Reconstruction of Late Pleistocene Paleoenvironments Using Bulk Geochemistry of Paleosols from the Lake Victoria Region

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 (94-36 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 750 ± 108 mm year −1 (CALMAG), 800 ± 182 mm year −1 (CIA-K), and 1,010 ± 228 mm year −1 (PPM 1.0 ) 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 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.

Ecomorphology and ecology of the grassland specialist,Rusingoryx atopocranion(Artiodactyla: Bovidae), from the late Pleistocene of western Kenya

“…Importantly, fossils of R. atopocranion co-occur with hominin fossils attributed to H. sapiens (Grine, 2016;Pearson et al, 2020) and Middle Stone Age (MSA) artifacts (Tryon et al, 2010(Tryon et al, , 2012(Tryon et al, , 2014Faith et al, 2015;Blegen et al, 2017;Jenkins et al, 2017), the latter providing the archaeological context of early modern humans in eastern Africa (Tryon and Faith, 2013;Tryon, 2019). Past work in the Lake Victoria Basin has documented the expansion of Serengeti-like grasslands across the region in the late Pleistocene (e.g., Tryon et al, 2010Tryon et al, , 2012Tryon et al, , 2016Faith et al, 2015;Garrett et al, 2015), likely in response to increased aridity and desiccation of the lake (e.g., Beverly et al, 2015aBeverly et al, , 2017Beverly et al, , 2020. This interpretation has been heavily influenced by the fossil faunas, including inferences based on the dominance of R. atopocranion, which was assumed to have had an affinity for open grassland habitats similar to extant alcelaphins (e.g., Faith et al, 2011;Faith, 2014).…”

“…Analyses of ancient soils, associated fossil taxa, and bathymetric reconstructions suggest that open and grassy habitats were widespread throughout much of the late Pleistocene in the Lake Victoria Basin. The region was considerably drier than modern times from ~100–36 ka, which probably resulted in the complete desiccation of Lake Victoria and an expansion of a Serengeti-like ecosystem across the basin (Tryon et al, 2014, 2016; Beverly et al, 2015a, b, 2017, 2020; Faith et al, 2015).…”

The Middle/Later Stone Age transition and cultural dynamics of late Pleistocene East Africa

“…For example, from~36 to 94 ka and 14 to 17 ka, at least Lake Victoria, the dominant water feature of the region, had either dried up completely or was substantially reduced in size, changing what is now a lake habitat to an extensive Serengeti-like grassland dominated by extinct and often strikingly distinctive animal communities ( Figure 4). 44,45 The rainforest is also subject to extensive contraction, fragmentation, or expansion caused by increases or decreases in rainfall 46 that may have contributed to intermittent hominin occupation of its margins. 44 Nearshore islands such as Zanzibar appear to have been occupied as early as the LGM~18-26 ka, possibly aided by lowered sea level.…”

A Late Pleistocene third molar ofHylochoerus(Suidae, Mammalia) from Rusinga Island, Kenya: paleoenvironmental implications and a note on the hypsodonty of African forest hogs

“…This is one of the most negative δ 13 C values for any fossil tooth enamel from Africa; other herbivores with such negative values are found in cave deposits in Southeastern Asia and are interpreted as being associated with closed canopy conditions (Bocherens et al 2017;Ma et al 2017Ma et al , 2019Bacon et al 2018). Thus, this particular sample presents an interesting puzzle for interpretation since there is no other clear evidence for closed-canopy conditions in the associated fauna or isotopes, and most lines of evidence point to relatively dry conditions and an expansion of C 4 grasslands across the Lake Victoria basin during the Late Pleistocene (e.g., Tryon et al 2010Tryon et al , 2012Tryon et al , 2016Faith et al 2015Faith et al , 2020Garrett et al 2015;Beverly et al 2017). The Rusinga Hylochoerus may derive from sediments reflecting a humid phase that is otherwise not represented by the faunas, but analysis of paleosols from Rusinga and nearby Karungu indicate that relatively dry conditions persisted from ~94-36 ka (Beverly et al 2017).…”

“…Thus, this particular sample presents an interesting puzzle for interpretation since there is no other clear evidence for closed-canopy conditions in the associated fauna or isotopes, and most lines of evidence point to relatively dry conditions and an expansion of C 4 grasslands across the Lake Victoria basin during the Late Pleistocene (e.g., Tryon et al 2010Tryon et al , 2012Tryon et al , 2016Faith et al 2015Faith et al , 2020Garrett et al 2015;Beverly et al 2017). The Rusinga Hylochoerus may derive from sediments reflecting a humid phase that is otherwise not represented by the faunas, but analysis of paleosols from Rusinga and nearby Karungu indicate that relatively dry conditions persisted from ~94-36 ka (Beverly et al 2017). The complete absence of Later Stone Age artefacts (dating from < 36 ka in the region; Tryon et al 2016;Blegen et al 2017) at Nyamsingula also suggests that it is unlikely the specimen derives from undocumented or eroded Holocene deposits that sample a more humid climate with closed habitats.…”