Paleoclimatic and paleoecological reconstruction of early Miocene terrestrial equatorial deposits, Rusinga and Mfangano Islands, Lake Victoria, Kenya

Abstract

Biological responses to climatic shifts are often studied to inform us on future anthropogenic-driven climate change. However, few of these climatic shifts occur over time scales appropriate to modern change and few occur with biota similar to modern. The Miocene Climatic Optimum is an ideal interval to study because of its rapid duration and because it occurred during the rise and proliferation of apes. The sediments on Rusinga and Mfangano Islands, Lake Victoria, Kenya were deposited between 18 and 20 Ma and record a changing equatorial climate just prior to the Miocene Climate Optimum. This location also offers an opportunity to use multiple proxies to constrain climate and landscape, including paleosol geochemistry, paleobotany and paleontology. Additionally, due to the rich fossil preservation on the islands, climatic shifts are framed within the context of early caterrhine evolution. Here, we report a climate shift recorded through three time slices spanning two formations over ~2 myr. The oldest unit, the Wayando Formation, records an arid, probably open ecosystem with pedogenic calcite rhizoliths, a high groundwater table, poorly-formed paleosols and permineralized sedges. The middle time slice, the Grit Member-Fossil Bed Member contact of the Hiwegi Formation, shows evidence of a local saline lake, with desiccation features, satin-spar after gypsum deposits and salt hoppers. Paleobotanical and sedimentological data from roughly contemporaneous strata indicate a warm, highly seasonal environment that supported a mixture of woodland and forested elements across the landscape. The youngest unit, which is within the Kibanga Member of the Hiwegi Formation, displays demonstrable evidence for a closed-canopy multistoried forest with the presence of tree-stump casts and permineralized root systems within a red-brown paleosol. Within the same paleosol horizon, the dental remains of the catarrhines Proconsul and Dendropithecus have been discovered in situ. This stratum is overlain by a sandstone with preserved leaves. Bulk soil geochemistry and paleobotanical proxies suggest the climate was warm (MAT estimates range = 22.6 - 34.5 °C) and relatively wet (MAP estimates range = 700 - 2,620 mm). This study suggests that tropical Africa may have had a much more dynamic climate leading up to the Miocene Climatic Optimum than previously suggested, and underwent considerable ecosystem reorganization both before and after the Miocene Climatic Optimum.