Late Glacial-Holocene diatom record of water chemistry and lake level change from the tropical East African Rift Lake Abiyata (Ethiopia)

摘要

Diatom study of a 12.6-m long sediment core (AB95II) from Lake Abiyata (Ethiopian Rift, ～8°N) revealed large amplitude changes in the water level and chemistry over the past 13.4 cal kyr BP. Past water conductivity, pH and water chemical facies, reflected byionic ratios, are quantitatively reconstructed, based on diatom transfer functions previously established. Conductivity has varied from 400 to 21 500 μS cm~(-1), pH from 8 to 10.5; the anion ([HCO_3~-+CO_3~(2-)]/[Cl~-+SO_4~(2-)] and cation ([Na~++K~+]/[Ca~(2+)+Mg~(2+)]) ratios ranged between ～0.9-4, and 0.5-275 respectively. A high water salinity oscillation is recorded at the base of the sequence; the extrapolated age of this interval (12.7-12.0 cal kyr BP) corresponds to the Yonger Dryas Chronozone. The Early-Middle Holocene registers a high-stand freshwater lake between 12 and 5.4 cal kyr BP. Changes in diatom assemblages suggest minor changes in water depth, lake mixing and/or nutrient content, during this interval. Post-5.4 cal kyr BP, shallow, saline conditions generally predominate. Short-term regressive events are recorded at 5.6 and 3.0 cal kyr BP. Our results are roughly consistent with previous works in the region. They refine the timing of the hydrochemical changes and quantify their amplitude, thus providing inputs for modelling approaches.