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

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 C-13 of Fe-R-associated OC (C-13(Fe-OC)) in surface sediments of the Changjiang Estuary and adjacent shelf. An estimated 7.43.5% 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 C-13(Fe-OC) relative to C-13 of TOC (C-13(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 15.66.5% of SOC is directly bound to Fe-R on a global scale, which is lower than the previous estimation (similar to 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.