The Holocene lake-evaporation history of the afro-alpine Lake Garba Guracha in the Bale Mountains, Ethiopia, based on delta O-18 records of sugar biomarker and diatoms

In eastern Africa, there are few long, high-quality records of environmental change at high altitudes, inhibiting a broader understanding of regional climate change. We investigated a Holocene lacustrine sediment archive from Lake Garba Guracha, Bale Mountains, Ethiopia, (3,950 m asl), and reconstructed high-altitude lake evaporation history using delta O-18 records derived from the analysis of compound-specific sugar biomarkers and diatoms. The delta O-18(diatom) and delta O-18(fuc) records are clearly correlated and reveal similar ranges (7.9 parts per thousand and 7.1 parts per thousand, respectively). The lowest delta O-18 values occurred between 10-7 cal ka BP and were followed by a continuous shift towards more positive delta O-18 values. Due to the aquatic origin of the sugar biomarker and similar trends of delta O-18(diatom), we suggest that our lacustrine delta O-18(fuc) record reflects delta O-18(lake water). Therefore, without completely excluding the influence of the 'amount-effect' and the 'source-effect', we interpret our record to reflect primarily the precipitation-to-evaporation ratio (P/E). We conclude that precipitation increased at the beginning of the Holocene, leading to an overflowing lake between ca. 10 and ca. 8 cal ka BP, indicated by low delta O-18(lake water) values, which are interpreted as reduced evaporative enrichment. This is followed by a continuous trend towards drier conditions, indicating at least a seasonally closed lake system.