Dynamic mobilization of redox sensitive elements Mo, U and V in seasonal hypoxic sediments off the Changjiang Estuary

Abstract

The coastal waters off the Changjiang Estuary are a seasonal hypoxic region, offering an ideal system for studying the geochemical cycles of redox sensitive elements (RSEs) at the sediment-water interface (SWI) and their response potential to redox changes. The profiles of porewater and solid RSEs and their chemical speciation and diffusive fluxes at five sites in two seasons were studied to evaluate the migration and removal of RSEs at the SWI off the Changjiang Estuary. Results showed that porewater and solid RSEs displayed the corresponding profile variations. The porewater Mo and U concentrations generally decreased with depth, attributed to the reductive removal. Particularly, the removal of Mo may be mediated by the Fe–S phase, and the removal of U appears to be via the microbially-mediated reduction. Porewater V, U and Mo were removed successively with depth, corresponding to the peaks of dissolved nitrate, Fe2+ and acid volatile sulfur (AVS). However, porewater V increased again in the deep layers due to its complexation with dissolved organic matter. The upward shift of RSE peaks in porewater and transformation depths of their oxidation states based on the model from spring to summer reflected the occurrence of hypoxia in summer. The downward diffusive fluxes of RSEs at hypoxic site in summer indicated their hypoxia-induced enrichment. The Mo–U covariation suggested that the seasonal hypoxic sites off the Changjiang Estuary were more favorable for the accumulation of authigenic U.