Spatial Distribution and Controls of Organic Carbon Bound to Reactive Iron Across the Pearl River Estuary–Shelf Continuum, Southern China

Organic carbon bound to reactive iron (OC-FeR) constitutes 10–80% of the organic carbon (OC) preserved in marine sediments. Estuaries are characterized by strong OC-FeR enrichment, but the distribution and preservation of OC-FeR in tropical estuaries remain poorly understood. Here, the total OC (TOC), reactive iron (FeR), OC-FeR, the proportion of OC-FeR to TOC (fOC-FeR), and the isotopic compositions of OC-FeR (δ13COC-FeR) were investigated across the Pearl River Estuary (PRE)–shelf continuum. In the river channels, TOC was relatively low, whereas fOC-FeR maintained moderate levels. In the estuarine bay, despite high FeR contents, the fOC-FeR was severely suppressed by intense hydrodynamic turbulence, frequent sediment resuspension, and rapid iron redox cycling. Conversely, the shelf area exhibited lower FeR abundances yet markedly higher fOC-FeR and OC:FeR molar ratios, demonstrating that a stable hydro-sedimentary regime promotes greater binding stability and enhanced preservation efficiency of OC-FeR. In summary, a pronounced spatial decoupling was identified between FeR abundance and OC-FeR preservation efficiency. Depleted δ13COC-FeR values, coupled with magnetic proxies in the shelf area, reveal that fine-grained, high-coercivity iron oxides (e.g., hematite) effectively protect terrestrial OC during long-distance seaward transport. Our study highlights the spatial heterogeneity of OC-FeR across a tropical compound estuarine–deltaic system and provides new insights into how Fe-bearing minerals from multiple sources couple with OC in river-dominated coastal systems.