Paleolimnology of a coastal lake on the Southern Cape coast of South Africa: Sediment geochemistry and diatom distribution

摘要

This study described the changes in paleosalinity over the last 10,000 yr in the coastal lake, Soetendalsv-lei, on the southern coast of South Africa. Sediment cores collected in 2006 were radiocarbon and (210)~Pb dated and analysed for geochemistry, stable isotopes and diatom assemblages. Results indicated that trophic status increased during periods of reduced salinity, as inferred from diatom assemblages and stable isotopes. As freshwater inflow increased to the lake, there was an increase in nutrients and terrestrial organic material input. Increases in salinity could be related to marine intrusion as a result of major marine transgression (~8000 yr and ~4000 yr BP) events for the South African coastline. A comparison of the results with historical records showed that drought and flood cycles greatly influenced the paleo-salinity of the lake. Poor diatom preservation possibly due to high alkalinity was also found in this study and highlighted the importance of using multiple proxies in the reconstruction of paleo-environmental conditions. The trophic development in relation to sea level change inferred from this study, is in close agreement with similar studies undertaken in South American coastal systems.