The exchange of geogenic materials between land and the ocean is a fundamental process that shapes seawater chemistry and regulates marine biogeochemical cycles through coupled interactions among the atmosphere, hydrosphere, geosphere, and biosphere. Yet the marine lead (Pb) cycle remains incompletely resolved. Global observations of deep-sea sediments require a substantial fluvial or boundary-derived Pb source, whereas recent coastal studies consistently identify continental margins as effective sinks for dissolved Pb. Resolving this apparent contradiction is critical for constraining the oceanic Pb budget, and for strengthening the application of Pb isotopes in tracing sources and interpreting climate–weathering interactions throughout Earth's history.

Here we present dissolved Pb concentrations and isotope compositions from seawater collected in equatorial Southeast Asia, a region that occupies a relatively small land area yet delivers nearly one-third of global riverine sediment discharge to the ocean. We demonstrate that reversible exchange between dissolved and particulate Pb exerts a primary control on the transfer of Pb across the land–ocean interface. Combined with numerical simulations, our results indicate that this exchange supplied approximately 30–40% of the Pb entering the ocean prior to the Anthropocene. These findings establish boundary exchange as a major mechanism governing the marine Pb cycle and provide new insight into the role of continental margins in global trace-element cycling.