Holocene palaeolimnology of Kajemamm Oasis, Northern Nigeria: an isotopic study of ostracodes, bulk carbonate and organic carbon

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Abstract
A high-resolution 5500 year palaeolimnological record from Kajemarum Oasis, a closed basin in the Manga Grasslands of northern Nigeria, provides evidence of environmental change in Subsaharan Africa during the Holocene. Palaeohydrological variations, mainly changes in the balance between precipitation and evaporation, are recorded by stable oxygen isotope ratios in bulk carbonate and ostracod calcite and by the Sr/Ca ratio in ostracode shells. Variations in the carbon isotope ratios in carbonates indicate changes in primary productivity in the lake, whereas the carbon isotope composition of organic carbon reflects ecophysiological processes within the lake and its surrounding catchment. Results indicate that there have been marked environmental changes in the Manga Grasslands over the last 5500 years. A more variable climate set in around 1500 cal. a BP. A prolonged drought between 1200 and 1000 cal. a BP, with reduced aquatic productivity, was followed by a switch to a wetter, but still unstable climate: moist conditions prevailed during the Little Ice Age. The results indicate that drought has affected the Sahel episodically over the last 1500 years and is not solely a twentieth-century phenomenon.