Facies, geochemistry and diatoms of late Pleistocene Olorgesailie tufas, southern Kenya Rift

This research reconstructs paleoenvironments of late Pleistocene tufas at Olorgesailie in the southern Kenya Rift and examines the relationship between climate, tectonism and tufa precipitation. The deposits crop out at three main sites, forming at 290. kyr and at least twice between 220 and 12. kyr ago. Ten carbonate facies comprise the bulk of the tufas, which accumulated at spring vents, in swamps and shallow lakes, along fissures, and along fluvial channels.The tufas contain diatoms dominated by shallow-water, swamp and fluvial taxa associated with fresh to mildly saline conditions. δ¹³C isotopes are relatively enriched (0.25 to 1.96‰) compared with other East African tufas, but uniform within each set of tufa deposits, suggesting similar sources of CO₂ derived from similar plants and soil organic matter during each interval of deposition. δ¹⁸O ranges from -7.17‰ to +5.16‰ suggesting variable evaporation and rainfall/groundwater sources or changing water temperatures. Variable REE patterns suggest spatial and/or temporal contrasts in groundwater chemistry.The former springs may have developed in response to climate changes, but there are no clear correlations with known East African wet phases. The deposition of tufa probably also relates to basin breakup and faulting, which likely influenced groundwater movements. Increased permineralized silica in plants in tufas and non-carbonate sediments reflects the introduction of deeper groundwaters than those during earlier Olorgesailie Formation times. Whether climatically or tectonically controlled, the tufa spring sites would have provided a supply of potable water, at least intermittently, during the Pleistocene, and would have provided attractive places for wildlife and possibly for Middle Stone Age human inhabitants of the region, especially during relatively dry times.