Lacustrine isotope archives of past climate and environmental changes in the tropics and subtropics: Some examples from Africa

The paper shows how isotopic data from lake sediments may contribute to an understanding of the response of continental systems to changes in major climate forcing factors. It focuses on the African tropics and subtropics. Selected examples highlight the interest of (1) carbon isotopes in plant material and (2) oxygen isotopes in carbonates in groundwater fed lakes. In organic rich lacustrine sediments, carbon isotope composition in plant material can provide insights into past changes in biome distributions and the carbon cycle. The major factors that control 13C/12C ratios in organic matter in peats and lake sediments are briefly summarized. In Africa, a major decrease (≥10 per mille) in d13C is observed in peats and lake records from both hemispheres during the last deglaciation. Comparison with other proxy data (notably pollen) and with past atmospheric CO2 concentrations suggests that the latter have been the major factor controlling the distribution of tropical biomes rather than available water. Carbonate rich sediments from groundwater fed lakes may provide a means of reconstructing climatic conditions that controlled the lake status. In appropriate systems, climatic parameters, e.g. ambient humidity, can be theoretically determined if a salt balance equation is introduced in addition to those for the water and isotopes and if information on inflow waters is available from groundwater records. This is exemplified with a small Holocene palaeolake from the northern Sahara. Lacustrine isotope archives represent a powerful tool for extending out knowledge of the amplitude and causes of climate changes. In many cases, interpretation of the isotope signal should be supported by a multi-proxy approach. Detailed studies of modern systems as references should be encouraged for establishing calibration functions that may be used to transfer isotope proxies in terms of quantitative environmental and climate variables. (author)