Zinc is an essential micronutrient and its concentration and isotopic composition in marine sediments represent promising tracers of the ocean carbon cycle. However, gaps remain in our understanding of the modern marine cycle of Zn, including an explanation of the heavy Zn isotopic composition of seawater relative to the known inputs, and the identity of a required missing sink for light Zn isotopes. Here we present Zn isotope data for organic-rich and trace metal–rich continental margin sediments from the east Pacific margins that together provide the first observational evidence for the previously hypothesized burial of light Zn in such settings. In turn, this light Zn output flux provides a means to enrich the seawater dissolved pool in heavy isotopes. The size and isotopic composition of the margin sink are controlled by the uptake of Zn into organic matter in the photic zone and the fixation of this pool, probably in the form of Zn sulfides, in sediments. An estimate of its significance to the overall Zn oceanic mass balance, both in terms of flux and isotopic composition, indicates that such settings can fulfill the requirements of the missing Zn sink. Taken together, these observations have important implications for the interpretation of Zn isotope data for marine sediments in the geologic record.

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