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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

Lithological and biological features of a fossiliferous tufa in the Kapthurin Formation, Baringo, Kenya, reveal the presence of a lush wetland in a semiarid environment during the Middle Pleistocene (ca 500 ka) in this portion of the East African Rift Valley. Four geological sections, each between 3 m and 8 m in thickness, exposed over a distance of 0AE5 km, reveal a 1 to 2 m thick paludal tufa which is composed of three carbonate beds, two dark grey silty claystones and a reddish-brown silty palaeosol. High resolution stratigraphic analysis, carbonate petrography, stable isotope and elemental geochemistry, clay mineralogy and fossil remains (molluscs, ostracods, diatoms and charophytes) reveal a ground water-fed system that fluctuated in depth and periodically disappeared altogether. Oxygen isotope ratios (d 18 O) of tufa matrix range from )4AE5& to )8AE0& (Vienna Pee Dee Belemnite) and become more positive up section, indicating the decreasing influence of fault-related fluids and increasing residence time or freshness of wetland water, rather than evaporative enrichment. This spring was situated on a lake margin during low lake levels, thrived during periods of increased ground water input and was ultimately replaced by an alkaline lake. The wetland would appear to have existed during a cool interval within the generally warm Marine Isotope Stage 13 or perhaps during the warm second half of Marine Isotope Stage 13. The ground water source of this wetland arose through a fault system. Thus, the position of the tufa deposit is controlled structurally but the timing and duration of the wetland system may have been influenced by both climatic and tectonic factors.

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