The Holocene lake-evaporation history of the afro-alpine Lake Garba Guracha in the Bale Mountains, Ethiopia, based on δ18O records of sugar biomarker and diatoms

Lucas Bittner,Hanno Meyer,Michael Zech,Henry F Lamb,Mikaela Weiner,Graciela Gil-Romera,Dai Grady,Eva Lorenz,Tobias Bromm,Bruno Glaser

doi:10.1017/qua.2021.26

Abstract

AbstractIn 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 δ18O records derived from the analysis of compound-specific sugar biomarkers and diatoms. The δ18Odiatom and δ18Ofuc records are clearly correlated and reveal similar ranges (7.9‰ and 7.1‰, respectively). The lowest δ18O values occurred between 10–7 cal ka BP and were followed by a continuous shift towards more positive δ18O values. Due to the aquatic origin of the sugar biomarker and similar trends of δ18Odiatom, we suggest that our lacustrine δ18Ofuc record reflects δ18Olake 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 δ18Olake 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.

Highlights

The climate of eastern Africa is driven by the position of the tropical rain belt
The eastward extent of Congo Air Boundary (CAB) movement is related to the seasonal changes of Indian Summer Monsoon (ISM) strength (Camberlin, 1997)
The similar trends of δ18Ofuc and δ18Odiatom results indicate that δ18Ofuc is a valuable proxy for δ18Olake water

Summary

Introduction

The climate of eastern Africa is driven by the position of the tropical rain belt. Increasing north hemisphere insolation during northern hemisphere summer forces the tropical rain belt and ITCZ northward and the Congo Air Boundary (CAB) migrates eastward (Nakamura, 1968; Hills, 1979; Davies et al, 1985). The eastward extent of CAB movement is related to the seasonal changes of Indian Summer Monsoon (ISM) strength (Camberlin, 1997). An increased W-E atmospheric pressure gradient between Africa and India during the northern hemisphere summer enhances the ISM (Wagner et al, 2018). The enhanced ISM redirects Indian Ocean air masses towards India preventing them from penetrating deeply into eastern Africa. The convergence zone of Indian Ocean air

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Discussion

Conclusion