Sr-Nd Isotopic Compositions and elemental ratios of fine-grained fluvial sediments in South China

Source-to-sink processes govern the characteristics of fluvial sediments delivered to estuaries, which in turn play a fundamental role in shaping coastal morphology and the stratigraphic record. A suite of chemical and mineralogical proxies is commonly employed to decipher sediment routing in fluvial systems. However, discrepancies in provenance signals across grain-size fractions can introduce significant uncertainty. Here, we present a comprehensive Sr-Nd isotope study on fine-grained fractions from river sediments in the South China. The provenance compositions of clay and fine silts are compared with reported data for coarse fractions in previous studies. The Sr isotopic compositions of these fluvial sediments differ between clay and fine silt fractions, with these differences ascribed to variations in chemical weathering intensity within the study area and mineral sorting during transport. In intensely weathered river systems, kaolinite-rich clay fractions display lower ⁸⁷Sr/⁸⁶Sr ratios than their corresponding fine silts. In contrast, kaolinite-poor clay samples from inner South China exhibit higher ⁸⁷Sr/⁸⁶Sr values than the associated fine silts. However, the Sr-Nd isotopic compositions of fine-grained riverine sediments are dominated by the geochemical features of their source rocks. Using Sr-Nd isotopic compositions of sediments from tributaries, Monte Carlo simulations indicate that sediments from the upper reaches contribute over 50% of the clay and 85% of the fine silts in the estuary of the Pearl River, the largest river in South China. In contrast, sand fractions are derived predominantly from the lower and middle reaches. Differential weathering rates of source rocks, superimposed on sediment transport dynamics, likely play a significant role in determining the provenance composition of estuarine sediments in the Pearl River. This study reveals the complex provenance differentiation across grain-size fractions in the river system and highlights the potential pitfalls of relying solely on mixed grain-size fractions in provenance analysis to trace the riverine evolution.