PALEOENVIRONMENTAL RECONSTRUCTION OF A PALEOSOL CATENA, THE ZINJ ARCHAEOLOGICAL LEVEL, OLDUVAI GORGE, TANZANIA

Paleosols record the geomorphic, biologic and paleoclimatic processes in the Earth’s Critical Zone and are archives of the ancient landscapes associated with archaeological sites. Detailed field, micromorphologic, and bulk geochemical analysis of paleosols were conducted near four sites at Olduvai Gorge, Tanzania within the same stratigraphic horizon as the Zinjanthropus (Paranthropus) boisei archaeological site. Paleosols are thin (<35 cm), smectitic, and locally zeolitic, and exhibit shrink-swell features demonstrating affinities with modern USDA Vertisols, such as slickensides and sepic-plasmic (bright-clay) microfabrics. Traced across the paleolandscape over distances of up to 1 km and just beneath Tuff IC, which defines a 1.845 Ma “time-line”, the paleosols record a paleocatena in which soil development on the landscape was strongly influenced by proximity to fault-controlled springs, which served as a source of supplemental soil moisture. Field evidence for this is seen in an abrupt lateral transition between correlative (< 1.5 m) paleosols at the PTK site in which paleosol B, formed nearest the spring system, is siliceous earthy clay, vs. paleosol A, formed from waxy clay. Bulk geochemical proxies (CIA-K, CALMAG) used to estimate a range of paleoprecipitation (during a 20 k Milankovitch cycle) from 733-944 mm/yr (SE +/- 172) that is higher than published paleohydrological calculations of 250 mm/yr to 700 mm/yr using δD soil values estimated from δD of lacustrine algal- and leaf-lipid biomarkers. The elevated soil moisture and enhanced pedogenic processes occur in groundwater discharge areas. Proxy and pedotransfer functions indicate high soil fertility but with salt accumulation that might have been vegetation-limiting. Paleopedological analysis contributes significantly to understanding the relationships of water budgets for soil systems in arid lands, especially where springs are important sources of freshwater for animals and early hominins.