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

Abstract

To better understand the role of reactive Fe (FeR) in the preservation of sedimentary OC (SOC) in estuarine sediments, we examined specific surface area (SSA), grain size composition, total OC (TOC), lignin phenols, FeR, FeR-associated OC (Fe-OC) and lignin phenols (Fe-lignin), and δ13C of FeR-associated OC (δ13 CFe-OC) in surface sediments of the Changjiang Estuary and adjacent shelf. An estimated 7.4 ± 3.5% of the OC was directly bound with FeR in the Changjiang Estuary and adjacent shelf. Unusually low TOC/SSA loadings and Fe-OC/Fe ratios in mobile-muds suggest that frequent physical reworking may reduce FeR binding with OC, with selective loss of marine OC. More depleted 13CFe-OC relative to 13C of TOC (13 Cbulk) in deltaic regions and mobile-muds showed that FeR was largely associated with terrestrial OC, derived from extensive riverine C 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., non deltaic shelf, anoxic basins, slope and deep sea), our findings suggest that about 15.6 ± 6.5% of SOC is directly bound to FeR 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 FeR plays an important role in the preservation and potential transport of terrestrial OC in the marine environment.