To better understand the role of reactive Fe (Fe_R) in the preservation of sedimentary organic carbon (SOC) in estuarine sediments, we examined specific surface area, grain size composition, total OC (TOC), lignin phenols, Fe_R, Fe_R‐associated OC (Fe‐OC) and lignin phenols (Fe‐lignin), and δ¹³C of Fe_R‐associated OC (δ¹³C_Fe‐OC) in surface sediments of the Changjiang Estuary and adjacent shelf. An estimated 7.4 ± 3.5% of the OC was directly bound with Fe_R in the Changjiang Estuary and adjacent shelf. Unusually low TOC/specific surface area loadings and Fe‐OC/Fe ratios in mobile muds suggest that frequent physical reworking may reduce Fe_R binding with OC, with selective loss of marine OC. More depleted ¹³C_Fe‐OC relative to ¹³C of TOC (¹³C_bulk) in deltaic regions and mobile muds showed that Fe_R was largely associated with terrestrial OC, derived from extensive riverine OC and Fe inputs. A higher proportion of hematite in the mobile muds compared to the offshore samples indicated that Fe oxides are likely subjected to selective sorting and/or become mature during long‐term sediment transport. When considering the percentage of Fe‐OC to SOC and SOC burial rates in different marine environments (e.g., nondeltaic shelf, anoxic basins, slope, and deep sea), our findings suggest that about 15.6 ± 6.5% of SOC is directly bound to Fe_R on a global scale, which is lower than the previous estimation (~21.5%). This work further supports the notion of a Rusty Sink where, in this case, Fe_R plays an important role in the preservation and potential transport of terrestrial OC in the marine environment.

The global carbon cycle is strongly modulated by organic carbon (OC) sequestration and decomposition. Whereas OC sequestration is relatively well constrained, there are few quantitative estimates of its susceptibility to decomposition. Fjords are hot spots of sedimentation and OC sequestration in marine sediments. Here, we adopt fjords as model systems to investigate the reactivity of sedimentary OC by assessing the distribution of the activation energy required to break OC bonds. Our results reveal that OC in fjord sediments is more thermally labile than that in global sediments, which is governed by its unique provenance and organo-mineral interactions. We estimate that 61 ± 16% of the sedimentary OC in fjords is degradable. Once this OC is remobilized and remineralized during glacial maxima, the resulting metabolic CO ₂ could counterbalance up to 50 ppm of the atmospheric CO ₂ decrease during glacial times, making fjords critical actors in dampening glacial-interglacial climate fluctuations through negative carbon cycling loops.