Particle dynamics constrained by uranium-series radionuclides in marine environments

<sec><p indent="0mm">Particulate matter largely governs the geochemical cycling and bioavailability of many elements by regulating their removal and regeneration, thereby affecting the interactions between the oceans and climate change. Particle dynamics encompasses the fundamental geochemical behavior of particulate matter and its associated elements, resident timescales, (re) cycling rates, and sinking fluxes of particulate components, thereby constituting a crucial component of marine biogeochemistry. As global change continues to accelerate changes in the ocean surface, understanding the responses and carbon capacity of the ocean interior has become imperative. Particles, which act as a quick linkage between the ocean surface and ocean interior, are key to understanding changes in the magnitude of carbon dioxide sequestration by the ocean. Uranium-series radionuclides are gaining importance because of their dominant role in tracking particle dynamics in the ocean. </sec><sec> This review provides a comprehensive synthesis of the applications of U-series radionuclides for constraining particle dynamics within the land–seabed continuum (i.e., bay–coast–euphotic zone–meso-/bathypelagic zone–bottom layer). The basic principle of radionuclides in tracing particle cycling and timescales is presented, with emphasis on the influence of observational conditions and intrinsic mechanisms on the applications of U-series nuclides, including physical processes, steady/non-steady conditions, algorithms, and the interaction between particles and radionuclides. Additionally, the challenges faced by U-series radionuclides in constraining particle dynamics, particularly within the ocean interior, are summarized. The study proposes a preferred strategy and model for constraining particle dynamics in terms of different oceanic regimes, and presents the questions that must be addressed to improve particle dynamics. </sec><sec> One of the major advances realized within the bay–coast continuum is the quantitative evaluation of particulate organic carbon sinking on a bay-scale in strong dynamic shallow waters based on monthly seasonal sampling, which significantly resolves the quantitative constraint of particle settling in technique-deficient environments and extends the application scope of U-series nuclides. The other is the accurate calculation of the actual riverine particulate components in coastal seas, which enables researchers to revisit the budgets for organic carbon and other elements in the coastal and shelf waters. </sec><sec> Reports on the euphotic zone allow the assessment of the influence of observational strategies and assumptions on the applications of U-series nuclides. Physically, single sampling is generally acceptable assuming steady-state conditions. By contrast, time-series observations are required for bloom events, upwelling, and scenarios with highly variable processes over short timescales. From an algorithmic perspective, the residence time method appears to be appropriate for regimes characterized by highly heterogeneous environments, whereas the ratio method seems to be better suited for steady environments. </sec><sec> Understanding of particle dynamics has been very limited in the ocean interior, i.e., from the mesopelagic zone to the seafloor, owing to the lack of techniques. However, in recent years, studies have provided insights into particle dynamics in the mesopelagic and bottom waters, establishing a foundation for examining the sinking and regeneration of organic carbon and carbon storage in the deep ocean. The application of U-series nuclides to constrain resuspension provides an approach for (quasi) quantitatively evaluating the material exchange between the seawater and sediment. </sec><sec> In conclusion, understanding of particle dynamics has significantly improved owing to the use of U-series nuclides in the ocean. Progress in shallow, meso- and bathypelagic, and bottom waters provides opportunities to utilize U-series nuclides for the investigation of particle dynamics in changing environments by using limited techniques and/or approaches, especially in bay/coastal areas and the ocean interior. By emphasizing particulate cycling over different timescales in the ocean interior, we can more effectively evaluate the response of the ocean to climate change and the role of the ocean in sequestrating carbon dioxide. </sec>