HIGH-RESOLUTION PALEOENVIRONMENTAL AND PALEOCLIMATIC RECONSTRUCTION OF A PLEISTOCENE CATENA USING PALEOPEDOLOGY AND GEOCHEMISTRY OF LAKE MARGIN PALEO-VERTISOLS, OLDUVAI GORGE, TANZANIA

Olduvai Gorge, Tanzania contains a rich record of Pleistocene paleoclimate and paleoenvironment, as well as an abundance of paleontological and archaeological data. The 2.2 myr volcaniclastic basin infill can be divided into time slices using dated tuffs, and each time slice can be studied in a paleolandscape context. Until ~1.75 Ma the sediments were deposited in a semi-arid closed rift basin that contained a shallow saline-alkaline lake that fluctuated with Milankovitch-driven climatic fluctuations. The lake periodically flooded depositing sediment, and clay-rich soils (Vertisols) developed on the lake margin flats.

A ~ 20 ka time slice at ~ 1.8 Ma was reconstructed using sedimentology and paleopedology. This time slice between Tuff IF and Ng’eju Tuff contains both stacked and cumulative paleosols over a ~1 km transect. Closer to the lake, these paleosols are thinner, vertically stacked, and separated either by thin discontinuous tuffs or tufa. Further from the lake margin, there is additional volcaniclastic input and paleosol thickness increases and is cumulative. These clay-rich paleosols form a catena and reveal an entirely new soil-forming environment, which contrasts strongly with the Aridisols and Andisols found on basin margins.

Macroscale, petrographic, and micromorphological data all show evidence of vertic features. Abundant slickensides, gilgai, and a variety of ped shapes were observed in the field. Micro-ped structures, stress cutans, and other pedogenic features sensitive to soil moisture conditions were identified in thin section. Although weakly developed, these paleo-Vertisols have distinct horizons defined by soil color changes, differing ped shapes, and major and trace element translocations (determined by mass balance calculations). Molecular weathering ratios show increased weathering, and mass balance indicates greater translocations (both positive and negative) through time in the stacked paleosols. In this lake margin environment, lower soil moisture created better drained conditions allowing for increased pedogenesis. The drying trend in this climosequence is consistent with other terrestrial records within Olduvai Gorge and provides additional paleoenvironmental and paleoclimatic data coinciding with the first hominin migrations out of Africa at ~1.8 Ma.