PALEOCLIMATIC OSCILLATIONS IN EAST-AFRICA INTERPRETED BY ANALYSIS OF TRACE-ELEMENTS IN NILE DELTA SEDIMENTS

Instrumental neutron activation analysis of rare-earth and other trace elements in the light-mineral fraction of sands from three cores in the Nile delta provides data about climatic changes that have affected the East African headwaters of the River Nile during the past 30,000 years. The sediments of the delta originate primarily from two distinct geologic terranes and geochemical sources, one source being the Ethiopian plateau and the other being the Central African plateau. These sources are reflected by different trace-element ratios found in the sediments, and the relative contributions of sediment from these sources have varied over time depending upon climatic conditions. Today's East African climatic belts, which are oriented in an east-west direction and differ markedly from one another, range from tropical and humid to warm and dry. These belts have oscillated north and south during the late Pleistocene and Holocene, a fact further confirmed by the distributions of chromium, scandium, lanthanum, and lutetium in the sediments, particularly the feldspars, of the Nile delta. The distributions of these trace elements reveal even greater detail about paleoclimatic oscillations than can be obtained by using heavy-mineral studies alone.