High-resolution tephrochronology resolves stratigraphic complexities in archaeologically significant Nariokotome tuffs, Turkana Basin

Abstract

The Turkana Basin is a renowned paleoanthropological region in Kenya and Ethiopia and is famous for discoveries of numerous hominin fossils and their associated cultural technologies. The Plio-Pleistocene sedimentary sequences hosting these important remains are interbedded with volcanic ash (tuff) beds that provide crucial bracketing age constraints. The Nariokotome Tuff Complex, comprising the Upper, Middle, and Lower Nariokotome Tuffs, preserves deposits that cover an important time interval during the late Early Pleistocene that saw milestone events for Homo erectus and associated technological development. Unfortunately, characterization of these tuffs has been hampered by a) overlapping published eruption ages and b) indistinguishable major element compositions. In addition, fluvial reworking of feldspar-bearing pumice clasts (the target rock for age determinations) from older volcanic deposits into younger tuff layers complicates correct age assignments. Here, we use multiple tephrochronological correlation tools, including high-resolution 40 Ar/ 39 Ar geochronology and grain-specific major- and trace-element geochemistry, to establish a well-characterized geochemical and geochronological framework for the Nariokotome tuffs. Utilizing a modern-generation mass-spectrometer, we report distinct ages for the Upper Nariokotome Tuff at 1,233.1 ± 1.3 ka (± 1.9 ka, 2σ; including external uncertainties), the Middle Nariokotome Tuff at 1,263.4 ± 1.2 ka (± 1.9 ka) and the Lower Nariokotome Tuff at 1,285.8 ± 1.0 ka (± 2.1 ka). In addition, high-spatial resolution Laser Ablation Inductively Coupled Mass Spectrometry trace element compositions provide distinct characterization of each tuff, aiding intrabasin correlation of these units. This combined methodology demonstrates the potential to resolve the stratigraphic complexities associated with assigning ages to key paleoanthropological sites.