Selenium (Se) isotopes has been widely used to track Earth's redox evolution, based on the assumption terrestrial δ^82/76Se is faithfully transported to the ocean. While dissolved Se isotope behavior in the estuary is poorly constrained which limits the robust interpretation of Se isotope variability. Therefore, we collected water and phytoplankton samples from a large river estuary system, Changjiang Estuary, and present the first dissolved inorganic Se (DISe) isotope dataset from a salinity gradient. The dδ^82/76DISe values increased from 0.96 to 1.74‰ along the salinity gradient in the CJE. The δ^82/76DISe showed non-conservative behavior during estuarine mixing. The heavy d^82/76DISe values (1.55 ± 0.20‰) in waters corresponding with light Se isotopes for phytoplankton (-0.35‰ ± 0.18‰) in diatom-bloom areas. The significantly increased δ^82/76Se signatures due to phytoplankton utilization of DISe were observed and reflected a Se isotopic enrichment factor (ͼ) of -1.92 ± 0.16‰ (Rayleigh model) or -2.14 ± 0.18‰ (steady state model). Biological activities alter to nonconservative activity of Se and also introduce large and variable Se isotopic fractionation.  Therefore, interpreting Se isotopes in the sedimentary record requires taking biological process into consideration.
 

Selenium isotopes, biological isotopic fractionation, diatom bloom, Changjiang Estuary, Estuarine processes