Abstract

The East African Rift System records a key interval in the evolution of modern African ecosystems, documenting significant floral changes and faunal dispersals in the context of environmental shifts. To date, Miocene-to-Pliocene data from eastern Africa have been derived primarily from richly fossiliferous rift basins along the far north of the Eastern Branch of the rift, with more limited windows emerging from the Malawi Rift and more recently, coastal Mozambique. Here, we present the first quantitative paleoclimate data for the Miocene–Pliocene transition from the Western Branch of the East African Rift System, based on analyses of paleosols from the Rukwa Rift Basin. Paleosols derived from the fossiliferous late Miocene–early Pliocene lower Lake Beds succession in southwestern Tanzania preserve a shallow lacustrine setting grading into a system of alluvial fans and braided rivers with abundant floodplain deposits. Paleoclimate reconstructions using bulk geochemistry and clay mineralogy reveal a highly seasonal, semiarid, mesic climate during the late Miocene, with increased moisture availability in the early Pliocene resulting in a shift to subhumid conditions. Stable-carbon-isotope composition of pedogenic carbonates document a woodland/bushland/shrubland paleoenvironment across the Miocene–Pliocene transition. Results support the presence of Pliocene subhumid to humid habitats, dominated by woody vegetation offering shade, food, and water for faunal dispersal along an inland corridor connecting northern segments of the East African Rift System with southern Africa.