Recent field work at Olduvai Gorge (Tanzania) using sedimentary geology, in particular high-resolution paleoenvironmental reconstruction and isotope geochemistry, has revealed that freshwater was in proximity to a number of the rich fossil sites in Beds I and II (̃ 2.0-1.0 Ma). This paper presents the fìrst geological evidence for springs associated with archaeological sites in this semiarid rift basin. The springs appear to be limited to a small area within the basin and were likely connected to faults that acted as conduits for groundwater. Tufas associated with ten archaeological sites have stable-isotope signatures occurring in a cluster bounded by δ 180 ratios from -6% to +1%, and the δ13C ratios from -5% to +2%. The δ180 values cluster around -4%, that of precipitation in the region, indicating a meteoric source. The longevity of the spring record reflects a hydrologic system that apparently persisted for hundreds of thousands of years. Previous landscape reconstructions depicted the archaeological sites on the lake margin of paleo Lake Olduvai, as an alkaline playa. The discovery of springs at or near the archaeological sites provides fresh insights for interpreting hominin behavior during this key time in evolution with respect to procuring food, water, and materials for stone tools, as well as hominin adaptation to climate change and paleoenvironmental change. The idea that spring deposits may be in proximity to archaeological sites could lead to discovery of new sites at other hominin fossil localities in the East African Rift System (EARS).
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