Iron, Nickel, Copper, Zinc, and their stable isotopes along a salinity gradient in the Pearl River Estuary, southeastern China

Although the crucial role that dissolved trace metals (dTMs) play in both carbon cycling and climate have been revealed over the past three decades, much about the oceanic budgets of dTMs remains unknown. Rivers are one of the most important dTM sources to the ocean, and estuaries are a key interface between land and ocean. However, our understanding of how dTMs behave as they pass through the estuarine transition is limited, leading to uncertainties in estimating fluvial metal fluxes to the ocean and balancing oceanic dTM budgets. Alongside concentrations, metal stable isotopes offer an additional dimension to constrain estuarine processes, but data on metal isotopes in estuaries are scarce. Here we present dTM concentrations and their stable isotope ratios along a full salinity gradient in the Pearl River Estuary (PRE), southeastern China. Concentration data show a large apparent loss of dFe (86%) during estuarine mixing, moderate losses of dNi (13%) and dZn (36%), and nearly conservative behavior for dCu. However, examination of metal isotope data (delta Fe-56, delta Ni-60, delta Cu-65, and delta Zn-66) reveals more complex biogeochemistry, requiring either a two-stage scenario or a three-endmember mixing process. The two-stage process involves fractionation of dTM isotopes at low salinities, likely driven by particle adsorption and colloidal flocculation, followed by conservative mixing between intermediate-salinity estuarine waters and South China Sea waters. Alternatively, the three-endmember mixing process includes influences from riverine, oceanic endmembers, and external sources such as benthic flux and industrial activities, shaping dTM isotope distributions. Specifically, capturing delta Fe-56 fully proves challenging, yet it appears strongly influenced by inputs of benthic Fe within the PRE, characterized by dFe > 13 nmol kg(-1) and delta Fe-56 < -0.80 parts per thousand. delta Ni-60 and delta Zn-66 can be described by either two-stage or there-endmember mixing processes. In the former, the Rayleigh fractionation with alpha(d-p) of 1.00004 for delta Ni-60 and 1.0001 for delta Zn-66, or the steady state fractionation with alpha(d-p) of 1.00005 for delta Ni-60 and 1.0002 for delta Zn-66, could be derived to explain their patterns at low salinity, likely driven by colloidal flocculation. Additionally, a third source with dNi > 33 nmol kg(-1) and delta Ni-60 > +1.26 parts per thousand and a third source with dZn > 13 nmol kg(-1) and delta Zn-66 > +0.96 parts per thousand, both influenced by human activities, could also shape the delta Ni-60 and delta Zn-66 patterns in the PRE, respectively. The description of delta Cu-65 is best achieved by a three-endmember mixing process, with the external source having dCu > 20 nmol kg(-1) and delta Cu-65 < +1.3 parts per thousand, indicative of processes like organic matter remineralization or discharge from wastewater treatment plant. Our study highlights the need for more extensive and detailed studies on estuarine settings to elucidate their potentially crucial role in global dTMs budgets.