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The thick Phanerozoic duricrust covering sub-Saharan Africa collapsed about 40-30 million years ago (Ma) across what is now the Congo Basin, driving accelerated erosion and flushing of underlying red bed sediments into the South Atlantic Ocean. Investigated red beds, boulders of silcretes and calcretes exposed around the margins and in river systems of the Congo Basin have Sr-87/Sr-86 values between 0.73 and 0.75. Leaching experiments from these rocks and sediments also reveal high Sr-87/Sr-86 (0.72-0.73). This implies a greater radiogenic Sr flux from the paleo-Congo River compared to its modern value that is influenced to a greater degree now by weathering of carbonates and basalts with relatively low Sr-87/Sr-86 (0.70-0.71). Modeling of our data during the mid-Cenozoic spill of the Congo Basin indicates that increased Sr river flux from the paleo-Congo contributed significantly to the rapid changes of dissolved Sr-87/Sr-86 in the global ocean, complementing at that time a similar effect of increased carbonate weathering linked to the Himalayan-Tibetan Orogeny.

Plain Language Summary Rapid increase of Sr-87/Sr-86 in global seawater since 40 million years ago (Ma), as observed in fossils and marine sequences, has been attributed to increased erosion of radiogenic Strontium-rich rocks from the Tibet-Himalaya Mountains system following collision between India and Asia. By contrast, across vast areas of south central Africa a prominent weathering profile developed during the Kalahari epeirogeny that created a thick layer of duricrust rocks. Collapse of this resistant carapace across the Congo Basin about 34 Ma led to accelerated erosion and flushing of underlying, largely unconsolidated, red bed sediments into the South Atlantic Ocean. New strontium analyses of the red bed sediments and duricrusts, linked to numerical modeling, suggest that this spill of the Congo Basin added significantly to the global ocean geochemistry and, at times, possibly more so than from the Tibet-Himalayan system.