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.

Millennial scale persistence of organic carbon bound to iron in Arctic marine sediments

“…To date, the mechanisms stabilising OC with Fe R in marine sediments have mainly been studied in surface sediments 13,[19][20][21][22][23] . These studies show that the fraction of the total OC bound to Fe R (fOC-Fe R ) is on average 10-20%, with values ranging from~0.5 to 40%.…”

Organic Carbon Stabilization Mechanisms in Mangrove Soils: A Review

“…Because inner-sphere complexation is strong and irreversible, this OM-Fe interaction is considered to result in the long-term preservation of OM in marine sediments. Similarly important roles that reactive Fe plays in the stabilization of OM have been reported in estuarine sediments [57,58]. Following these studies, Dicen et al for the first time investigated the role of reactive Fe in mangrove soils [39].…”

Factors Controlling Storage, Sources, and Diagenetic State of Organic Carbon in a Prograding Subaerial Delta: Wax Lake Delta, Louisiana

“…The composition of OM in WLD is ultimately linked to elevation, but the stability of OM within soils and sediments is determined by a combination of chemical and environmental factors including compound structure, mineral association, redox state, and the presence of microbial heterotrophs (Baldock & Skjemstad, ; Hedges & Oades, ; Schmidt et al, ; Zhao et al, ). In WLD sediment samples, TOC exhibits a strong positive correlation with the silt + clay fraction in a semilog relationship ( r 2 = 0.82, p < 0.05; Figure a).…”