Geochemical Behavior and Provenance of Rare Earth Elements in Sediments from the Mekong River Basin: Implications for Weathering Processes and Anthropogenic Disturbance

Abstract:

This study investigates the geochemical behavior, provenance, and environmental significance of rare earth elements (REEs) in surface sediments (0–10 cm) from 42 sampling sites across the Mekong River Basin (MRB). Samples were collected between 2021–2023, covering the upper, middle, and lower reaches, including mainstream and major tributaries. REE concentrations (ΣREE) range from 18.7 to 126.3 mg/kg, with an average of 64.2 mg/kg, generally lower than upper continental crust (UCC) values. Chondrite-normalized REE patterns exhibit light REE (LREE) enrichment (LaN/YbN=3.2–8.7), weak negative Eu anomalies (Eu/Eu=0.72–0.94), and flat heavy REE (HREE) distributions. Provenance analysis indicates that sediments are predominantly derived from felsic igneous rocks in the Tibetan Plateau and Indochina Peninsula, with minor contributions from mafic rocks. Weathering indices (CIA, CIW) and REE ratios (La/Sm, Gd/Yb) suggest moderate chemical weathering in the upper reaches and intense weathering in the lower reaches. Anthropogenic disturbance (mining, agriculture, dam construction) is identified as a key modifier of REE distributions in middle and lower reaches, with elevated LREE concentrations and altered Eu anomalies. This study provides new insights into REE geochemical dynamics in large river basins, supporting environmental monitoring and sustainable management of the MRB.