Iron geochemistry in surface sediments of a temperate semi-enclosed bay, North China

Iron (Fe) speciation and microbial reducible Fe(III) oxides (MR-Fe(III)) in surface sediments of semi-enclosed Jiaozhou Bay were quantified to reveal Fe geochemistry in the settings subjected to anthropogenic perturbations. Results indicate that sedimentary Fe in the bay is mainly of natural weathering source, without appreciable anthropogenic Fe inputs, as indicated by a generally good linear coupling of total Fe to aluminum. Among the three highly reactive Fe(III) (Fe(III)HR) pools, well crystalline Fe(III) oxides (Fe(III)wc) were always the predominant phase, followed by poorly crystalline Fe(III) oxides (Fe(III)pc), and amorphous Fe(III) oxides (Fe(III)am) were only of minor importance. The dominance of non-sulfidized Fe(II) over sulfidized Fe in the sediments points to the importance of microbial iron(III) reduction (MIR) in the free sulfide-starved conditions. High riverine inputs of TOC leads to outliers in the Fe(III)HR versus total organic carbon (TOC) ratio compared the rest of the bay. OM-dependent MIR as the common driving force has rendered all Fe(II)-bearing phases linearly coupled to TOC. MR-Fe(III) in the surface sediments covered all Fe(III)am and a fraction of less reactive Fe(III) phases, while Fe(III)wc was at most a minor contributor. Highly reactive Fe appears to be enriched to some extent in the temperate semi-enclosed bay, as in the wet-tropical counterparts. •All Fe(II)-bearing phases are linearly coupled to total organic carbon (TOC).•High inputs of wastewater caused a decoupling of highly reactive Fe(III) from TOC.•Microbial reduction is an important pathway of Fe(III) reduction.•Highly reactive Fe is enriched to some extent in temperate semi-enclosed bay.