PLEISTOCENE HOMININ PALEOENVIRONMENTAL RECONSTRUCTIONS: SEDIMENTOLOGY, MICROMORPHOLOGY, AND STABLE ISOTOPES OF RIVERINE TUFAS FROM THE RUSINGA AND KARUNGU REGIONS OF WESTERN KENYA

Although the effects of changing paleoclimate and paleoenvironment on the evolution of Pleistocene Homo sapiens continues to be heavily debated, few data from equatorial East Africa are available prior to the Last Glacial Maximum (LGM). Lake Victoria (LV) spans the equator between the eastern and western branches of the East African Rift and is the largest tropical freshwater lake by surface area (~64,400 km²) but is only 68 m deep. The shallow depth and dependence on direct precipitation to maintain lake levels makes LV sensitive to changes in precipitation. Middle to Late Pleistocene deposits on the shoreline of eastern LV preserve an abundance of vertebrate fossils and Stone Age artifacts, and riverine tufas identified at the base of the deposits are ideal for paleoenvironmental reconstructions. Here we present new data on tufas identified at Rusinga Island and on the mainland near Karungu, Kenya. Tufas were described and sampled for micromorphology, mineralogy, bulk geochemistry, stable isotopes and U-series at four sites: Nyamita, Aringo, Kisaaka, and Onge. Additional dates are in progress but Aringo has a U-series age of 455±45 ka (95% CI). Seven carbonate facies and three clastic facies cemented by syn-depositional calcite were identified. Poor sorting of clastic sediments indicates flashy, ephemeral discharge, but barrage tufas and paludal areas with δ¹³C values of ~-10‰ are indicative of C₃ macrophyes (e.g., Typha). Other sites have a mixed C₃/C₄ signature that suggests a semi-arid C₄ grassland surrounding these spring-fed rivers. The δ¹⁸O of tufa from Nyamita is on average ~1‰ more negative than calcite precipitated from modern rainfall in the LV Basin, which suggests less contribution of catchment rainfall and more enriched monsoonal input similar to the LGM. Positive increases in both δ¹³C and δ¹⁸O at other sites suggest increased evaporation or CO₂ degassing. Both Nyamita and Aringo (~40 km apart) record cyclicity in isotopic composition and stromatolite lamina type likely tied to sub-Milankovitch cycles, such as El Niño. The tufas indicate that climate was significantly drier than modern LV, but the presence of spring-fed rivers would have afforded animals a more reliable water source (aquifer vs. precipitation), allowing for a more diverse plant and animal community in an otherwise arid environment.