Recurrent spring‐fed rivers in a Middle to Late Pleistocene semi‐arid grassland: Implications for environments of early humans in the Lake Victoria Basin, Kenya

Abstract

The effect of changing palaeoclimate and palaeoenvironment on human evolution during the Pleistocene is debated, but hampered by few East African records directly associated with archaeological sites prior to the Last Glacial Maximum. Middle to Late Pleistocene deposits on the shoreline of eastern Lake Victoria preserve abundant vertebrate fossils and Middle Stone Age artefacts associated with riverine tufas at the base of the deposits, which are ideal for palaeoenvironmental reconstructions. New data from tufas identified on Rusinga Island and on the mainland near Karungu, Kenya are provided from outcrop, thin sections, mineralogical, stable isotopic and U-series dating analyses. Tufa is identified in four sites: Nyamita (94Á0 AE 3Á3 and 111Á4 AE 4Á2 ka); Kisaaka, Aringo (455 AE 45 ka); and Obware. The age ranges of these tufa deposits demonstrate that spring-fed rivers were a recurrent, variably preserved feature on the Pleistocene landscape for ca 360 kyr. Poor sorting of clastic facies from all sites indicates flashy, ephemeral discharge, but these facies are commonly associated with barrage tufas, paludal environments with d 13 C values of ca 10& indicative of C 3 plants and fossil Hippopotamus, all of which indicate a perennial water source. Other tufa deposits from Nyamita, Obware and Aringo have a mixed C 3 /C 4 signature consistent with a semi-arid C 4 grassland surrounding these spring-fed rivers. The d 18 O values of tufa from Nyamita are on average ca 1& more negative than calcite precipitated from modern rainfall in the region, suggesting greater contribution of depleted monsoonal input, similar to the Last Glacial Maximum. Microdebitage and surface-collected artefacts indicate that early modern humans were utilizing these spring-fed rivers. The presence of springÀfed rivers would have afforded animals a reliable water source, sustaining a diverse plant and animal community in an otherwise arid environment. 1611

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

“…Bulk geochemical data FIGURE 2 | Stratigraphy of paleosols used in this study. The dates (and associated lab samples numbers) identified on this figure are discussed further in Blegen et al (2015Blegen et al ( , 2016, Beverly et al (2015b), and Tryon et al (2010Tryon et al ( , 2012. The Nyamita and Wakondo Tuffs at the Nyamita AV1002, Nyamita 1, and Wakondo localities are discontinuous due to their proximity to fluvial channels and are shown in their correlated positions within the paleosols.…”

“…Four named tuffs are used to correlate the sections between Rusinga Island and Karungu: Wakondo, Nyamita, Nyamsingula, and Menengai. The Wakondo Tuff is >68 ± 5 ka based on overlying optically stimulated luminescence (OSL) dates and <94.0 ± 3.3 based on the U-series age of the underlying tufa deposits (Beverly et al, 2015b;Blegen et al, 2015). Additional data on the tufas are presented in Beverly et al (2015b).…”

“…The Wakondo Tuff is >68 ± 5 ka based on overlying optically stimulated luminescence (OSL) dates and <94.0 ± 3.3 based on the U-series age of the underlying tufa deposits (Beverly et al, 2015b;Blegen et al, 2015). Additional data on the tufas are presented in Beverly et al (2015b). The Nyamita Tuff is constrained by OSL dates above and below the tuff that indicate deposition at ∼49 ka .…”

“…Tephra deposits can be correlated between sites on the basis of geochemical compositional similarity and have been dated by multiple radiometric methods (Tryon et al, 2010;Beverly et al, 2015b;Blegen et al, 2015Blegen et al, , 2016). Four named tuffs are used to correlate the sections between Rusinga Island and Karungu: Wakondo, Nyamita, Nyamsingula, and Menengai.…”

“…Here we present data from stratigraphy, micromorphology (petrography), and bulk geochemistry from six Late Pleistocene sites from northeastern Lake Victoria in Kenya that were deposited between 94 and 36 ka Beverly et al, 2015b;Blegen et al, 2015). These sites sample an approximately 55 km-long north to south transect along the eastern margin of the modern lake: Rusinga Island (n = 2), Mfangano Island (n = 1), and Karungu (n = 4) (Figure 1; Tryon et al, 2014Tryon et al, , 2016Beverly et al, 2015a,b;Blegen et al, 2015;Faith et al, 2015;Garrett et al, 2015).…”