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 (Fe-R) plays an important role in the preservation of organic carbon (OC) in coastal sediments, yet changes in the OC bound to Fe-R (OC-Fe-R), 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-Fe-R from estuarine suspended particulate matter (SPM) to coastal sediments, to further understand the role of Fe-R in the preservation of terrestrial OC exported from large rivers into marginal seas. We examined OC and its carbon isotopic composition (& UDelta;14Cbulk, 813Cbulk), specific surface area (SSA), grain size composition, lignin phenols, Fe-R, Mossbauer spectroscopy, and isotopic signatures of OC-Fe-R (& UDelta;14COC-Fe-R, 813COC-Fe-R) in SPM and surface sediments of the Changjiang Estuary. Particulate OC (POC) and Fe-R 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 Fe-R largely occurs at the sediment-water interface due in part, to rapid Fe cycling. The percentage of OC-Fe-R (fOC-FeR) in SPM (6.6 & PLUSMN; 1.9%) is similar to that in mobile-mud sediment (8.8 & PLUSMN; 1.8%). There are no significant differences in OC-Fe-R content (p > 0.05) from SPM to mobile-mud sediments, but non-OC-Fe-R largely decreases, suggesting that terrestrial OC-Fe-R has greater stability compared to terrestrial non-OC-FeR. Both 813COC-Fe-R and & UDelta;14COC-Fe-R are lower than bulk OC, indicating that Fe(R )is mainly associated with pre-aged soil OC of terrestrial plant origin, especially in estuarine SPM and mobile-mud sediments. Taken together, binding with Fe-R is a potential long-term protection mechanism for terrestrial OC. Both & UDelta;14Cbulk and & UDelta;14COC-Fe-R 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 Fe-R and its associated OC. Based on literature comparisons of soils, estuarine SPM, and marine sediments, OC-Fe-R associations are controlled mainly by sedimentary regimes, Fe(R )compositions, and OC sources. This work supports the notion that Fe(R )plays an important role in the stabilization and transport of river-derived terrestrial OC.