The Role of Reactive Iron in the Preservation of Terrestrial Organic Carbon in Estuarine Sediments

Abstract 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 δ 13 C of Fe R ‐associated OC (δ 13 C 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 13 C Fe‐OC relative to 13 C of TOC ( 13 C 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.