Revisiting the paleoclimate, paleoecology, and age of the Middle Miocene Nyakach Formation, Western Kenya

The Nyakach Formation of western Kenya provides a rare Middle Miocene (ca. 14–16 Ma.) terrestrial record bearing the monkey, Victoriapithecus macinnesi, and associated vertebrate faunas. Here we integrate sedimentology, clay mineralogy, paleosol geochemistry, phytoliths, and δ¹³C of soil organic matter to refine the age, depositional setting, and paleoecology of the Nyakach Formation. Stratigraphic and lithological evidence indicates subaerial deposition within a rift-margin fluvial system influenced by episodic volcanism, with eastern and western sites reflecting multiple sediment sources and variable degrees of reworking. Smectite-rich, base-saturated paleosols with high CaO concentrations point to carbonatitic parent material and a seasonally variable, but overall humid hydroclimates. Proxy-based precipitation estimates from paleosol geochemistry yield subhumid–humid conditions (ca. 1100–1400 mm/yr), which are substantially wetter than earlier reconstructions, but similar to consensus values from the Miocene Model Intercomparison project (MioMIP1). Phytolith assemblages dominated by forest-indicator morphotypes, together with δ¹³C analyses on paleosol organic matter indicating predominantly C₃ vegetation and only limited C₄ biomass, support the reconstruction of a warm, tropical, seasonally wet forest with localized grassy openings. These findings highlight a heterogeneous but forest-dominated ecosystem during the Langhian stage, shaped by volcanism, tectonism, and fluvial dynamics, and provide refined ecological context for Middle Miocene primates and faunas inhabiting the Nyanza Rift region.