Chinese blue carbon ecosystems store substantially less soil organic carbon than the global averages reported for mangroves, salt marshes and seagrasses, yet this carbon persists on millennial timescales. We attribute this paradox of low stocks and high stability to a mineral-carbon pump driven by high sediment supply and allochthonous organic inputs. Synthesizing a national dataset of more than 500 surface soils and 56 sediment cores spanning China's latitudinal gradient, we resolved the sources, molecular composition and turnover of coastal sedimentary carbon. Allochthonous riverine and marine material supplies 50 to 70% of this carbon. Refractory black carbon contributes a further 10 to 20%, with radiocarbon ages reaching 10,000 to 15,000 years. Mineral-associated organic matter dominates the stored carbon, rising from 52% of the total in mangroves to 79% in salt marshes. Hydrophobic lipids form the most abundant molecular class across all three habitats. Particulate organic carbon increases with temperature and precipitation, whereas mineral-associated carbon scales with sediment accretion rate, so mineral burial simultaneously enriches the stable pool and dilutes fresh plant-derived inputs. Radiocarbon-constrained turnover times reach roughly 2,200 years in salt-marsh topsoils and exceed 11,000 years at depth, with mineral reactivity and microbial reworking jointly governing persistence. These patterns indicate that long-term carbon stability in sediment-rich coasts rests on mineral protection of imported pre-aged organic matter rather than on in-situ plant production. Protecting the sediment conveyor that buries and stabilizes this carbon, including adjacent unvegetated tidal flats and managed coastal forests, should be central to blue carbon accounting and management.