Trace elements are powerful tracers - and in some instances drivers - of ocean interactions with the atmosphere, the hydrological cycle, the geology of the seafloor, and life on Earth. The concentration and the isotopic composition of trace elements are, therefore, diagnostic tools for the state of the ocean and its role as part of Earth's dynamic system. Dissolved and particulate transport mechanisms determine how fast the ocean responds to change. The new wealth of data from the international GEOTRACES programme reveals new sources and sinks at all ocean boundaries, highlighting a much more dynamic equilibrium between the seafloor and the ocean than previously thought.