Magnetic properties of Baltic Sea ferromanganese concretions

Ferromanganese concretions are abundant in many parts of the coastal Baltic Sea and hence provide an important yet underused archive to investigate these shallow sea areas in detail. Coastal sea areas can be best described as filters, and they have a focal role in the biogeochemistry of riverine nutrients and suspended particles during their transport to open sea. Colloidal iron and manganese oxyhydroxides, originating from the surrounding catchment area and/or from adjacent anoxic sediments, accumulate and ultimately lead to the authigenic formation of ferromanganese concretions at an unusually fast growth rate, generally in low-carbon environments with limited sedimentation. The porous concretions come in various sizes and shapes, and host diverse microbial communities with reductive and oxidative metabolisms that can affect both the concretion growth and dissolution. Despite that, the specific formation mechanisms and (bio)mineralization processes of different concretion morphotypes are poorly constrained.

We have investigated the magnetic properties of Baltic Sea ferromanganese concretions of different morphotypes, depositional environments, water depths and geographical locations. While the magnetic mineral concentrations change according to salinity and water depth, the concentrations are also seemingly controlled by the concretion morphotype. Preliminary results of laboratory remanences also indicate the magnetic mineral assemblages are dominated by SD-sized ferrimagnetic particles with magnetic properties similar to those of magnetosomes. The possible presence of magnetotactic bacteria within the concretions provides novel insights into the biogeochemistry of low sediment accumulation and low-carbon coastal environments of the Baltic Sea.

This research is part of the Fermaid project, funded by the Academy of Finland grant 332249.