The Geochemical Behavior Of Cu And Mo In The Yangtze Estuary And Its Adjacent Sea

Estuaries represent an important interface between rivers and oceans within which the flocculation,sinking,complexation and adsorption/desorption of the materials carried by rivers may occur because of the variations of physicochemical and biological conditions.These processes may result in different characteristics of watermasses for rivers and oceans,which induces some uncertainties for the estimate of the oceanic isotopic mass balance model.Estuaries characterized by different geological and environmental settings make divergent influences on the transport of isotopes.Therefore,it is important to obtain more representative data of estuarine systems for the further understanding of global isotopic cycling,which can also provide valuable insights for the evaluation of riverine flux to the oceans.The fractionation of Cu isotopes is controlled by various factors such as weathering,biological activities and redox conditions,while the fractionation of Mo isotopes is mainly controlled by the redox conditions without powerful influences from other factors.The simultaneous studies of the geochemical behavior of Cu and Mo can provide a more comprehensive understanding of the mixing process of freshwater and seawater.The Yangtze River is the third longest river in the world,which contributes a lot of terrestrial materials to the ocean every year.The complicated topography and plenty of human activities make the Yangtze Estuary become an ideal setting for the studies of the river-ocean interactions,anthropogenic impacts on environments and isotopic cycling processes.In this study,the geochemical behaviors of Cu and Mo in the Yangtze Estuary and its adjacent sea were systematically analyzed.Determination of the copper isotopic composition(?⁶⁵Cu)of seawater is difficult due to low Cu concentrations(0.3 to 7.6 n M)and high salt content.In order to analyze the?⁶⁵Cu of saline fluid samples,we built a new method for the preconcentration of Cu in seawater using a Cu-selective resin column.Cu can be totally retained in the Cu-selective resin column while other matrix elements(e.g.,Na,Mg,K and Ca)can be easily eluted at pH=4-9.However,Cu can be easily eluted in the conditions of 2-6 M HCl and acquire a quantitative recovery of 102.3±5.0%(mean,2SD,n=5),which effectively avoid the fractionation of Cu isotopes.Due to the substandard purity of Cu solutions obtained from this preconcentration method,an AGMP-1M anion exchange resin column was used for further separation of Cu isotopes.In this study,seawater samples can be loaded on the Cu-selective resin column under a wide range of pH conditions(from pH=4 to 9)instead of the narrow acidic pH range required in previous studies.The whole procedure is simple and has a low blank of 1.28 ± 0.48 ng Cu(n =4).Results obtained from a series of doping experiments of seawater with an Alfa Cu standard confirmed the accuracy of the proposed method.This new method was then applied in the analysis of Cu isotopic compositions of saline water samples collected in the adjacent sea of Yangtze Estuary.The distribution of Cu in the study area shows discontinuous trends in the freshwater and saline water zones(boundary:?122°E).Aqueous Cu concentrations of freshwater(25.0-36.0 n M)are higher than that of saline waters(1.4-12.1 n M),while the former has an increasing trend and the latter shows a declining trend in the oceanward direction.Similarly,the?⁶⁵Cu of fluid phase(+0.14-+1.87‰)shows an increasing trend in the freshwater zone and a decreasing trend in the saline water zone,with a rapid dive near the boundary of these two zones.Thus,there is a positive relationship between the?⁶⁵Cu and Cu concentrations of the fluid phase.Furthermore,the biological parameters(nutrients and chlorophyll)and Cu concentrations also show a positive relationship,indicating that the preferential complexation of heavy Cu isotopes with organic ligands released by phytoplankton may control the behavior of Cu in the Yangtze Estuary and its adjacent sea.The Cu concentrations of suspended particulate matter(SPM),ranging from about 20 to 65?g/g,decrease gradually with the increase of salinity,and the SPM has a?⁶⁵Cu range of-0.13-+0.31‰.The SPM of the freshwater and seawater endmembers has the same?⁶⁵Cu of?+0.20‰,while the?⁶⁵Cu of SPM in the mixing zone of freshwater and seawater has big negative deviations.The synchronous decreases of?⁶⁵Cu for fluid and solid phases are mainly constrained by the intensive simultaneous adsorption and desorption of Cu from SPM in the Turbidity Maximum Zone(TMZ)and the anthropogenic light Cu inputs.The relatively stronger intensity of Cu adsorption in the TMZ results in the depletion of dissolved Cu and a small flat curve within an overall decreasing trend of SPM Cu concentrations.The Mo concentration and isotopic data for both aqueous and solid phases(i.e.,SPM)were also analyzed in this study.Aqueous Mo concentrations increase rapidly from?13.9 n M(river water)to?115 n M(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?⁹⁸Mo values increase rapidly within the mixing zone from+1.15‰(river water)to+2.3‰(seawater),whereas the?⁹⁸Mo of SPM exhibits a smaller increase in the oceanward direction(from ?0.25 to +0.27 ‰).Aqueous ?⁹⁸Mo 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 bring the dynamic exchange of materials between fluid phase and SPM into the analysis of isotopic fractionation and provide new insights into the geochemical behavior of isotopes in estuarine systems.