Element geochemistry of offshore sediments in the northwestern South China Sea and the dispersal of Pearl River sediments

• Study offshore sediment sources in northwest South China Sea using elemental chemistry.
• Identify main controlling factors influencing bulk elemental compositions.
• Ratios of two element pairs, Ti–Nb and Zr–Hf, are the best chemical tracers.
• Pearl River sediments are mostly found within the modern and relict deltas.
• Pearl River sediments account for a small proportion in offshore area west of Xiachuan Island.

The northwestern South China Sea is drained by the Pearl River and several small rivers in South China. The dispersal of these riverine sediments on the shelf has not been clearly identified via bulk elemental geochemistry due to intensive chemical weathering and the lack of contrasting bedrock geology. A comprehensive elemental geochemistry study was conducted to examine the potential sources of offshore sediments in the northwestern South China Sea. Multivariate statistical analyses of sediment geochemistry reveal that lithogenic element concentrations are strongly positively correlated with fine-grained fractions. Trace metals in offshore sediments, particularly Cu, Zn and Pb, are associated with diagenetic Fe–Mn oxides and the enrichments have been partly caused by anthropogenic activities. Offshore sediments in the northwest Guangzhou Bay exhibit the highest Zr and Hf concentrations, especially in coarse silt and very fine sand fractions. To detect sediment sources, the ratios of immobile element pairs (Ti–Nb, Zr–Hf and Th–Sc) are identified by using graphical analyses. The effects of grain size and sediment sorting on the three elemental ratios are evaluated using correlation analyses. All three elemental ratios reveal that sediments from the Pearl River contain a higher proportion of weathered materials from felsic rocks and are mainly predominant within the modern and relict deltas to the east. Geochemical evidence of the provenance indicates that offshore sediments in the west are mainly from small rivers. Fine-grained sediments escaping out of the Pearl River estuary account for a very small proportion of sediments in western offshore waters of the northwestern South China Sea.