The Zircon Story of the Niger River: Time‐Structure Maps of the West African Craton and Discontinuous Propagation of Provenance Signals Across a Disconnected Sediment‐Routing System.

The Niger River drains a large part of the West African Craton, where rocks ranging in age from Paleoarchean to recent offer an unexcelled opportunity to map the diverse time structures of sediment sources and provide essential information for provenance diagnoses. In this study, U‐Pb zircon dating is complemented with bulk‐sand geochemical (Zr, Hf, REE) and Nd‐Hf isotope data to pin‐point parent rocks of zircon grains and draw inferences on sediment generation across sub‐Saharan western Africa. In Upper Niger sand, zircon ages pass from exclusively Archean in Guinea headwaters to dominantly Paleoproterozoic in the Inner Delta in Mali, testifying to the progressive dilution by tributary sediment derived from the Birimian domain. Zircon ages abruptly change to dominantly Neoproterozoic downstream of the Inner Delta, becoming indistinguishable from those in Saharan eolian dunes across the Sahel. Most of the sediment generated in the headwaters is thus dumped in the marshlands and bedload is reconstituted downstream by recycling eolian sand. Zircon grains in the Lower Niger yielded virtually the same U‐Pb ages as in Benue sediment, indicating an overwhelming supply from the Benue tributary. In the Niger Delta, however, Archean zircons reappear, and both εNd and εHf values become notably more negative, indicating extensive reworking of sand deposited along the coastal plain at earlier times of wetter climate, when artificial barriers to the sediment flux did not exist in the upper to middle Niger River catchment. Plain Language Summary: Across sub‐Saharan western Africa, the Niger River drains rocks with ages spanning more than 3 billion years. In such a vast region, diverse techniques can be combined to identify processes of erosion and transport, starting from the analysis of sediments and the zircon grains they contain. The very old ages of zircons derived from Guinean headwaters (all older than 2650 million years) are gradually diluted by younger zircons generated in SW Mali (from 1900 to 2200 million years), whereas a sharp change to more recent zircon ages (from 500 to 700 million years) is documented downstream of the Inner Delta. Most sediment generated in the headwaters is thus dumped in these marshlands and subsequently replaced with sand eroded from eolian dunes along the southern edge of the Sahara Desert. Rigid segmentation of fluvial sediment transport has become the rule after the extensive construction of dams worldwide, but can also be caused by natural processes. Marshlands formed at the subequatorial edge of tropical deserts can represent very efficient natural barriers to sediment transport, preventing a large part of the detritus generated in the highlands from reaching the ocean, with consequent impact on both sediment budgets and sediment composition. Key Points: The Niger River offers an unexcelled opportunity to use detrital geochronology to map the diverse time structures of sediment sourcesSand generated in the headwaters is dumped in the Inner Delta as indicated by a significant change in the U‐Pb signature and εNd‐εHf valuesReworking of sand along the coastal plain testifies the absence of artificial or major natural barriers to the sediment flux in earlier times [ABSTRACT FROM AUTHOR]