Selective Destabilization of Mineral-Protected Organic Carbon Accompanies Iron Reduction across a Coastal Wetland Salinity Gradient

Climate-driven seawater intrusion is intensifying salinization in coastal wetlands, but its consequences for the stability of sediment organic carbon (OC) remain poorly constrained. Here, we combined field observations along a natural salinity gradient with spectroscopic and mineralogical analyses, mineral sorption–desorption experiments, and manipulative incubations to examine the stability of mineral-protected OC. Salt-marsh sediments contained 9.23% less OC than freshwater sediments, with a disproportionate difference in reactive iron-bound OC (FeR-OC). Across the 36 independent field plots, FeR-OC represented 9.80–22.95% of total OC, while the FW–SM FeR-OC difference was numerically equivalent to 64.28% of the corresponding bulk-OC difference. This disproportionate response was associated with a shift from reactive Fe(III)-bearing phases toward Fe(II)-rich products, as well as a weakening of mineral protection, supported by spectroscopic, mineralogical, and geochemical evidence. In salt-marsh incubations, molybdate addition decreased FeR-OC mobilization by 39.51%, identifying a molybdate-sensitive contribution consistent with the sulfate-reduction branch. Together, these results support an iron-unlocking pathway in sulfate-rich coastal wetlands, in which Fe reduction weakens mineral protection and mobilizes previously stabilized OC. This mechanism identifies iron-mediated mineral protection as an important geochemical vulnerability of blue carbon under increasing seawater influence, while the subsequent mineralization, export, or recapture of mobilized OC remains unresolved.