Insights into the genesis and geological significance of iron-manganese precipitates in the Baltic Sea, Gulf of Finland seafloor

Iron-manganese (oxy)hydroxide precipitates (Fe-Mn concretions) occur at the ocean and sea floors all over the world, typically in regions with low sedimentation rates. In shallow water environments like the Baltic Sea, Fe-Mn concretions form in areas where bottom currents prevent active sediment accumulation. The shallow brackish conditions and periodical saltwater inflows from the North Sea make the Baltic Sea a unique environment for concretion formation. Fe-Mn concretions in the Baltic Sea also grow much faster than oceanic concretions, resulting in different mineralogical, chemical, and isotopic compositions compared to deep-sea nodules. One of the areas in the Baltic Sea where Fe-Mn concretions are widespread is the Gulf of Finland, where the concretions form where Late Pleistocene glaciolacustrine varved clays, glacial till, or crystalline bedrock is exposed on the seafloor. 

Due to redox-driven precipitation processes, the spherical or disc-like Fe-Mn concretions forming symmetrically around a nucleus are composed of alternating Fe- and Mn-rich layers. In addition, crust-like concretions can grow in areas with higher background sedimentation rates. During their formation, Fe-Mn concretions record the geochemical status of the sedimentary environment, making them potential archives of the geological history of sedimentary basins. Following the Last Glacial Maximum, the Baltic Sea basin has been through multiple phases of fresh and saltwater conditions before the establishment of the modern brackish Baltic Sea. The Fe-Mn concretions in the Baltic Sea have potentially recorded those changes in their chemical and stable isotopic composition, helping us to understand their formation mechanisms and growth phases through layer-by-layer sampling of the concretions from the centre outwards. 

This study examines the morphological, chemical, stable isotopic and mineralogical properties of the Fe-Mn concretions in the Baltic Sea, Gulf of Finland seafloor and has an overarching aim to assess the complex geological processes controlling the formation of concretions, as well as get a closer look at the growing phases through the finer sampling of their layered structure, which could offer a new perspective on the timing of formation of these concretions relative to the development of the Baltic sea and the Gulf of Finland.