EQUATORIAL PALEOENVIRONMENT LEADING INTO THE MIOCENE CLIMATIC OPTIMUM: LESSONS LEARNED FROM RUSINGA AND MFANGANO ISLANDS, LAKE VICTORIA KENYA

During the Cenozoic, there is a gradual decline in CO₂ and a subsequent global cooling that is punctuated by rapid warming events. The most recent of these pronounced warming intervals is the Miocene Climatic Optimum, which sets the stage for Neogene and Quaternary biota and climate. The strata on Rusinga and Mfangano Islands record a series of complex, changing ecosystems in the early Miocene from ~17.5 – 19.7 Ma, just prior to the Miocene Climatic Optimum. The Wayando Formation (~18.5 – 19.7 Ma) has poorly developed paleosols with groundwater carbonates and permineralized sedges. The d¹³C of soil organic material varies between -21.5‰ and -24.6‰ (n=5), indicating either water-stressed C₃ photosynthesizing plants or mixed C₃ and C₄ vegetation. The paleosols, with vadose and early diagenetic phreatic cements and weak development suggest a subhumid to semiarid climate and the sedges indicate at least seasonally available standing water. This formation is contrasted with the ~18.2 – 17.5 Ma Hiwegi Formation, which records a dynamic, fluctuating climate. The only palesol in the Grit Member preserves soft powdery carbonate masses indicating subhumid to semiarid conditions. Fossils leaves from the Grit Member also suggest a hot, relatively dry climate with seasonal precipitation. The Grit Member-Fossil Bed Member contact preserves evidence of a local, possibly closed, shallow saline lake, with salt hoppers, satin-spar after gypsum deposits and other desiccation features, indicating an arid to semi-arid climate. The upper Fossil Bed and Kibanga Members have multiple paleosols that contain thick, laterally extensive permineralized roots, and in one paleosol tree stump casts, and fossil leaves that indicate the presence of a relatively widespread, dense, closed canopy forest. The d¹³C of soil organic material varies between -24.0‰ and -25.8‰ (n=12). These values are indicative of strictly C₃ vegetation, such as are found in forests and semiarid to humid climates. Taken together, this study suggests a long-term trend towards increasing precipitation in equatorial East Africa in the early Miocene indication that equatorial African climates may have been changing considerably leading into the Miocene Climatic Optimum.