Reconstructing vegetation history of the Olorgesailie Basin during the Middle to Late Pleistocene using phytolith data

The Olorgesailie basin, located in the East African Rift System (EARS), southern Kenya (1.5–1.6°S, 36.4–36.5°E, 940–1040 m asl), is an important site for palaeoanthropological, palaeontological and geological research, with sediments dating back more than 1 Ma. Little is known about the palaeovegetation and palaeoenvironmental context of this important site and how this varied through space and time. Here we use phytolith data to reconstruct the vegetation history through the Middle and Late Pleistocene (c. 670 ka to c. 64 ka). The analysis of 24 samples from 86palaeosols within the Olorgesailie and Oltulelei Formations uses diagnostic phytoliths to plot relative phytolith abundance diagrams documenting temporal and spatial vegetation variations. We use three phytolith indices (climate index, aridity index and tree density index) to estimate temperature, precipitation, and vegetation structure from sediments that are well-constrained chronologically by ⁴⁰Ar/³⁹Ar dating. Spatial vegetation variation is captured through examining phytoliths from three distinct localities (Loc. OLT, Loc. B and Loc. G) over a distance of about 5 km. Results suggest that local vegetation changes approximately correspond with stratigraphic units. Phytolith indices reflect warm and moist conditions c. 670 ka, with cool and dry conditions from c. 650 ka during which time riparian and a variety of other habitats were present. An unconformity, which extends over c. 180,000 years (500 to 320 ka), is interpreted as largely caused by tectonic processes coupled with precipitation variability. A shift from woody vegetation to C₄ grasslands is evident following the unconformity, and riparian habitats were again present. Considerable climate variability is apparent thereafter, with inconsistent precipitation conditions until c. 220 ka when more stable and moist conditions set in until c. 64 ka. The study documents vegetation dynamics directly associated with the Middle to Late Pleistocene palaeontological and archaeological record of Olorgesailie.