Preferential preservation of pre-aged terrestrial organic carbon by reactive iron in estuarine particles and coastal sediments of a large river-dominated estuary

Reactive iron (FeR) plays an important role in the preservation of organic carbon (OC) in coastal sediments, yet changes in the OC bound to FeR (OC-FeR), during transport and deposition, remain poorly understood. The main goal of this work is to investigate the variation of the age and composition of OC-FeR from estuarine suspended particulate matter (SPM) to coastal sediments, to further understand the role of FeR in the preservation of terrestrial OC exported from large rivers into marginal seas. We examined OC and its carbon isotopic composition (Δ14Cbulk, δ13Cbulk), specific surface area (SSA), grain size composition, lignin phenols, FeR, Mössbauer spectroscopy, and isotopic signatures of OC-FeR (Δ14COC-FeR, δ13COC-FeR) in SPM and surface sediments of the Changjiang Estuary. Particulate OC (POC) and FeR concentrations in SPM are significantly higher than in surface sediments, with no significant differences between surface- and bottom-water SPM. This indicates that loss of OC and FeR largely occurs at the sediment–water interface due in part, to rapid Fe cycling. The percentage of OC-FeR (fOC-FeR) in SPM (6.6 ± 1.9%) is similar to that in mobile-mud sediment (8.8 ± 1.8%). There are no significant differences in OC-FeR content (p > 0.05) from SPM to mobile-mud sediments, but non-OC-FeR largely decreases, suggesting that terrestrial OC-FeR has greater stability compared to terrestrial non-OC-FeR. Both δ13COC-FeR and Δ14COC-FeR are lower than bulk OC, indicating that FeR is mainly associated with pre-aged soil OC of terrestrial plant origin, especially in estuarine SPM and mobile-mud sediments. Taken together, binding with FeR is a potential long-term protection mechanism for terrestrial OC. Both Δ14Cbulk and Δ14COC-FeR decrease with an increase in the ratio of hematite to (super)paramagnetic Fe3+, indicating that high-crystallinity iron oxide is largely associated with pre-aged terrestrial OC, and there is a potential joint maturation mechanism between FeR and its associated OC. Based on literature comparisons of soils, estuarine SPM, and marine sediments, OC-FeR associations are controlled mainly by sedimentary regimes, FeR compositions, and OC sources. This work supports the notion that FeR plays an important role in the stabilization and transport of river-derived terrestrial OC.