Diatom records of climate variability from Ethiopian crater lakes

Closed-basin lakes respond to changes in the balance of precipitation and evaporation by changes in water depth and salinity. In turn, lacustrine organisms vary in species composition and abundance in response to the depth and salinity changes. Diatoms, which have silica shells that are readily preserved in most lake sediments, provide a stratigraphic record of past variations in lake hydrochemistry. Their biological responses to the changing lake environment are probably complex, involving threshold responses to several environmental variables (salinity, stratification, mixing, catchment disturbance, nutrient loading, water depth, light penetration etc). Nevertheless, in combination with other palaeoclimatic indicators, diatom stratigraphic data can be used to reconstruct past lake hydrology, including climate-related variability.

The goal of this project is to reconstruct  lake hydrochemistry for the last 2000 years from the diatom stratigraphy of sediment cores from crater lakes in Ethiopia. The five crater lakes at Debre Zeit, (8°45’N 38°59’E; ~1900m altitude) present a unique opportunity to exploit the sedimentary archive of hydroclimatic variability in a semi-arid, monsoonal environment. Although similar in size and hydrology, and sharing a common climatic,  geological and land-use context, the five lakes form a series of contrasting pairs in terms of water chemistry and depth. These contrasts will allow controlled comparisons of  the responses of the lakes to the same events, and thus a greater understanding of the controls on limnic processes.  Carbonate varves in the sediments of two of the lakes provide a timescale with annual resolution. In combination with stable isotope analysis of the carbonates from the same cores, and modelled responses of the lakes to rainfall, the diatom data will provide a high-resolution record of climate change that will be critical to understanding and predicting climate variability in an area that is especially vulnerable to droughts and floods.

The research student appointed to this project will help obtain cores from the lakes during two field expeditions. Working in collaboration with collagues from the University of Addis Ababa, he/she will also monitor monthly changes in the water chemistry and diatom flora of at least one of the lakes. At Aberystwyth, short cores of surface sediments will be freeze dried, resin-embedded and the laminations counted and measured. The microstratigraphic distribution of diatoms, chrysophyte cists, charcoal from summer fires, pollen and aragonite will be described, and compared to seasonal variation in the phytoplankton. Longer cores will be sampled at lower resolution, and the diatoms extracted and counted. Using available diatom hydrochemical transfer functions, the hydrochemistry of the lakes will be reconstructed, and used to infer climate variability during the last 2000 years.

If you would like to discuss this project, please do not hesitate to contact Dr Henry Lamb (hfl@aber.ac.uk).

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