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Iron isotopes reveal colloidal release as the dominant sedimentary flux from oxic inner continental shelf
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DOI：10.1016/j.gca.2026.07.045.png)
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Abstract
En 中文
Continental shelf and estuarine sediments are critical carbon sinks and significant sources of dissolved iron (dFe). Although the mechanism of Fe reductive dissolution (RD) under low-oxygen conditions is well documented, the processes governing Fe dissolution and supply in oxic sediments remain poorly understood. Here, we combine porewater colloidal Fe (cFe; 0.02–0.2 μm) and the δ56Fe signatures of dFe in China’s largest estuary to reveal distinct dFe speciation patterns. Nearshore oxic-nitrogenous porewaters are dominated by cFe with near-crustal δ56Fe of dFe (∼0‰), whereas porewaters in ferruginous zones of all sites and offshore hypoxic zones are dominated by soluble Fe (sFe, <0.02 μm) with lighter δ56Fe of dFe values (−2.32‰ to − 0.19‰). The Rate Estimation from Concentrations (REC) model, combined with an isotopic mass balance model, demonstrates that cFe predominantly derives from lithogenic weathering via non-reductive dissolution (NRD), while sFe production is driven by RD, suggesting that sFe oxidation is not a major cFe source. The offshore shift from cFe dominance (with near-crustal δ56Fe) to sFe dominance (with light δ56Fe) indicates variable contributions from RD and NRD pathways across the river-dominated inner shelf. This pattern of NRD dominance in nearshore oxic sediments and RD dominance in hypoxic offshore regions is driven by cross-shelf gradients in total organic carbon, dissolved oxygen (DO) and terrestrial weathering inputs. In nearshore oxic sediments, benthic cFe fluxes account for > 60% of benthic dFe fluxes and exhibit a positive correlation with bottom-water DO, revealing NRD-generated cFe as the primary dFe source under oxic conditions. Our findings indicate that NRD may be an important mechanism for dFe supply in young, terrestrial-influenced coastal sediments, a pathway that is potentially underrepresented in current oceanic Fe cycle models.
Keywords:
Colloidal iron
Iron isotopes
Non-reductive dissolution
Estuarine oxic sediments
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