Paleopedology and Paleohydrology of a Volcaniclastic Paleosol Interval: Implications for Early Pleistocene Stratigraphy and Paleoclimate Record, Olduvai Gorge, Tanzania

Abstract

ABSTRACT A cumulative red paleosol interval developed on volcaniclastic parent material under semiarid conditions in Olduvai Gorge, Tanzania. It contains a complex 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 25 ka of the 50 kyr duration of lowermost Bed II, and is defined by early Pleistocene ( 1.75 Ma) Tuffs IF and IIA. The paleosol interval records a paleocatena related to both landscape and drainage--the slope position on a pyroclastic fan relative to an alkaline lake, the proximity to freshwater wetlands, and the position of water table. Biogenic paleosol structures include grass and sedge root traces, zeolite rhizocretions, and soil fauna (termite and ant) traces. Abundant pedogenic features sensitive to soil moisture conditions, including redoximorphic mottles in the paleosol matrix, Fe oxide glaebules, grain and pore coatings, illuviated clay grain and pore coatings, and vadose siliciclastic and zeolite crystal silt, record episodic water-table fluctuations. The geochemistry of whole-rock samples distinguishes two parent materials (early low Ti/Zr, weathered volcaniclastic sediment; and late high Ti/Zr, tuffaceous sediment), which represent two distinct pedogenic phases. The Lower Paleosol developed at both sites, whereas the Upper Paleosol developed only at the upslope site. Mass-balance calculations indicate greater weathering, higher Eh and pH, and greater zeolite precipitation at the upslope site than at the downslope site. These relationships are compatible with the upslope site having had a lower overall water table and better-drained conditions than the downslope site, which had a higher water table and poorly drained conditions. The Lower Paleosol provides evidence of a fluctuating water table consistent with a wetter climate followed 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 the paleosol interval, is used to make a tentative correlation at 1.75 Ma with global climate (dust) records (wet/dry cycles).