Phytoliths as paleoenvironmental indicators, West Side Middle Awash Valley, Ethiopia

Phytolith assemblage analysis offers the potential to refine our knowledge of paleoecosystems where grasses and sedges predominate. Ln this work, Holo cene and Pleistocene sediments from an arid tropical region in Ethiopia hav e been analyzed for their phytolith content, presented as detailed counts a nd diagrams according to the Twiss classification. The aim is to test the u sefulness of phytolith assemblages to indicate paleoenvironments at Middle Awash, where fossil pollen grains are poorly preserved in sediments that yi elded abundant archaeological remains. The vegetation in the Middle Awash s ubdesertic valley is currently a shrub steppe dominated by C4 rasses adapte d to arid conditions, with a narrow riparian forest limited to the Awash Ri ver. Our results show that modern surface samples, Holocene and Pleistocene sediments contain well-preserved and different phytolith assemblages, and therefore that no translocation processes from modem soil to geological str ata seem to occur. Fossil records and modern assemblages are interpreted us ing phytolith ratios to estimate the density of the tree cover, the aridity and the proportion of C3 versus C4 grasses, as applied to phytolith assemb lages from North America and West Africa. The phytolith assemblages from mo dem soil samples correctly reflect the proportion of trees and shrubs versu s grasses, different in the riparian vegetation and the shrub steppe. Modem phytolith assemblages appear to be a mixed signature of local and regional vegetation. Phytolith analysis of the Holocene sample suggests a grassland , where the grass community is constituted by the Chloridoideae subfamily, adapted to warm and dry conditions and where C3-Pooideae cover the highland s. Phytolith analysis of the Pleistocene sample evidences grassland formati on with scattered woody elements, where C4-Panicoideae grasses, adapted to warm and humid conditions dominate the grass cover. However, these conclusi ons need to be confirmed by more complete study on phytolith assemblages fr om modern vegetation from Ethiopia. (C) 1999 Elsevier Science B.V. All righ ts reserved.