UNRAVELLING STRATIGRAPHIC COMPLEXITIES IN THE NARIOKOTOME TUFF COMPLEX USING A COMBINATION OF HIGH-RESOLUTION TEPHROCHRONOLOGICAL TOOLS

The Turkana Basin, Kenya, is renowned for the discoveries of numerous Plio-Pleistocene archaeological sites containing multiple hominin species and their associated cultural technologies. The sedimentary sequences in the Turkana Basin are interbedded with volcanic ash layers (tuffs) that provide age constraints for their enclosed palaeontological and archaeological artefacts. The Nariokotome Tuff Complex in West Turkana comprises three closely spaced tuffs: the Lower, the Middle and the Upper Nariokotome. These tuffs are important for recognising temporal equivalent strata for intra-basin correlation. In addition, they provide age constraints for the remarkably complete skeleton, Nariokotome Boy, as well as the rarely documented Lower-early Middle Pleistocene archaeological sites in East Africa, the Nadung’a sites. Despite their importance, the correct identification and age assignments of these tuffs are hindered by these existing stratigraphic complexities: a) the inability to distinguish the eruption ages of the three tuff units (because published age constraints overlap), b) the similar major element compositions of the Middle and the Upper Nariokotome tuffs, c) fluvial reworking of pumice clasts (the target rock for age determinations) from older into younger tuff layers resulting in erroneous age estimates for a given layer, further inhibiting correct age assignments for a specific site.

Here, we address these problems by combining single-grain, ultra-high resolution ⁴⁰Ar-³⁹Ar geochronology of feldspars from pumices enclosed within tuffs, and novel LA-ICP-MS trace element geochemistry applied to tuff and pumice glass samples. Utilising modern day mass-spectrometers we report the first distinguishable ages for the three Nariokotome tuffs; Lower (1,284.3 ± 2.4 ka; 2σ), Middle (1,263.4 ± 1.9 ka) and Upper (1,228.3 ± 1.2 ka) Nariokotome. In addition, by application of LA-ICP-MS trace element geochemistry to individual tuff glasses and pumice clasts, we obtain unique geochemical fingerprints for these temporally distinct layers and confirm the comagmatic origin of pumice clasts selected for dating. The combined methodology therefore refines the regional tephrostratigraphy, provide tighter age constraints, and demonstrates the potential to resolve the stratigraphic complexities associated with assigning ages to archaeological sites.