Isotope tracking of anthropogenic Cu and Zn in urbanized coastal environments: A review

Copper (Cu) and zinc (Zn) are common trace metal contaminants in marine environments that, despite their importance for the health of marine organisms, can be toxic. Recently, the stable isotopes of these elements have emerged as powerful tracers for studying their cycles. Thus, this review aims to connect urban and marine interfaces under a "land-sea continuum" framework to understand the complex sources, pathways, and transformations of Cu and Zn in urbanized coastal environments, a perspective currently lacking in the literature. Here, we briefly establish the sampling and analytical aspects of isotope measurement of these elements in the natural matrices of marine realms, providing a recent compilation of reference materials for analytical control. The principles of isotope fractionation are introduced and contextualized within coastal ecosystems. We summarize the isotopic signatures of major anthropogenic sources-including road-deposited sediments, non-exhaust traffic-related emissions, industrial waste, and metallurgical byproducts-and highlight the challenges of distinguishing them when isotopic ranges overlap. In parallel, natural baselines such as ores, soils, suspended particulate matter (SPM), and riverine inputs are also reviewed to contextualize environmental variability. The review examines how source isotope signatures evolve with further isotope fractionation during estuarine transport and transfer processes in the water-sediment-biota interface. Finally, this review also identifies future research directions to trace the sources, pathways, and sinks of these contaminants along the land-sea continuum, as well as the legacy and ongoing impact of urban metal pollution at catchment-to-coastal scales.