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Divided by Geochemistry, United by Geochronology: Tephrochronological Challenges in Turkana Basin, Kenya

Saini Samim , Hayden Dalton, David Phillips, Janet Hergt, Alan Greig

Abstract

The Turkana Basin, NW Kenya, is a renowned paleoanthropological site preserving fossiliferous sequences intercalated with volcanic ash (tuff) layers that provide critical age constraints for hominin species and their cultural technologies. Establishing a tephrostratigraphic framework within the basin relies heavily on 40Ar/39Ar geochronology derived from feldspars found in entrained pumice clasts and relating pumice glass chemistry to its host tuff. However, heterogeneous geochemical signatures found within volcanic products from a single eruption can complicate temporal correlation of tuffs. Here, we investigate two key tuff horizons --- the Morutot and the Orange tuffs --- spanning a critical period of lithic technological advancement in the Turkana Basin (e.g., from developed Oldowan to early Acheulean). Utilizing shard-specific major and LA-ICP-MS geochemistry, we identify at least two and four distinct glass geochemical signatures for Morutot and Orange tuffs, respectively. Despite these geochemical heterogeneities, we are able to reconcile the diverse geochemical products by obtaining high precision 40Ar/39Ar ages utlising new-generation multi-collector mass spectrometers (e.g., ARGUS VI+) and tie them to distinct eruption events. High-precision 40Ar/39Ar ages yield distinct eruption ages for the Morutot Tuff at 1,618.5 ± 0.6 ka (±1.2 ka, 2σ, including external uncertainties) and Orange Tuff at 1,758.5 ± 0.4 ka (± 0.8 ka). This integrated approach not only refines the tephrostratigraphic framework of the region, but also provides insights into silicic magma dynamics, highlighting the potential of combining geochronology with detailed geochemical analysis to address existing challenges in tephrostratigraphy.