Deposition of particulate organic matter (POM) induces diagenetic hot spots at the sediment-water interface (SWI). Here we explore the effects of intensive POM degradation for metal mobilization at the SWI. By using a combined planar optode-DGT (diffusive gradient in thin-films) sensor we obtained simultaneous measurements of dissolved O₂ and trace metal dynamics around an aggregate of reactive organic matter placed on the SWI of a sediment mesocosm. The aggregate induced a rapid, highly localized, decrease in O₂ concentration, resulting in an anoxic feature at the SWI. Co-located with this feature, we observed intense Fe and Mn mobilization, removal of Co, Ni and Zn and found evidence for the concurrent release and precipitation of Pb within a small confined volume. We also identified two small microniches in the anoxic sediment below the SWI, defined by elevated trace metal mobilization. Differences between the metal release rates in these two microniches indicate that they were formed by the mineralisation of different types of organic matter buried in the sediment. Our results provide direct empirical evidence for the potential importance of POM-induced reactive microniches when considering the fluxes of metals from and within aquatic sediments, and suggest that other elements' cycles may also be affected.