PALEOENVIRONMENTAL EVOLUTION OF SOUTH-EASTERN ETHIOPIAN HIGHLANDS (BALE MOUNTAINS) DURING THE LAST 13,000 YEARS

Lakes in the main Ethiopian Rift Valley are now considered as one of the reference sites for paleoclimatic reconstruction in the tropics. They provide evidence for a regional pattern of lake highstands between 10-5 kyr BP, interrupted by abrupt arid intervals resulting from global mechanisms. However, recent work indicates that some of the environmental changes were caused by non-climatic, local factors. Lakes and swamps at high altitudes have proved suitable for paleoclimatic reconstruction. Most Ethiopian mountains were affected by Pleistocene glaciation, which retreated after ~14 kyr BP. Earlier cores collected from the south Ethiopian mountains were either discontinuous, short or have provided low-resolution data. We recently investigated a new site in the Bale Mountains of Ethiopia, on the Southeastern Plateau east of the Main Ethiopian Rift Valley. In May 2001, we obtained a 15.82m core with a basal radiocarbon age of ~13 kyr BP from lake Garba Guratsch at 4000m. This core has the potential to provide the first high resolution, continuous record spanning the Late Glacial to the Holocene for the Ethiopian mountains.

Initial results of analysis of sediment, organic matter and pollen are presented here. The sediment consists of greenish-grey silt with layers of sand and gravel from the base to 9m (~13-10 kyr BP), and organic gytta from 9m to the top (10-0 kyr BP). Total organic carbon is up to 3% in the lower clastic sediment and reaches 11% in the upper organic unit. Pollen analysis shows the dominance of Amaranthaceae/Chenopodiaceae, Poaceae and Artemisia in the Lateglacial sediment, perhaps indicating cold steppe vegetation on a deglaciated landscape. The high sedimentation rate between 11-10 kyr BP (5m in 1000 yrs) could be due to high sediment transport from a sparsely vegetated landscape around the lake. The rise in Ericaceae after 10 kyr BP, coincident with the deposition of organic gyttja, marks the beginning of the Holocene. This may indicate a warm and humid climate with stabilization of the slopes and soil development. The fall in Ericaceae and the rise in Podocarpus and Juniperus (now growing on the northern drier slope of the Bale mountains) above 4m in the core could relate to drying of the mid-Holocene climate. The rise in Plantago and the fall in Podocarpus above 2m may be evidence of human impact.