Fractionation of Dissolved Selenium Isotopic composition during a Phytoplankton Bloom in an Estuary

Selenium (Se) isotopes have been widely used to track the Earth's redox evolution based on the assumption that terrestrial Se is faithfully transported at the land-ocean interface without nonconservative behavior or biological fractionation affecting its isotope composition. However, dissolved Se isotope behavior in estuaries is poorly constrained, which calls this assumption into question. In this study, we present the first dissolved inorganic Se (DISe) isotope dataset along a salinity gradient during transport of the Changjiang river. Water and phytoplankton samples from a bloom event were collected. The δ82/76DISe values increased from 0.96‰ to 1.74‰ along the salinity gradient (from 0.1 to 34.5). The nonconservative behavior of DISe was revealed by large offsets of δ82/76DISe from theoretical mixing of the bloom waters. The dynamics of δ82/76DISe in the Changjiang Estuary (CJE) were controlled by the combination of physical mixing and biological utilization. In those bloom waters, the heavy δ82/76DISe values (1.66‰ ± 0.07‰) in the waters corresponded to the light Se isotopes for phytoplankton (−0.35‰ ± 0.18‰). The significantly increased δ82/76DISe signatures in the bloom waters due to the utilization of DISe by phytoplankton led to an Se isotope fractionation factor (reported as ε82/76Se = 1000 × (α82/76Se − 1) in) of −1.92‰ ± 0.16‰ (Rayleigh model) or −2.14‰ ± 0.18‰ (steady-state model). We propose that biological activities modify the nonconservative activity of Se and induce significant Se isotope fractionation with a factor of approximately −2‰ during transport. Our findings highlight that interpreting Se isotopes in the sedimentary record requires the consideration of nonconservative behavior and biological fractionation of Se.