Sr-Nd isotopic constraints on source-to-sink processes of fine-grained fluvial sediments in the South China

Source-to-sink processes of river systems play a fundamental role in shaping coastal morphology and the stratigraphic record. Sr Nd isotopes are commonly employed to decipher sediment routing in fluvial systems. However, discrepancies in provenance signals across grain-size fractions introduce significant uncertainty. Here, we present a comprehensive Sr Nd isotopic study of the clay and fine-silt fractions within fine-grained river sediments from South China. Although the strong correlation between 87 Sr/ 86 Sr and εNd values in all fine-grained fluvial sediments from South China indicates a dominant provenance control, the 87 Sr/ 86 Sr offsets between clay and fine-silt fractions (Δ 87 Sr/ 86 Sr (clay-fine-silt) ) show a moderate positive correlation with CIA (Chemical Index of Alteration) values and clay mineral compositions. These differences in Δ 87 Sr/ 86 Sr (clay-fine-silt) are therefore attributed to variations in chemical weathering intensity across the study area. In the poorly to moderately weathered regions of inland South China, illite-rich clay fractions generally exhibit higher 87 Sr/ 86 Sr ratios than the corresponding fine silts. In contrast, illite-poor clay fractions from intensely weathered river systems display lower 87 Sr/ 86 Sr values relative to their associated fine silts. Principal component analysis (PCA) of the clay and fine-silt fractions using trace elemental ratios as well as Sr Nd isotopic compositions identifies three major potential sediment sources for the Pearl River, the largest river system in South China. Monte Carlo simulations further indicate that sediments derived from the upper reaches contribute more than 60% of the clay and fine-silt fractions delivered to the estuary, whereas sand fractions are predominantly sourced from proximal regions. Differential weathering rates of source rocks, superimposed on sediment transport dynamics, play a significant role in determining the provenance composition of estuarine sediments. This study reveals the complex provenance differentiation across grain-size fractions in one large river system and highlights the potential pitfalls of relying solely on mixed grain-size fractions in provenance analysis to trace the riverine evolution.