Radiocarbon and molecular constraints on the sources and fates of dissolved organic matter in the Bohai Sea.

The ocean contains a reservoir of dissolved organic matter (DOM) comparable to the atmospheric carbon pool, yet molecular characterization remains challenging owing to its chemical complexity. We measured the radiocarbon content (Δ14C) and molecular composition of solid-phase extracted DOM (SPE-DOM) from the Bohai Sea using accelerator mass spectrometer and Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS), respectively. Bulk dissolved organic carbon concentrations and SPE-DOM Δ14C gradients indicated pre-aged terrestrial carbon inputs near the Yellow River mouth, with molecular shifts reflecting 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. Negatively correlated formulas (8-14% of total intensity) were highly oxidized and low-molecular-weight, with an apparent Δ14C value of -751 ± 181‰ (corresponding to ∼3000-35,000 yr BP with 95% confidence interval), suggesting pre-aged terrestrial or sediment-associated carbon as important sources. Conversely, positively correlated formulas (14-23% of total intensity) were less oxidized and higher-molecular-weight, with an apparent Δ14C value of -74 ± 100‰. Their partial overlap (approximately 44%) with known terrestrial formulas indicates mixed autochthonous and terrestrial sources. Comparison with the Yangtze River Estuary and a global open-ocean dataset demonstrated that the same molecular formulas can be associated with DOM having a wide range of radiocarbon ages and environmental histories. Only 20% of negatively correlated formulas were widely distributed globally, whereas 60% of positively correlated formulas were detected at all depths throughout the Atlantic and Pacific Oceans, overlapping with known terrestrial formulas. This divergence underscores that terrigenous organic matter exhibits highly variable reactivity depending on its molecular characteristics and sources.