Holocene climate and vegetation change in the Main Ethiopian Rift Valley, inferred from the composition (C/N and δ 13C) of lacustrine organic matter

Abstract

In order to track changes in the relative abundance of C 3 and C 4 plants in savanna vegetation, C/N and δ 13C values were measured on bulk organic material in an 8840 14C-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 δ 13C values thus provide a proxy for changes in catchment vegetation that are supported by pollen data. δ 13C values in the early Holocene are relatively low, reflecting the dominance of C 3 vegetation (woody plants) and a more humid climate. δ 13C shows no response to a known regional arid interval at 7800 yr BP, suggesting that woody vegetation was able to survive relatively prolonged dry periods. A gradual, rather than sharp, δ 13C response to the end of the early Holocene humid interval at ∼4500 yr BP further supports this. Higher δ 13C values at ∼2800–2300 and ∼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 δ 13C 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.