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

International audiencePhytolith assemblage analysis offers the potential to refine our knowledge of paleoecosystems where grasses and sedges predominate. In this work, Holocene and Pleistocene sediments from an arid tropical region in Ethiopia have been analyzed for their phytolith content, presented as detailed counts and diagrams according to the Twiss classification. The aim is to test the usefulness of phytolith assemblages to indicate paleoenvironments at Middle Awash, where fossil pollen grains are poorly preserved in sediments that yielded abundant archaeological remains. The vegetation in the Middle Awash subdesertic valley is currently a shrub steppe dominated by C4 grasses adapted to arid conditions, with a narrow riparian forest limited to the Awash River. 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 modern soil to geological strata seem to occur. Fossil records and modern assemblages are interpreted using phytolith ratios to estimate the density of the tree cover, the aridity and the proportion of C3 versus C4 grasses, as applied to phytolith assemblages from North America and West Africa. The phytolith assemblages from modern soil samples correctly reflect the proportion of trees and shrubs versus grasses, different in the riparian vegetation and the shrub steppe. Modern 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 highlands. Phytolith analysis of the Pleistocene sample evidences grassland formation with scattered woody elements, where C4-Panicoideae grasses, adapted to warm and humid conditions dominate the grass cover. However, these conclusions need to be confirmed by more complete study on phytolith assemblages from modern vegetation from Ethiopia