2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 171-2
Presentation Time: 1:55 PM


ASHLEY, Gail M., Earth and Planetary Sciences, Rutgers University, 610 Taylor Road, Piscataway, NJ 08854, gmashley@rci.rutgers.edu

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.