THE RECORD OF SPRINGS AT DK ARCHEOLOGICAL SITE, OLDUVAI GORGE, TANZANIA

The DK archaeological site has the oldest evidence of hominins (1.85 Ma) in Olduvai Gorge, Tanzania. The Olduvai region is hyper-arid to arid (evapotranspiration, 2500 mm/yr; precipitation, 250-700 mm/yr over a precession cycle). The location of the site, on an alluvial fan at the margin of an alkaline playa lake, has long been a puzzle and its origin controversial (a hominin “living floor” vs. carnivore kill site?). Ten trenches recently excavated in the 2-meter thick sequence of siliciclastic and carbonate sediments (along a 450 m long outcrop) provide evidence of the paleoenvironmental context of DK. Siliciclastic data (sedimentary structures, grain size, and mineralogy) suggest the sequence records climate change (dry-wet-dry). The sediments are bracketed by argon-argon volcanics (basalt and Tuff IB) separated by 20,000-25,000 years. The time recorded is consistent with the length of a precession cycle.

The archaeological site occurs within a dry interval. It is at the base of the section immediately overlying the basalt. A freshwater carbonate deposit (δ¹³C -5.49‰; δ¹⁸O -4.60‰) occurs on top of the basalt and is 50 m west of the DK site, but in the same stratigraphic horizon. The water appears to have discharged from a fracture (or fault) at DK. This spring system was long lived as it was the source of water for a second freshwater carbonate higher in the DK section. The upper carbonate (δ¹³C -4.34‰; δ¹⁸O -5.06‰) also occurs during a dry interval. It is a bedded limestone (lime-mud with fossils such as ostrocods, gastropods, and oogonia) deposited from carbonate-rich groundwater. Carbonates occur only during the declining limb (wet to dry portion) of the precession cycle. In summary, the site, situated on a low gradient, distal portion of a volcanic alluvial fan was spring-fed. The groundwater discharge flowed from a fracture created localized ponded water that was a potential freshwater resource for hominins and other vertebrates during dry intervals.