Tracing Nile sediment sources by Sr and Nd isotope signatures (Uganda, Ethiopia, Sudan)

Abstract

Strontium and neodymium isotopes, measured on diverse mud and sand fractions of sediment in transit along all major Nile branches, identify detritus sourced from Precambrian basements, Mesozoic strata, and Tertiary volcanic rocks exposed along the shoulders of the East African rift and in Ethiopian highlands. Sr and Nd isotopic ratios reflect the weighted average of detrital components generated in different catchments, allowing us to discriminate provenance, calculate sediment budgets, and investigate grain-size and hydraulic-sorting effects. 87Sr/ 86Sr and 143Nd/ 144Nd range, respectively, from as high as 0.722 and as low as 0.5108 for sediment derived from Archean gneisses in northern Uganda, to 0.705 and 0.5127 for sediment derived from Neoproterozoic Ethiopian and Eritrean basements. 87Sr/ 86Sr and 143Nd/ 144Nd, ranging 0.705–0.709 and 0.5124–0.5130 for Blue Nile tributaries, are 0.704–0.705 and 0.5127–0.5128 for largely volcaniclastic sediments of River Tekeze–Atbara, and 0.705–0.706 and 0.5126–0.5127 for main Nile sediments upstream Lake Nasser. Model mantle derivation ages ( t DM), oldest in Uganda where sediment is principally derived from the Congo Craton (3.4–3.0 Ga for Victoria and Albert Nile), progressively decrease northward across the Saharan Metacraton, from 2.6 Ga (Bahr el Jebel in South Sudan), to 2.4–2.2 Ga (Bahr ez Zeraf across the Sudd), and finally 1.6–1.3 Ga (White Nile upstream Khartoum). Instead, t DM ages of Sobat mud increase from 0.9 to 1.5 Ga across the Machar marshes. T DM ages are younger for sediments shed by Ethiopian (1.2–0.7 Ga) and Eritrean basements (1.5–1.2 Ga), and youngest for sediments shed from Ethiopian flood basalts (0.3–0.2 Ga). Integrated geochemical, mineralogical, and settling-equivalence analyses suggest influence on the Nd isotopic signal by volcanic lithic grains and titanite rather than by LREE-rich monazite or allanite. Because contributions by ultradense minerals is subordinate, intrasample variability of Sr and Nd ratios is minor. In Blue Nile, Atbara and main Nile sediments of mixed provenance, however, the Nd ratio tends to be higher and t DM ages lower in largely volcaniclastic mud than in mixed volcaniclastic/metamorphiclastic sand. The complete geochemical database presented here, coupled with high-resolution bulk-petrography and heavy-mineral data, provides a key to reconstructing erosion patterns and detrital fluxes across the whole Nile basin, and to investigate and understand how sources of sediment have changed in the historical and pre-historical past in relation to shifting climatic zones across arid northern Africa.