4-5 Revisiting the Paleoclimate and Paleoecology of Middle Miocene Nyakach Formation, Western Kenya

Session: Recent Advances in Soil and Paleosol Science

Presenting Author:

Venanzio Munyaka

Authors:

Munyaka, Venanzio¹, Peppe, Daniel J.², Strömberg, Caroline A. E.³, McNulty, Kieran P.⁴, Deino, Alan L.⁵, Muteti, Samuel⁶

(1) Baylor University, Department of Geosciences, Waco, TX, USA, (2) Baylor University, Department of Geosciences, Waco, TX, USA, (3) Department of Biology & Burke Museum of Natural History and Culture, Seattle, WA, USA, (4) Department of Ecology, Evolution & Behavior, Minneapolis, MN, USA, (5) Berkeley Geochronology Center, Berkely, CA, USA, (6) National Museums of Kenya, Department of Earth Sciences, Nairobi, Kenya; Department of Ecology, Evolution & Behavior, Minneapolis, MN, USA,

Abstract:

It is often suggested that during the Middle Miocene, eastern Africa was dominated by open, seasonally arid wooded savannas, shaped by local tectonics and climatic changes associated with the Mid Miocene Climatic Optimum (MMCO) and the Middle Miocene Climatic Transition (MMCT). To test this hypothesis, we present a multiproxy reconstruction of paleoclimate and paleoenvironment from the Middle Miocene Nyakach Formation along the southern flanks of the Nyanza Rift in western Kenya. We report results from localities east and west of Sondu town. Eastern sites are characterized by pink to red polymictic conglomerates with quartz and granodiorite clasts and abundant paleosols. Western sites show fewer, poorly developed paleosols and buff to green sandstones containing basalt clasts and amphibole crystals. Despite lithological differences, vertebrate fossils consistently occur within coarse-grained sandstones and conglomerates across all sites. Bulk organic carbon concentrations are low (~0.04 wt.%), with δ¹³C_OM values ranging from –17‰ to –32‰ (mean –23‰), suggesting C₃ vegetation. Precipitation estimates from a random forest model for mean annual precipitation (RF‐MAP_1.0) that uses paleosol geochemistry as input data, range from 500 to 2000 mm/yr (average ~1400 mm/yr, humid), while the E2MMIO paleoclimate simulations under CO₂ scenarios of 200–800 ppm yields narrower MAP estimates (750–1160 mm/yr, average ~1100 mm/yr). Despite model uncertainties, the data are consistent with a humid, tropical seasonal forest environment. This interpretation is corroborated by sparse phytolith assemblages, which include a high proportion of forest indicators (>50%) and relatively low representation (~20%) of open-habitat grasses from the PACMAD (Panicoideae, Arundinoideae, Chloridoideae, Micrairoideae, Aristidoideae, Danthonioideae) clade, alongside rare palm phytoliths. Taken together, the Nyakach sites were part of a fluvially dominated tropical seasonal forest, with an understory of PACMAD grasses and palms, challenging the assumption of widespread open savanna during this interval in eastern Africa.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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