Reconstruction of Late Pleistocene paleoenvironments of the Lake Victoria region using paleosols and freshwater tufa.
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Access changed 2/12/18.
Beverly, Emily Jane.
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Lake Victoria is the largest freshwater lake in the tropics by surface area (68,800 km2) and is currently a biogeographic barrier between the eastern and western branches of the East African Rift. Lake Victoria has desiccated in the past at ~17 ka and at ~15 ka, but little is known about its history prior to the Last Glacial Maximum. The Middle to Late Pleistocene deposits exposed on the shoreline of eastern Lake Victoria, which preserve abundant vertebrate fossils and Middle Stone Age (MSA) artifacts, are ideal for paleoenvironmental and paleoclimatic reconstructions to understand the effect of changing environment during the migration of early modern humans within and out of Africa. Measured stratigraphic sections reveal fluvial and tufa deposits that directly overlie Miocene paleotopography. The ages (94.0±3.3, 111.4±4.2, and 455±45 ka) of these tufa deposits demonstrate that spring-fed rivers were a recurrent, variably preserved feature on the Pleistocene landscape for ~360 kyr, but tufa precipitation abruptly ceased at ~94 ka. Above the riverine tufas, three well-exposed and laterally continuous paleosols with intercalated tuffs allow for reconstruction of modern soil properties and estimates of mean annual precipitation (MAP). The oldest paleosol is a smectitic paleo-Vertisol with vertic features and saline and sodic properties that indicate seasonal precipitation. Higher in the section, the paleosols are tuffaceous paleo-Inceptisols with Alfisol-like soil characteristics (illuviated clay). Paleosol MAP proxies indicate that MAP was 44% of modern between ~94 ka and >35 ka. These paleoprecipitation estimates were applied to a water budget model to understand the effects of prolonged aridity on the Lake Victoria region. When MAP is 44% of modern, Lake Victoria desiccates within centuries, but refills slowly (e.g., >10 kyr) until precipitation is >94% of modern, at which point it can be refilled in centuries. Therefore, it is likely that Lake Victoria was desiccated for most, if not all, of the interval between 94 and >35 ka. Prolonged desiccation would have removed a major barrier for the movement of fauna, including early modern humans, and provided a long-term dispersal corridor between the rifts and across the equator.