The roles of trace elements and their isotopes in the marine environment are essential and diverse (e.g., nutrients, tracers of oceanic processes, anthropogenic pollutants), but are not well understood yet. In this thesis, two elements and their isotopes were investigated in two distinct marine environments: cadmium (Cd) in the Siberian shelf seas and neodymium (Nd) in the western North Atlantic Ocean. Whereas the role of Cd as nutrient in the open ocean is reflected by Cd isotope data, its behaviour in estuaries is poorly constrained. Chapter 2 of this thesis presents new Cd isotopes and concentrations of 19 water samples from the mixing zone of Siberian rivers with the Arctic Ocean. The results provide the first constraints on the isotope composition of natural riverine Cd fluxes to the ocean and the cycling of Cd in a shelf environment. Chapter 3 describes the method developed to isolate and analyse seawater Nd isotopes and concentrations in the MAGIC laboratories. The Nd isotopic composition of seawater is a promising tracer for ocean circulation and exchange between the continental margins and the oceans. By analysing 12 seawater depth profiles from the Dutch GEOTRACES transect GA02 (Chapters 4 and 5), we demonstrate that Nd features different behaviour in regions close to the formation area of deep water masses compared to export areas, where it behaves conservatively away from continental margins. Besides, we show that upper- North Atlantic Deep Water (NADW) and lower-NADW have distinct Nd isotopic compositions, and that the signature of lower-NADW is significantly more radiogenic than the commonly accepted value, which should be taken into account for future applications. Overall, the present thesis underlines the important role that isotope analyses can play in deciphering marine biogeochemical cycles of trace elements.