Abstract
The impact of changing environments on the evolution and dispersal of Homo sapiens is highly debated, but few data are available from equatorial Africa. Lake Victoria is the largest freshwater lake in the tropics and is currently a biogeographic barrier between the eastern and western branches of the East African Rift. The lake has previously desiccated at ~17 ka and again at ~15 ka, but little is known from this region prior to the Last Glacial Maximum. The Pleistocene terrestrial deposits on the northeast coast of Lake Victoria (94 to 36 ka) are ideal for paleoenvironmental reconstructions where volcaniclastic deposits (tuffs), fluvial deposits, tufa, and paleosols are exposed, which can be used to reconstruct Critical Zones (CZ) of the past (paleo-CZs). The paleo-CZ is a holistic concept that reconstructs the entire landscape using geologic records of the atmosphere, hydrosphere, lithosphere, biosphere, and pedosphere (the focus of this study). New paleosol-based mean annual precipitation (MAP) proxies from Karungu, Rusinga Island, and Mfangano Island indicate an average MAP of 750108 mm yr-1 (CALMAG), 800182 mm yr-1 (CIA-K), and 1010228 mm yr-1 (PPM1.0) with no statistical difference throughout the 11 m thick sequence. This corresponds to between 54 and 72% of modern precipitation. Tephras bracketing these paleosols have been correlated across seven sites, and sample a regional paleo-CZ across a ~55 km transect along the eastern shoreline of the modern lake. Given the sensitivity of Lake Victoria to precipitation, it is likely that the lake was significantly smaller than modern between 94 ka and 36 ka. This would have removed a major barrier for the movement of fauna (including early modern humans) and provided a dispersal corridor across the equator and between the rifts. It is also consistent with the associated fossil faunal assemblage indicative of semi-arid grasslands. During the Late Pleistocene, the combined geologic and paleontological evidence suggests a seasonally dry, open grassland environment for the Lake Victoria region that is significantly drier than today, which may have facilitated human and faunal dispersals across equatorial East Africa.
Highlights
The interdisciplinary Critical Zone (CZ) research initiative opened up a new way of thinking about near surface environments where complex interactions between rock, soil, water, air, and living organisms shape the Earth’s surface (National Research Council, 2001)
The results presented in this study indicate that the paleosols of the late Pleistocene deposits in the northeastern Lake Victoria Basin record a much drier climate (54–72% of modern mean annual precipitation (MAP)) along the northeastern coast of Lake Victoria during the Late Pleistocene (94–36 ka)
Paleosol features and bulk geochemistry indicate that the paleo-Critical Zones (CZ) were seasonally dry but fertile soils; high salinity and sodicity would likely have limited the vegetation to those tolerate of higher levels
Summary
The interdisciplinary Critical Zone (CZ) research initiative opened up a new way of thinking about near surface environments where complex interactions between rock, soil, water, air, and living organisms shape the Earth’s surface (National Research Council, 2001). Proxies for the atmosphere (e.g., mean annual precipitation, mean annual temperature), biosphere (photosynthetic pathway), hydrosphere (flood frequency and magnitude), and pedosphere (redox, pH, soil fertility) have all been developed to reconstruct past land surfaces (Nordt and Driese, 2013). The combination of these interdisciplinary methods allow for the reconstruction of biogeochemical interactions within a past landscape comparable to modern CZ studies (Amundson et al, 2007; Brantley et al, 2007; Dere et al, 2015) and termed the paleo-Critical Zone (paleo-CZ) by Nordt and Driese (2013). The paleo-CZ concept has continued to gain traction in the sedimentary geology community and especially those interested in paleosols
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