This article discusses the role of reactive iron in marine sediments, focusing on its mineralogy, concentrations, and reactivity towards sulfide. The study examines two contrasting environments: the Fe-poor FOAM site in Long Island Sound and the Fe-rich sediment of the subaqueous Mississippi Delta. Results show that oxide minerals are the primary Fe phases in early diagenetic pyrite formation, but lepidocrocite and ferrihydrite are the most significant in pore waters. Despite active sulfate reduction, dissolved sulfide is rarely present in pore waters due to the high concentrations of dissolved Fe. Experimental results and diagenetic modeling suggest that bacterial reduction of iron oxides is a key source of dissolved Fe in some sediments, even when sulfide reacts with iron oxides. The study highlights the existence of distinct microenvironments in marine sediments, where microbial iron reduction leads to Fe migration into solution, while sulfate reduction produces sulfide that reacts with Fe oxides to form Fe sulfides. The results emphasize the importance of reactive iron in controlling pore water chemistry and the influence of bacterial activity on Fe liberation. The study also presents a pore-water iron model, which predicts iron profiles based on liberation rates and compares them with actual measurements, supporting the hypothesis that bacterial iron reduction is a major source of dissolved Fe in sediments where sulfate reduction is absent.This article discusses the role of reactive iron in marine sediments, focusing on its mineralogy, concentrations, and reactivity towards sulfide. The study examines two contrasting environments: the Fe-poor FOAM site in Long Island Sound and the Fe-rich sediment of the subaqueous Mississippi Delta. Results show that oxide minerals are the primary Fe phases in early diagenetic pyrite formation, but lepidocrocite and ferrihydrite are the most significant in pore waters. Despite active sulfate reduction, dissolved sulfide is rarely present in pore waters due to the high concentrations of dissolved Fe. Experimental results and diagenetic modeling suggest that bacterial reduction of iron oxides is a key source of dissolved Fe in some sediments, even when sulfide reacts with iron oxides. The study highlights the existence of distinct microenvironments in marine sediments, where microbial iron reduction leads to Fe migration into solution, while sulfate reduction produces sulfide that reacts with Fe oxides to form Fe sulfides. The results emphasize the importance of reactive iron in controlling pore water chemistry and the influence of bacterial activity on Fe liberation. The study also presents a pore-water iron model, which predicts iron profiles based on liberation rates and compares them with actual measurements, supporting the hypothesis that bacterial iron reduction is a major source of dissolved Fe in sediments where sulfate reduction is absent.