Paleoclimate Reconstructions of the Early Miocene from Stratigraphic and X-ray Diffraction Analysis of Paleosols from the Kiahera Formation, Rusinga Island, Lake Victoria, Kenya

Abstract

The early Miocene deposits on Rusinga Island, Lake Victoria, Kenya have been continuously studied for over 80 years to learn more about the paleontology, including early hominids, such as the ape Ekembo. Most of the previous research has focused on the Hiwegi Formation, where the vast majority of fossils have been discovered, leading to understudying other formations including the Kiahera Formation which recently has been revealed as invaluable for the preservation of climate information. The Kiahera Formation contains many fossils and fossilized soils (paleosols), and a climatic history can be reconstructed through a combination of stratigraphic mapping and x-ray diffraction of the clay minerals of the paleosols. Lower members of the site observed, R73, are composed of volcaniclastics including igneous rock fragments, volcanic breccias, volcanic bombs, and ash which we interpret as evidence of active volcanism. The younger member is made up of alternating strata of cross-bedded sandstones that scoured into the underlying layers and paleosols which we interpret as evidence of the volcano ceasing to erupt, and evidence of climate. Within the Kiahera Formation two paleosol types are found: Calcisols and Vertisols; the former suggests a semi-arid to arid climate in which evaporation was greater than precipitation and the latter suggest a seasonal water budget surplus and deficit. X-ray diffraction analysis of the paleosols determines a composition of montmorillonite, kaolinite, and illite which is similar in composition to modern Vertisols. These results suggest paleoclimates dominated by monsoonal conditions, possibly the East African Monsoon which today dominates the region.