The Role of Fe(II) Minerals in Carbon Stabilization in Coastal and Estuarine Anoxic Environments

Fe(III) (oxyhydr)oxides are well-known for their role in organic carbon (OC) stabilization in terrestrial soils. Coastal and estuarine soils typically act as iron sinks and receive a high input of OC. However, tidal submersion induces anoxic and reducing conditions that favor the microbial reductive dissolution of Fe(III) (oxyhydr)oxides, followed by the partial formation of Fe(II) minerals. However, the potential of these minerals—such as siderite, vivianite and iron sulfides—to stabilize C has only recently received attention. In this review, we (a) elucidate methodological constraints in Fe(II) mineral analysis, (b) highlight formation mechanisms of Fe(II) minerals and (c) their interactions with organic matter (OM) and inorganic C and (d) explore their role in C stabilization. Fe(II) minerals interact with OM through surface complexation, coprecipitation or physical entrapment, despite their typically low surface charge. These interactions are facilitated by surface-reactive chemical species such as potential-determining ions, divalent cations and functional surface groups, enabling Fe(II) minerals to bind or occlude OC in anoxic settings. During Fe(II) mineral formation, dissolved inorganic C can be exported as total alkalinity or precipitated as stable carbonates, both contributing to long-term inorganic C sequestration. Emerging research indicates that Fe(II) mineral interactions with organic and inorganic C likely binds 5.8–16.6 Tg C yr⁻¹, a potential overlooked global C sink with yet unexplored long-term stability. This review thus emphasizes the geochemical relevance of Fe(II) minerals beyond transient redox products as they may constitute a persistent and quantifiable carbon sink in anoxic sediments, warranting further exploration.