COLLAPSE OF THE AFRICAN WEATHERING SURFACE ACROSS THE CONGO BASIN TRIGGERED CHANGES IN CENOZOIC OCEAN CHEMISTRY

Rapid increase of ⁸⁷Sr/⁸⁶Sr in seawater over the last 40 million years (Ma) is attributed mostly to exhumation of the Himalayas following collision and orogeny between India and Eurasia. These processes eroded abundant rocks with elevated radiogenic Sr isotope ratios (0.73-0.77), the detritus of which shed into the Indian Ocean to form the Bengal fan (>10 million km³). By contrast, across subSaharan Africa an elevated peneplanation surface known as the African Surface developed during the Kalahari epeirogeny between 120 and 80 Ma, after which intense chemical weathering created a cover of hard duricrusts that protects this plateau from any significant exhumation of crystalline basement. However, across the Congo Basin, thick sequences (100-1000 m) of poorly consolidated Cretaceous red sandstone directly underlying the duricrust initiated extensive subterranean erosion and collapse of the African carapace during the Cenozoic. This rapid erosion across the Congo Basin is matched against the onset of voluminous offshore sedimentation of the Congo fan (>0.7 million km³) along the Atlantic margin during the Eocene-Oligocene transition, at ca. 34 Ma, following a long period (55 Ma) of very subdued sedimentation. Because the source terrains for the red sandstones are largely recycled from a vast system of Precambrian mountain belts, including abundant metamorphic rocks and granitoids with elevated Sr isotope ratios (0.71-0.86), as evidenced by U-Pb detrital zircons provenance analysis, we propose that the sudden late Cenozoic discharge of the Congo system (> 50 million tonnes/yr) could have in turn triggered significant changes in the Sr composition of the ocean.

We present new Sr isotope analyses to test the effects of such large scale erosion processes on ocean chemistry: Kalahari calcretes ⁸⁷Sr/⁸⁶Sr= 0.71-0.73 (0.714 in average); red sandstones ⁸⁷Sr/⁸⁶Sr= 0.73-0.75 (0.739 in average); boulders of silcrete/calcrete ⁸⁷Sr/⁸⁶Sr= 0.73-0.74 (0.733 in average); and dilute acid leach experiments from these endmember rock samples have ⁸⁷Sr/⁸⁶Sr= 0.72-0.73 (0.727 in average). Our results support interpretation of considerable dissolved Sr fluxes from the paleo-Congo River, and our modeling reveals that over the last 34 Ma these Sr fluxes at times surpassed those from the Ganges-Brahmaputra system.