Late Neogene terrestrial climate reconstruction of the central Namib Desert derived by the combination of U–Pb silcrete and terrestrial cosmogenic nuclide exposure dating

Abstract

The Cenozoic “Namib Group” of the Namib Desert relies on relative chronology and lacks direct radiometric dating. Therefore, the landscape evolution and paleoclimate of the central Namib Desert remains imprecise, hindering the detailed search for global and/or local forcing factors for the aridification of the Namib. The broad presence of silcretes and calcretes in the Namib Desert allows the application of the of the U–Pb laser ablation dating technique on silcretes and calcretes to date important phases of landscape stability and to retrieve crucial paleoclimatic and environmental information on desertification and its paleoclimatic variability. Microscale silcrete formation (maximum of 8 mm), as a result of pressure solution by expanding calcrete cementation, grants the opportunity to date multiple phases (multiple generations of silcrete as growing layers) of silcrete formation. Groundwater silcrete and calcrete formation took place at our study site during the Pliocene, an epoch of relatively stable climate and landscape evolution under semi-arid to arid conditions. Terrestrial cosmogenic nuclide (TCN) exposure dates from flat canyon rim surfaces show the remission of groundwater calcrete formation due to river incision during Late Pliocene–Early Pleistocene. This incision is a consequence of a large-scale landscape rejuvenation caused by a climate shift towards more arid conditions in the Pleistocene, which can be connected to global climate patterns. This study shows the feasibility of applying U–Pb laser ablation to groundwater calcretes and silcretes, discusses important issues associated with this technique, and opens up the possibility of dating numerous sedimentary sequences with silcretes and calcretes in arid environments. Our study redefines and improves the generally accepted Late Cenozoic chronostratigraphy of the Namib Desert (Miller, 2008).