A study on reactive iron in marine sediments explores the mineralogy, concentrations, and reactivity of iron towards sulfide in two contrasting environments: the Fe-poor FOAM site and the Fe-rich Mississippi Delta sediment. Results show that oxide minerals, particularly lepidocrocite and ferrihydrite, are more reactive towards sulfide than goethite and hematite. In high Fe oxide concentrations, dissolved sulfide is nearly absent from pore waters, even with active sulfate reduction. Experimental and diagenetic modeling suggest that in some sediments, pore water Fe originates from bacterial reduction of iron oxides, not from sulfide reactions. Distinct microenvironments exist where Fe reduced by bacteria migrates into solution, while in others, sulfide reacts with Fe oxides to form Fe sulfides. The FOAM site, located in Long Island Sound, has a sedimentation rate of 0.1 cm yr⁻¹ and is characterized by low dissolved sulfide despite active sulfate reduction. The Mississippi Delta sites, with sedimentation rates of 2 cm yr⁻¹ and 1 cm yr⁻¹, show higher dissolved Fe and lower sulfide. Solid phase analysis reveals that iron oxides, particularly ferrihydrite and lepidocrocite, are more reactive towards sulfide than goethite and hematite. Iron liberation experiments show that in the absence of sulfate reduction, bacterial activity can release Fe into solution. In the presence of sulfate reduction, Fe is primarily released through reactions with sulfide. Unamended sediment incubations show that sulfate reduction leads to the formation of Fe sulfides, with dissolved Fe concentrations increasing in the absence of sulfate reduction. Sulfide uptake experiments demonstrate that ferrihydrite and lepidocrocite react rapidly with sulfide, while goethite and hematite react more slowly. These results suggest that reactive iron oxides play a key role in controlling pore water chemistry by reacting with sulfide. The study also highlights the importance of bacterial iron reduction in the origin of pore water Fe, particularly in sediments where sulfate reduction is absent. The depth distribution of reactive iron is influenced by bacterial activity and the availability of Fe oxides. The model for pore water Fe profiles suggests that Fe originates from bacterial liberation reactions, not from sulfide reactions with iron minerals. The study underscores the complex interplay between iron oxides, sulfide, and bacterial activity in marine sediments.A study on reactive iron in marine sediments explores the mineralogy, concentrations, and reactivity of iron towards sulfide in two contrasting environments: the Fe-poor FOAM site and the Fe-rich Mississippi Delta sediment. Results show that oxide minerals, particularly lepidocrocite and ferrihydrite, are more reactive towards sulfide than goethite and hematite. In high Fe oxide concentrations, dissolved sulfide is nearly absent from pore waters, even with active sulfate reduction. Experimental and diagenetic modeling suggest that in some sediments, pore water Fe originates from bacterial reduction of iron oxides, not from sulfide reactions. Distinct microenvironments exist where Fe reduced by bacteria migrates into solution, while in others, sulfide reacts with Fe oxides to form Fe sulfides. The FOAM site, located in Long Island Sound, has a sedimentation rate of 0.1 cm yr⁻¹ and is characterized by low dissolved sulfide despite active sulfate reduction. The Mississippi Delta sites, with sedimentation rates of 2 cm yr⁻¹ and 1 cm yr⁻¹, show higher dissolved Fe and lower sulfide. Solid phase analysis reveals that iron oxides, particularly ferrihydrite and lepidocrocite, are more reactive towards sulfide than goethite and hematite. Iron liberation experiments show that in the absence of sulfate reduction, bacterial activity can release Fe into solution. In the presence of sulfate reduction, Fe is primarily released through reactions with sulfide. Unamended sediment incubations show that sulfate reduction leads to the formation of Fe sulfides, with dissolved Fe concentrations increasing in the absence of sulfate reduction. Sulfide uptake experiments demonstrate that ferrihydrite and lepidocrocite react rapidly with sulfide, while goethite and hematite react more slowly. These results suggest that reactive iron oxides play a key role in controlling pore water chemistry by reacting with sulfide. The study also highlights the importance of bacterial iron reduction in the origin of pore water Fe, particularly in sediments where sulfate reduction is absent. The depth distribution of reactive iron is influenced by bacterial activity and the availability of Fe oxides. The model for pore water Fe profiles suggests that Fe originates from bacterial liberation reactions, not from sulfide reactions with iron minerals. The study underscores the complex interplay between iron oxides, sulfide, and bacterial activity in marine sediments.