The geochemical behavior of molybdenum in the modern Yangtze Estuary and East China Sea shelf

Rivers are the main source of molybdenum (Mo) to the oceans, and estuaries represent an important interface between rivers and oceans within which Mo can be sequestered or isotopically fractionated. Determining the behavior of Mo in estuaries is thus essential for a full understanding of the global Mo cycle. Here, we present a case study of the modern Yangtze Estuary and adjacent East China Sea shelf, generating Mo concentration and isotopic data for both aqueous and solid phases (i.e., suspended particulate matter, or SPM, primarily consisting of clay minerals and Al-Fe-Mn-oxides). Aqueous Mo concentrations increase rapidly from similar to 13.9 nM (river water) to similar to 115 nM (seawater) through the mixing zone of the innermost continental shelf. Minor deviations from a salinity-based mixing relationship (positive at < 22 psu to negative at 22-34 psu) indicate a shift from net desorption to net adsorption of Mo on SPM with increasing salinity. Aqueous delta Mo-98 values increase rapidly within the mixing zone from +1.15 parts per thousand (river water) to +2.3 parts per thousand (seawater), whereas the delta Mo-98 of SPM exhibits a smaller increase in the oceanward direction (from -0.25 to +0.27 parts per thousand). Aqueous delta Mo-98 exhibits large negative deviations from a simple two-component (i.e., freshwater-seawater) mixing model due to operation of an additional process, inferred to be Mo exchange between the fluid and solid phases. Simulations using a reaction-transport model (PHREEQC) demonstrated that a combination of two processes (i.e., freshwater-seawater mixing, and fluid-solid Mo exchange) can account for observed patterns of Mo concentrations and isotopes in the Yangtze Estuary. The results of this study thus provide valuable insights into Mo cycling in estuarine systems and indispensable quantitative data for global Mo budgets.

Automated summary

This study investigates the behavior of molybdenum in the Yangtze Estuary and East China Sea shelf, highlighting the importance of freshwater-seawater mixing and fluid-solid molybdenum exchange processes in influencing molybdenum concentrations and isotopic compositions. The results provide valuable insights into molybdenum cycling in estuarine systems and offer indispensable quantitative data for global molybdenum budgets.