Geochemical—Mineralogical Constraints on the Provenance of Sediment Supplied From South African River Catchments Draining Into the Southwestern Indian Ocean

Abstract

This study utilizes the radiogenic isotopes of neodymium (Nd) and strontium (Sr) measured in river mud —which serves as a proxy for the suspended load—to examine how source-rock lithology and weathering intensity impact the composition of material delivered to marine sediments along the Southeast African continental margin. Sediments were sampled from 22 river catchments between Durban and Cape Town in South Africa. Two distinct endmembers (i.e., geographical source areas) characterizing the regional river systems were identified. According to endmember modeling, rivers of the Eastern Cape drain soils weathered of the Karoo Supergroup and Drakensberg basalts (⁸⁷Sr/⁸⁶Sr of 0.74049 and εNd of −8.53), whereas rivers of the Southern Cape drain the Cape Supergroup, exhibiting more radiogenic ⁸⁷Sr/⁸⁶Sr ratios (0.74596) and unradiogenic εNd values (−10.09). River sediments from the Karoo Supergroup are finer-grained compared to those from the Cape Supergroup. We analyzed clay mineralogy and two grain size fractions (<2 μm; 2–32 μm) from co-registered samples, and the results suggest that the spatial variability in the ⁸⁷Sr/⁸⁶Sr values is due to the lower degree of chemical weathering of silicate rocks and increased illite abundances toward the south, which a simple mixing model can explain. Kaolinite abundance decreased from Northeast to Southwest within South African coastal river catchments. In contrast, illite abundances increased due to diminished chemical weathering in the southern regions driven by the weaker rainfall regime. As demonstrated in this study, combining geochemical tools provides a reliable foundation for provenance studies of offshore marine sediment studies.