Reconstruction of a Pleistocene paleocatena using micromorphology and geochemistry of lake margin paleo-Vertisols, Olduvai Gorge, Tanzania

Abstract

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 Ma of volcaniclastic infill can be divided into time-slices using dated tuffs. Sediments were deposited in a semi-arid, closed rift basin containing a shallow saline–alkaline lake until ∼1.75 Ma. Four trenches in uppermost Bed I sample a ∼20 ka time-slice between Ng'eju Tuff (base, 1.818 ± 0.006 Ma) and Tuff IF (top, 1.803 ± 0.002 Ma). They form a ∼1 km transect of the lake margin flat that contains both stacked and cumulative paleosols that are interpreted as a heterogeneous paleocatena. Closer to the lake, paleosols are thinner, vertically stacked, and separated by thin tuffs or tufa. Further from the lake margin, there is additional volcaniclastic input, and paleosols are thicker and cumulative. Macroscale and micromorphological features identify these clay-rich paleosols as paleo-Vertisols. Abundant pedogenic slickensides and a variety of ped shapes were observed in the field, as well as micro-ped structures and stress cutans in thin section. Although weakly developed, these paleo-Vertisols also have distinct horizons defined by soil color changes, differing ped shapes, and bulk geochemistry and are indicative of monsoonal precipitation seasonality.Paleosols and parent materials were analyzed for bulk geochemistry of major, rare, and trace elements. Geochemical proxies reveal changing hydrology not definitively identifiable in the field or micromorphology. Molecular weathering ratios show increased weathering, and mass-balance calculations indicate greater translocations (positive and negative) through time. Lower soil moisture due to a drier climate likely created better-drained conditions allowing for increased pedogenesis. The records of FeMn oxides and zeolites precipitated in rhizoliths are further evidence for changing redox conditions and water chemistry. Redox-sensitive elements such as Fe and Mn were mobilized during saturated soil conditions and precipitated during drier conditions. As the hydrology changed due to increased aridity, zeolites precipitated as rhizoliths due to capillary rise and evaporative pumping of saline–alkaline water. The fluctuating hydrology identified in the paleo-Vertisols deposited between 1.79 and 1.81 Ma is indicative of a precession cycle and consistent with records from Olduvai Gorge and throughout East Africa. This 20 ka time-slice of the 2 Ma drying trend identified in Africa coincides with the first hominin migrations out of Africa at ∼1.8 Ma and suggests that hominins were living in a water-stressed environment with annual precipitation seasonality and large variations in precipitation due to precession.