Paleopedology and papleohydrology of a volcaniclastic paleosol interval: Implications for Early Pleistocene stratigraphy and paleoclimate record, Olduvai Gorge, Tanzania

A cumulative red paleosol interval developed on volcaniclastic parent mater ial under semiarid conditions in Olduvai Gorge, Tanzania, It contains a com plex history of pedogenesis that was affected by: (1) episodic pyroclastic and debris fan processes and (2) episodic expansions and contractions of an adjacent alkaline lake and the associated fluctuations in the water table. The paleosol interval is 130-320 cm thick, represents similar to 25 ka of the similar to 50 kyr duration of lowermost Bed II, and is defined by early Pleistocene (similar to 1.75 Ma) Tuffs IF and IIA, The paleosol interval r ecords a paleocatena related to both landscape and drainage-the slope posit ion on a pyroclastic fan relative to an alkaline lake, the proximity to fre shwater wetlands, and the position of water table. Biogenic paleosol struct ures include grass and sedge root traces, zeolite rhizocretions, and soil f auna (termite and ant) traces, Abundant pedogenic features sensitive to soi l moisture conditions, including redoximorphic mottles in the paleosol matr ix, Fe oxide glaebules, grain and pore coatings, illuviated clay grain and pore coatings, and vadose siliciclastic and zeolite crystal silt, record ep isodic water-table fluctuations. The geochemistry of whole-rock samples dis tinguishes two parent materials (early low Ti/Zr, weathered volcaniclastic sediment; and late high Ti/Zr, tuffaceous sediment), which represent two di stinct pedogenic phases. The Lower Paleosol developed at both sites, wherea s the Upper Paleosol developed only at the upslope site. Mass-balance calcu lations indicate greater weathering, higher Eh and pH, and greater zeolite precipitation at the upslope site than at the downslope site. These relatio nships are compatible with the upslope site having had a lower overall wate r table and better-drained conditions than the downslope site, which had a higher water table and poorly drained conditions. The Lower Paleosol provid es evidence of a fluctuating water table consistent with a wetter climate f ollowed by a prolonged arid period, The Upper Paleosol began to form after a return to wetter conditions and ended under arid conditions. The position of the Olduvai Subchron, C2n (1.942-1.785 Ma) in Bed I, directly beneath t he paleosol interval, is used to make a tentative correlation at similar to 1.75 Ma with global climate (dust) records (wet/dry cycles).