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

Abstract

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 ~13.9 nM (river water) 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 δ98Mo values increase rapidly within the mixing zone from +1.15‰ (river water) to +2.3‰ (seawater), whereas the δ98Mo of SPM exhibits a smaller increase in the oceanward direction (from −0.25 to +0.27‰). Aqueous δ98Mo 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.