Paleosol diversity in the Olduvai Basin, Tanzania: Effects of geomorphology, parent material, depositional environment, and groundwater on soil development

Abstract

Olduvai Gorge, Tanzania is known for its fossil and cultural record of early hominins. The archaeological records are typically found within pedogenically modified sediments, thus the interpretation of paleosols provides important paleoenvironmental context information. The Gorge contains a rich and diverse record of paleosols that vary spatially and temporally, however the stratigraphy can be divided into time slices using dated tuffs and studied in a paleolandscape context. Sediments were deposited in a semi-arid closed rift basin containing paleo Lake Olduvai, with volcanoes supplying volcaniclastic material to an alluvial fan system on the eastern side and quartzofeldspathic fluvial sediments derived from weathered basement rocks on the western side. The shallow saline-alkaline lake in the basin center and groundwater levels in the surrounding uplands fluctuated with Milankovitch-driven climatic (precession) cycles of ∼20,000 years. The rift basin paleolandscape (at ∼1.8 Ma) is reconstructed using sedimentology, stratigraphy and paleopedology. Standard field descriptions of physical and biogenic paleosol structures are combined with stable isotope ratios of carbonates, and whole rock geochemistry of sediments, to reveal diversity of paleosols within the basin. There are at least three distinct types of paleosols that record a paleocatena related to both landscape and drainage differences. Red tephra-rich Andisols developed on the volcaniclastic alluvial fan to the east of the paleolake, calcium-carbonate-rich, silty Aridisols developed on the interfluves and floodplain of the fluvial plain on the west, whereas clay-rich paleosols (Vertisols) developed on the lake margin and lake in the center of basin. Variances in geomorphology, depositional environment, parent material, and depth to the water table are reflected in the development of distinctly different soil types that can provide key data needed for high-resolution reconstruction of the landscape known to be utilized by early hominins.