Holocene climate and vegetation change in the Main Ethiopian Rift Valley, inferred from the composition (C/N and δ13C) of lacustrine organic matter

In order to track changes in the relative abundance Of C-3 and C-4 plants in savanna vegetation, C/N and delta(13)C values were measured on bulk organic material in an 8840 C-14-year record from Lake Tilo, Ethiopia. Between 8840 and 2500 BP, high C/N ratios Suggest that input to the lake was predominantly from terrestrial plants. The corresponding delta(13)C values thus provide a proxy for changes in catchment vegetation that are supported by pollen data. delta(13)C values in the early Holocene are relatively low, reflecting the dominance of C-3 vegetation (woody plants) and a more humid climate. delta(13)C shows no response to a known regional and interval at 7800 yr BP, Suggesting that woody vegetation was able to Survive relatively prolonged dry periods. A gradual, rather than sharp, delta(13)C response to the end of the early Holocene humid interval at similar to 4500 yr BP further supports this. Higher delta(13)C values at similar to2800-2300 and similar to 1000 yr BP correspond to increases in sedge pollen, thought to be growing in freshwater springs, exposed as lake-level fell. The C/N and delta(13)C Composition of bulk organic material complements the pollen evidence and may be useful in other lakes in savanna regions as indicators of terrestrial vegetation change. (C) 2004 Elsevier Ltd. All rights reserved.