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Factors influencing reactive Fe cycling and its protection of organic carbon (OC) in sediments are poorly understood. Here we comparatively study Fe speciation and Fe‐associated OC (Fe‐OC) in surface sediments of the Bohai Sea (BHS) and southern Yellow Sea (SYS), two seas with common sediment sources but different depositional regimes. Though significant sequestration of highly reactive Fe (FeHR) is expected in the estuarine system stream from the river‐dominated BHS, this pool is, however, slightly enriched in the BHS sediments relative to their source material. This reconfirms a previous speculation of sedimentary FeHR enrichment in semi‐protected settings. Relative to the BHS, the SYS sediments are depleted of FeHR, despite common sediment sources of these two areas. Estuarine pre‐enrichment and subsequent redistribution of FeHR in the BHS, and aging of Fe‐bearing authigenic clays during transport from the BHS to the SYS are potential mechanisms for the depletion. The fractions (fFe‐OC) of Fe‐OC in total OC in sediments of the two seas are at the lower end for soils and sediments, indicating Fe being a minor “rusty sink.” 13CFe‐OC fractionations indicate preferential sequestration of terrestrial OC by Fe oxides in the BHS, in contrast with preferential retention of marine OC in the SYS. Different fractionations of 13CFe‐OC in the two seas are a net result of selective adsorption of OC by Fe oxides and selective stabilization of OC during Fe reductive dissolution. Preferential sequestration of terrestrial OC may exert an important influence on distribution and compositions of OC buried in the river‐dominated system.

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