Sr-Nd isotopic fingerprints of Red River sediments and its implication for provenance discrimination in the South China Sea

The Red River has a sediment discharge of 130 Mt/yr and is one of the most important sources of fluvial ma-terials to the South China Sea (SCS). However, due to the paucity of Sr-Nd isotopic data of the Red River delta sediments, the Sr-Nd isotopic fingerprints of the Red River sediment have not been well defined, which hinders the provenance discrimination of modern sediment in the SCS. This study presents the grain size, element, and Sr-Nd isotope data of Red River delta sediments to examine the controlling factors of sediment Sr-Nd isotopic compositions and better characterize the sediment discharge of the modern Red River to the SCS. Compiled rock Sr-Nd isotopic data in the Red River catchment indicate that catchment bedrock lithology exerts a dominant control on distinct sediment Sr-Nd isotopic features among its main branches, Song Da, Thao River, and Song Lo. The Sr-Nd isotopic compositions of Red River delta sediments are concentrated and between these upstream branches, indicating a result of the mixing process. A Bayesian mixing model shows that the Song Lo, Song Da and Thao River make an approximately equal contribution to the modern Red River delta. However, in delta sediments, the correlation between the 87Sr/86Sr and mean grain size suggests a hydrodynamic sorting effect on sediment 87Sr/86Sr ratios, which should be a concern in the calculation of mixing models. Combining newly measured and published data in the Red River delta, we show that the Sr-Nd isotopic fingerprints of Red River sediment are 0.7304 +/- 0.0027 (1a, n = 32) and -12.0 +/- 0.4 (1a, n = 32) respectively. Published Sr-Nd isotopic compositions of delta sediments in the Pearl River and Mekong River are also compiled to make a comparison with the Red River. The Mekong River sediment has less radiogenic 87Sr/86Sr of 0.7212 +/- 0.0008 (1a, n = 7) and higher eNd values of -10.1 +/- 0.7 (1a, n = 8), and Pearl River sediment shows higher 87Sr/86Sr ratios of 0.7378 (n = 1) and eNd values of -11.8 +/- 0.4 (1a, n = 6), which is similar to those of Red River sediment. This study provides new insight into the reliable assessment of sediment Sr-Nd isotopic fingerprints for large rivers in SE Asia, which is favorable for the provenance discrimination of modern SCS sediment.