Diatomaceous sediments and environmental change in the Pleistocene Olorgesailie Formation, southern Kenya Rift Valley

The Olorgesailie Formation is comprised of lacustrine, volcaniclastic and alluvial sediments that formed in the southern Kenya Rift between about 1.2 million and 490,000 years ago. Diatoms are common in much of the sequence and preserve a record of environmental change within the basin. A high-resolution diatom stratigraphy has been developed for these deposits. The data document the presence of freshwater and saline lakes as well as wetlands. Transfer functions indicate that these water bodies ranged in conductivity between about 200-20,000 μS cm^{- 1}, with pH varying between about 7.5 and 10.3. Pedogenesis affected multiple horizons within the succession, reflecting periods of emergence that encompass much of the time represented by the Olorgesailie Formation. A variety of other sedimentological indicators (carbonates, rhizoliths, mudcracks, erosional channels) also record periods when the sampled portions of the basin were dominated by terrestrial conditions. Stone tools are common at several levels, indicating the use of the area by hominins. Lakes and wetlands were potentially usable as sources of potable water by hominins for part of the basin history, but at other times were undrinkable. Other water sources (springs, rivers) would have been necessary during these periods. Paleoenvironments in the basin were complex and changed frequently with time. Such shifts in resources and habitat distribution during Olorgesailie Formation time seem likely to have influenced the behavior and evolution of local plant and animal populations, including Homo.