A GROUNDWATER-FED MEGA WETLAND: A PERENNIAL SOURCE OF WATER FOR HOMININS, UPPER BED II, OLDUVAI GORGE, TANZANIA

The Olduvai basin in northern Tanzania archives rich archaeological and paleontological records including the remains of three hominin species (Paranthropus boisei, Homo habilis and Homo erectus). The reason for the unusual dense concentration of life in this arid region has been elusive, as it is within the rain shadow of the Ngorongoro Volcanic Highlands, had ephemeral rivers and a nonpotable saline-alkaline lake. Paleo-precipitation is estimated to be from 500-900 mm/yr in response to precession-scale orbital forcing of the monsoonal system; the lake level oscillated on same time scale (~20 kyr). Recent detailed geologic mapping and high-resolution landscape reconstruction at Olduvai Gorge have yielded new information on basin hydrology and the identification of numerous springs, seeps and groundwater-fed wetlands in sediments dating 2 Ma to 1 Ma.

A large paleo wetland (~150,000 m²) occurred along the margin of paleo Lake Olduvai during Upper Bed II time (~1.2 Ma). It is part of a broad lacustrine delta system formed at the distal end of 20 km long fluvial plain draining the 3000 m high Volcanic Highlands. The sediment record of the paleo wetland fines upward from silt to siliceous silty-clay (with root casts) overlain by thick tufa and capped with clay. This distinctive package is interpreted as sedimentation triggered by a rising water table linked to rise in lake level. The paleo wetland was sourced from high precipitation (>1200 mm/yr) on the Highlands. The water moved both in surface streams and in groundwater into the basin. Oxygen isotope values of tufas indicate meteoric water and thus the groundwater-fed wetland was likely a freshwater resource in the basin.

The record of the paleo wetlands in this volcaniclastic setting are not peats, but rather siliceous and/or carbonate deposits; composition depends on system geochemistry. Rivers carried calcium-rich volcaniclastic sediments. In hydrologically closed wetlands, evaporative concentration can quickly supersaturate waters with respect to carbonate, resulting in calcite precipitation. However, plant decomposition and microbial respiration produces CO2 and if not efficiently degassed (such as in a thickly vegetated wetland) CO2 can build up. CO2 suppresses pH, prevents carbonate precipitation, and enhances preservation of biogenic amorphous silica.