| Groundwater-fed wetlands are common features of many lake margins, and they may leave characteristic deposits that record hydrologic conditions at their time of formation. Observations of Tanzanian wetlands adjacent to Lakes Eyasi, Makat (Ngorongoro), and Natron show that they produce distinct lithofacies in various depositional subenvironments including channels, marshes, hippopotamus flats, pools, and alkaline flats. Sediment supply, aqueous geochemistry, large mammal activity, and vegetation are primary controls on sedimentation. Wetland sediments are laterally restricted to a few tens of meters along strike, but are more extensive down-gradient, and interfinger with adjacent fluvio-lacustrine deposits. In all basins, these deposits overlie a regressive disconformity that records the onset of late Holocene aridity.; Chemical analyses of waters in these settings show evidence of evaporative concentration and solute loss. Solute Na+, SO4 2−, Cl−, and alkalinity follow conservative evaporative trajectories, whereas solute Ca2+, SiO2, and K+ are lost to mineral precipitation. Solute Mg 2+ is highly variable, and is affected by pH. Field data and model calculations show that PCO2 is an important influence on pH, and therefore on magnesium silicate solubility. Mass balance calculations of evaporating waters suggest that CO2 enrichment due to biotic respiration leads to magnesium silicate dissolution, and degassing produces precipitation. Only minor evaporation is required for kerolite supersaturation, suggesting that interstratified clays are an important phase in dilute as well as saline-alkaline systems. Associated <0.2μm clay chemistries are intermediate between endmember dioctahedral and trioctahedral smectites or kerolite.; Lateral variation over ∼1.5 km in the sedimentary geochemistry of basal Bed II claystones of Olduvai Gorge (∼1.8 Ma) reflects variation in paleo-water quality analogous to modern lake-marginal wetlands. Bulk Ba/Sr, Na2O + K2O + MgO/Al2O3, MgO/Al 2O3, and Mg/Al ratios of <0.2μm clays point to freshest conditions around Locality 43 (HWK-East), moderate conditions several hundred meters to the east (Loc. 40-MCK), and high salinity/alkalinity several hundred meters to the west at Locs. 85 (VEK) and 45 (FLK). Clay chemistry and artifact abundance are well correlated in laterally-distributed excavations (r = −0.67, p < 0.005), suggesting a relationship between paleo-water quality and hominid paleoecology. This pattern is consistent with ecologically-based predictions in which artifact loss is highest near localized freshwater sources where carcass scavenging opportunities are greatest. This quantifies a relationship between artifact density distribution and a paleoecological proxy over a landscape scale for the first time in Early Stone Age archaeology. |
