Distribution and fractionation of rare earth elements in suspended sediment of the Zhujiang River, Southwest China

摘要

Purpose The geochemistry of rare earth elements (REEs, La to Lu) in suspended sediments (SS) is generally controlled by weathering processes and the water environment. Although the concentrations and fractionations of REEs in SS have been reported in the Zhujiang River, China, their variations and controlling factors have not been researched. Thus, the objective of this study was to identify the REE controlling factors in SS through their variations of concentrations and fractionations during 14 years (from 2000 to 2014). Material and methods The concentrations of REEs, light REE (LREE, La to Nd), middle REE (MREE, Sm to Ho), and heavy REE (HREE, Er to Lu) in the SS of the Zhujiang River were investigated, and REE fractionation proxies including n-ary sumation LREE/ n-ary sumation HREE, n-ary sumation MREE/ n-ary sumation HREE, and Ce and Eu anomalies were calculated. Furthermore, the Pearson correlation relationships between physicochemical parameters, REE concentrations, and REE fractionation proxies were analyzed. Results and discussion The n-ary sumation REE concentrations of SS in the Zhujiang River varied from 35.9 to 396.4 mg kg(-1) (mean 229.6 mg kg(-1)), higher than the world's rivers (mean 174.8 mg kg(-1)). Moreover, n-ary sumation REE, n-ary sumation LREE, n-ary sumation MREE, and n-ary sumation HREE concentrations increased along the flow direction. PAAS-normalized REE ratios showed that MREE in SS were enriched relative to LREE and HREE. The concentrations of all REEs in SS significantly correlated with Fe, Al, and K concentrations positively, while significantly correlating with Ca and Na concentrations in a negative trend. These correlation relationships between the concentrations of REEs and major metal elements were attributed to the spatial variation of rock distribution based on the same source (weathering processes and soil erosion) of REEs and major metal elements in SS. The physicochemical properties affected REE fractionation in SS, such as clay minerals, water pH, and dissolved oxygen (DO). During 14 years from 2000 to 2014, the decrement of soil erosion reduced the REE concentrations of SS in the upper reaches. Meanwhile, river water acidification resulted in the weakening negative Ce anomaly and positive Eu anomaly of SS. These results suggested that the variations of REE concentrations and fractionations in SS were mainly controlled by soil erosion and water pH in the Zhujiang River from 2000 to 2014. Conclusions The concentrations of REEs in SS mainly depend on weathering processes, while REE fractionations are closely associated with river water physicochemical properties in the Zhujiang River.