Comparison of rare earth element geochemistry and Nd isotopes in two adjacent tropical estuaries: Van Uc and Cam-Nam Trieu, Northern Vietnam.

Abstract

Concentrations of rare earth elements (REE) were measured in surface waters collected from the Van Ucand Cam estuaries, from the inner river to the estuarine transition zone. Additionally, the neodymiumisotope composition (Nd-IC) of two river water samples was analyzed to compare the two adjacent riversunderdifferentgeochemical conditionsandanthropogenicinfluences.Watersampleswerepreconcentrated using Nobias® resin prior to REE analysis by Sector Field-ICP-MS, while Nd-IC measurementswere performed on a Thermal Ionization Mass Spectrometer (TIMS) after several treatments. The total REEconcentrations(ƩREE) in the Van Uc estuary ranged from 10.5 to 23.6 nmol/kg, significantly higher thanthose in the Cam-Nam Trieu estuary, which ranged from 2.14 to 6.34 nmol/kg. Both Van Uc and Camriverine and estuarine water are enriched in the middle REE (MREE) relative to light REE (LREE), withdepletion observed in heavy REE (HREE) when normalized to shale compositions. No negative ceriumanomalies (Ce*/Ce) were observed in any sample, reflecting the predominance of riverine influence in theestuarine mixing zone. Gadolinium anomalies (Gd*/Gd) were uniformly positive but low(≤ 1.3), implyingminimal inputs from gadolinium-based MRI contrast agents or sufficient dilution to suppress detectableanthropogenic Gd signals. Nd-IC of the Van Uc water wasεNd = -10.5while of Cam-Nam Trieu water was lessradiogenic, withεNd = -11.3.