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A record of vegetation dynamics and lake level changes from Lake Emakat, northern Tanzania, during the last c. 1200 years

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Abstract

Analyses of down-core variations in pollen and charcoal in two short cores of lake sediment and wood samples taken from the in situ remains of Nuxia congesta from Lake Emakat, a hydrologically-closed volcanic crater lake occupying the Empakaai Crater in northern Tanzania, have generated evidence of past vegetation change and lake level fluctuations. Eight AMS radiocarbon (14C) dates on bulk samples of lake sediment provide a chronological framework for the two cores and indicate that the sediment record analysed incorporates the last c. 1200 years. The in situ remains of a Nuxia congesta tree, now standing in deep water, were dated with three additional AMS 14C dates, suggesting tree growth within the interval ∼1500–1670 AD. Down-core variations in pollen from terrestrial taxa, particularly the montane forest trees Hagenia abyssinica and Nuxia congesta, indicate a broad period of generally more arid conditions in the catchment to c. 1200 AD and at a prolonged period between c. 1420 and 1680 AD. Variations in pollen from plants in lake margin vegetation indicate low lake levels, presumably as a result of reduced effective precipitation, contemporary with indications of relatively dry conditions mentioned above, but also during the late 18th and the late 19th centuries. The presence of charcoal throughout both cores indicates the frequent occurrence of vegetation fires. An increase in burning, evident in the charcoal data and dated to the early to mid second millennium AD, could relate to an expansion of human population levels and agricultural activity in the region.

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Acknowledgements

The authors are grateful for the financial support for this research provided by the Swedish Research Council (VR), the Granholm Foundation, the Swedish Society for Anthropology and Geography (SSAG), Lillemor and Hans W:son Ahlmann, Axel Lagrelius, Carl Mannerfelt and Margit Ahltins foundations, and the Swedish Foundation for International Cooperation in Research and Higher Education (STINT). The radiocarbon dates were provided by Ångström Laboratory, Div. of Ion Physics, 14C-lab, UPPSALA, Sweden and Poznań Radiocarbon Laboratory, ul. Rubież 46, 61–612 Poznań, Poland. Helena Öberg kindly provided us with a sediment date. We thank our collaborator in Tanzania, Alfred Muzuka, Institute of Marine Science, University of Dar es Salaam. We would also like to thank Guillaume Buchet for his assistance at the palynological laboratory at CEREGE. The Tanzania Commission for Science and Technology (COSTECH) and Tanzania Wildlife Research Institute (TAWIRI) kindly authorised research permission and fieldwork. Particular thanks for assistance while in the field are due to Victor Runyoro, a conservation authority officer at Ngorongoro. Finally, we are grateful to two anonymous referees of an earlier version of this paper for their highly constructive comments.

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  1. Department of Physical Geography and Quaternary Geology, Stockholm University, 10691, Stockholm, Sweden

    Maria Ryner & Karin Holmgren

  2. Department of Geography, Trinity College, University of Dublin, Dublin 2, Ireland

    David Taylor

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  1. Maria Ryner
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Correspondence to Maria Ryner.

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Ryner, M., Holmgren, K. & Taylor, D. A record of vegetation dynamics and lake level changes from Lake Emakat, northern Tanzania, during the last c. 1200 years. J Paleolimnol 40, 583–601 (2008). https://doi.org/10.1007/s10933-007-9184-0

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