The ocean contains a reservoir of dissolved organic matter (DOM) comparable in size to the entire atmospheric carbon pool, with a mean radiocarbon age of 5,500 years. Despite its central role in fueling marine microbial activity, detailed molecular characterization of this material remains challenging owing to its chemical complexity. Here, we measured the radiocarbon content (Δ¹⁴C) of solid-phase extracted DOM (SPE-DOM) from the Bohai Sea with ultra-high-resolution mass spectrometry (FT-ICR-MS) to investigate DOM cycles. Bulk dissolved organic carbon concentrations and SPE-DOM Δ¹⁴C gradients indicated distinct inputs of pre-aged terrestrial carbon near the Yellow River mouth, accompanied by molecular shifts due to both physical mixing and localized biogeochemical processes. Covariance between individual formula intensities and Δ¹⁴C values provided the basis for classifying the DOM pool into two regimes. The negatively-correlated molecular formulas accounted for 8–13% of the total intensity and consisted of highly oxidized, low-molecular-weight formulas (weighted average m/z 315–318) with an extrapolated Δ¹⁴C end-member of −760 ± 158‰ (~11,400 yr BP), suggesting that ancient soil-derived carbon from the Loess Plateau may be an important terrestrial source of this DOM fraction. Conversely, the positively-correlated molecular group, which accounted for 14–20% of the total intensity, was characterized by less oxidized, higher-molecular-weight (weighted average m/z 441–445) formulas with a modern Δ¹⁴C signature (−18 ± 115‰), reflecting autochthonous production. Cross-regional comparison with the Yangtze River Estuary and a global open-ocean dataset demonstrated that identical molecular formulas do not have fixed radiocarbon ages and can possess vastly different radiocarbon ages and environmental histories depending on their local or regional setting. Crucially, validation using global open-ocean dataset revealed a distinct dynamic separation; when filtered for high-frequency formulas detected across more than 95% of open-ocean samples, only 12.4% of the negatively-correlated formulas persisted, whereas 59.7% of the positively-correlated structures survived and exhibited extensive overlap with previously reported persistent terrestrial formulas (Terr-MFs; Yi et al., 2025), highlighting that these specific molecular configurations preferentially persist within the universal oceanic carbon reservoir.

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