Study of the sources and dispersion of sediments on the Sunda Shelf based on the investigation of rare earth element concentrations and Nd isotope compositions

Abstract

The Sunda Shelf receives substantial sediment inputs from the Tibetan Plateau and Southeast Asia's mountainous islands, impacting southern South China Sea sedimentation during Quaternary sea-level changes. Due to multiple river basins contributing to the shelf, the origin and distribution of its surface sediments remain poorly defined. Here, we analyzed major and trace element concentrations and Nd isotope compositions (¹⁴³Nd/¹⁴⁴Nd expressed in epsilon units, εNd) of sediments from rivers around the Sunda Shelf and its surface sediments, focusing on <63 μm and < 2 μm fractions to evaluate grain size effects on rare earth element (REE) patterns and εNd compositions and to identify sediment sources. Significant differences in UCC-normalized REE patterns between <2 μm and < 63 μm river sediment fractions indicate a notable grain size effect. However, εNd variations between these fractions are typically less than 1 εNd unit, suggesting minimal grain size impact on Nd isotope compositions of the detrital fractions. Surface sediments on the western Sunda Shelf derive from the Malay Peninsula, Mekong River, and northern Thai rivers based on the δEu_UCC-(Gd/Yb)_UCC diagram. εNd spatial distribution of the detrital fraction indicates nearshore sediments of the Sunda Shelf are influenced by neighboring rivers (from −15.8 to −6.6), while the central region is affected by the Mekong and northern Thai rivers (from −15.8 to −10.3). Notably, unradiogenic εNd in central shelf sediments indicates Thai river sediments were transported there via paleo-channels during low sea levels before the Gulf of Thailand fully flooded. A large eddy in the central Sunda Shelf would have conserved these relic sediments and redistributed modern sediments by sorting and transporting sediments from nearby rivers.