolithic sites of Dungo IV and V (Baia Farta, Benguela) have delivered a rich pre-Acheulean lithic industry testifying the antiquity of the hominin settlement in western Africa despite the current absence of any hominin fossil in the area. In Dungo IV, the Paleolithic level is located on a conglomeratic paleo-beach (104 m a.s.l.) buried under an at least 3 m thick sandy layer. In Dungo V, two unearthed large whale fossils are associated with numerous lithic tools intimately mixed with the whale bones. This is the oldest evidence of stranded marine mammal scavenging by hominins in this part of Africa. The lack of volcanism and fossils makes chronological constrain difficult. Considering its configuration, the Dungo IV site may be relevant for a dating based on both the 10Be and 26Al cosmogenic nuclides. For this purpose, a depth profile all along the sandy layer overlying the archeological layer has been sampled. Statistical treatments performed on the 26Al/10Be ratios obtained for the depth profile demonstrate that they all belong to the same population. If we consider that the samples have always been at or close to their sampling depth, the regression modeling allows computing that the surface sedimentary layer has been emplaced at least 614 ka ago and less than 662 ka ago. On the other hand, if we consider that the surface deposit has been truncated, burial durations ranging from 585 ka to 786 ka and truncations lower than 4 m result from the modeling of the evolution of the 10Be and 26Al concentrations as a function of depth.The analyses of four pre-Acheulean artefacts lead to a minimum burial duration of 730 ka and a maximum burial duration of 2.11 Ma.The low pre-burial denudation rates modeled from the data acquired for the stone tools as well as for the overlying layer (1–16 m.Ma−1) imply large inherited 26Al and 10Be concentrations. The post-depositional maximum denudation rate of 71 m.Ma−1 associated with both the lithic artefacts and the surface sedimentary layer (considering that the samples have always been at or close to their sampling depth) as well as the deduced maximum uplift rate of ∼170 m.Ma−1 are in agreement with the known tectonic evolution and the climatic variability of this area.This study confirms the antiquity of the hominin presence in western Africa more than 2000 km away from the closest old hominin fossil sites.