THE input of high-temperature fluids to the ocean along sea-floor spreading centres plays an important part in controlling the composition of sea water1-3. Available data1-3 show that hydrothermal systems are major sources of lithium, manganese, rubidium, iron and silicon and sinks for magnesium and sulphur. Furthermore, reactive precipitates formed in vent systems or emerging plumes4-6 scavenge elements from sea water, thereby also influencing the composition of the sea water. Our work on the Mid-Atlantic Ridge, combined with data from the Pacific Ocean, shows that scavenging by vent-derived iron oxides helps to control the concentration and cycling of vanadium in the ocean. We estimate that 10-60% of the riverine input of vanadium to the ocean is removed from sea water by this mechanism and that such a process is also important for other elements.