Selenium isotopic constraints on biogeochemical cycling of selenium in the Western Pacific Ocean

Selenium (Se) is an essential micronutrient for marine organisms and its isotopes can be used as a tracer of past environmental conditions. It has been assumed that internal oceanic processes do not cause significant Se isotopic fractionation. However, there have been no studies of the dissolved Se isotopic composition in the oceans, even though Se can undergo various reactions in seawater that might fractionate Se isotopes. Here we present the first full-depth dissolved inorganic Se (DISe) isotopic compositions from the open ocean, which vary from 0.08‰ to 1.50‰ in the Western Pacific Ocean. The δ82/76DISe values exhibit a broadly decreasing trend with water depth. Distinct DISe concentrations and isotopic compositions for different water masses were obtained, with deep waters having nearly constant δ82/76DISe values of 0.20‰ ± 0.06‰. The inverse correlation between δ82/76DISe and ln[DISe] values is controlled mainly by vertical Se cycling, with preferential uptake of lighter DISe isotopes by phytoplankton. Application of closed-system (Rayleigh) and open-system steady-state models to the euphotic zone yields an apparent fractionation factor (ε_app) of −0.78 to −1.00‰ and an intrinsic fractionation factor (ε_intrinsic) of −1.33 to −1.48‰, representing the first field-based estimates of Se isotopic fractionation during biological uptake in the open ocean. Our findings highlight that understanding the Se isotopic systematics in sedimentary rocks requires the consideration of biological fractionation of Se in the euphotic zone in the open ocean. This provides new constraints on the key geochemical processes that affect Se isotopes and their use as a paleoenvironmental proxy.