Global seafood demand is increasing while oceans continue to receive substantial anthropogenic mercury, heightening concerns about the toxic methylmercury bioaccumulation in seafood. Presently, the fate of mercury in the ocean remains uncertain, hindering comprehensive assessments of marine mercury dynamics and seafood safety. Here we leverage an observation-driven dataset to demonstrate that approximately \({1{,}290}_{-400}^{+680}\,{\rm{Mg}}\) of mercury is buried annually in continental shelves, substantially reducing its bioaccumulation potential in marine food webs. This flux is sixfold greater than that in the United Nations Environment Programme’s last report and twofold to sevenfold that of deep-sea sediment burial, making continental shelves the largest marine mercury sinks. Since industrialization, mercury levels in surface shelf sediments have tripled, indicating that most buried mercury is of anthropogenic origin. However, this sink is increasingly threatened by climate-related processes, bottom trawling and dredging, which physically remobilize mercury through diffusion, stirring, redistribution and off-shelf transport. Empirical extrapolations suggest that ongoing trawling, dredging and warming may transform coastal sediments from mercury sinks to net sources. This shift may have already occurred in parts of Europe’s shelves, though additional verification is required. Our findings highlight the urgent need to reduce anthropogenic mercury and greenhouse gas emissions and balance fishery demand with ecosystem conservation to sustain these critical mercury sinks.
