Diagenesis Decreasing the Mo Isotopic Composition in Estuarine Systems: Implications for Constraining Its Riverine Input to Ocean

Understanding the geochemical behavior of the Mo isotopes in estuarine systems is essential for determining the isotopic composition of riverine inputs to the ocean and for assessing the historical oxidation state of Earth's ancient oceans. However, the extent and mechanisms of Mo isotope fractionation during estuarine processes are not yet fully understood. This study systematically investigated the seasonal and spatial variations in aqueous and particulate delta 98Mo values within the Pearl River Estuary (PRE). Our research found that aqueous delta 98Moaque. values in both summer and winter deviated significantly from the theoretical mixing line for the PRE. Based on the geochemical characteristics of the water column, particulate matter, and pore water in the PRE, we first propose that diagenesis release from sediments is the predominant factor resulting in lower than anticipated aqueous delta 98Moaque. values. Given the prevalence of suboxic and anoxic sediments in estuarine and coastal areas, such diagenetic release may substantially decrease the global riverine influx of aqueous Mo isotopes to the ocean. Additionally, particulate delta 98MoSPM values exhibit an increasing trend (from 0.02 to 1.62 parts per thousand) with increasing salinity in both seasons, suggesting that the terrestrial input particulate delta 98MoSPM value would be heavier than the mean value for UCC. We hypothesize that the adsorption and desorption processes involving Fe (hydro) oxides predominantly influence this trend. This study advances our understanding of the mechanisms of aqueous and particulate Mo isotopic fractionation in estuarine systems and would be helpful in constraining Mo isotopic compositions of rivers and oceans. The reliability of using Mo isotopes to trace historical shifts in global ocean redox depends heavily on precise constraints of the Mo isotopic composition of riverine inputs to the ocean. Estuaries, serving as dynamic interfaces between terrestrial and marine environments, can substantially alter the Mo isotopic composition of riverine inputs to the ocean. To more accurately constrain the Mo isotopic composition of riverine inputs to the ocean, understanding the extent and mechanisms of Mo isotope fractionation during estuarine processes is crucial. Addressing this knowledge gap, our study investigates the seasonal and spatial variations in delta 98Mo values of aqueous and particulate Mo in the Pearl River Estuary (PRE), China. Our findings reveal that aqueous delta 98Moaque. values are markedly lower than the theoretical mixing-line values in both summer and winter seasons, whereas particulate delta 98MoSPM values demonstrate a distinct increasing trend from the upper to lower reaches of the PRE. This suggests that estuaries can profoundly modify the Mo isotopic composition of riverine inputs to the ocean. In conclusion, we propose that the reductive release from sediments likely explains the lower aqueous delta 98Moaque. values, while the adsorption and desorption of Fe (hydro) oxides primarily drive the observed increase in particulate delta 98MoSPM values. Aqueous delta 98Mo values in different seasons were substantially lower than theoretical mixing-line values in the Pearl River Estuary (PRE) Reductive release from sediments is responsible for the aqueous delta 98Mo values being lower than theoretical levels Fe (hydro) oxides are the dominant factors controlling the increasing trend in the particulate delta 98Mo values with salinity