Morphology-Driven Magnetic Characteristics of Shallow-Water Ferromanganese Concretions

Ferromanganese concretions, ubiquitous both in deep ocean environments and shallow-water coastal regions worldwide, are the subject of renewed scientific interest due to their multifaceted importance as underwater habitats, critical raw material sources, and invaluable repositories of paleoceanographic information. The magnetic properties of ferrimagnetic minerals within concretions, as well as the origins of their natural remanent magnetization, represent areas of study that are still in their early stages of exploration. Recent findings have unveiled the role of biogenic magnetite in the development of biogeochemical remanent magnetization within deep ocean crusts and nodules, pointing to the influence of microbial catalysis in their precipitation. While extensive research on magnetic properties of deep ocean Fe-Mn deposits has been conducted, similar investigations in fast-growing shallow-water concretions have remained notably absent. Furthermore, the specific mechanisms governing the formation and (bio)mineralization of diverse concretion morphotypes (crust-like, discoidal and spheroidal) in shallow-water setting remain enigmatic.

Our work focuses on ferromanganese concretions in shelf areas and seas, with samples from the the Baltic Sea. We report here the magnetic characteristics, microstructure, and origin of Baltic Sea concretions, which sheds light on their formation processes and environmental implications. To achieve this, we combined nano- and micro-computed tomography imaging, electron microscopy, micro-X-ray fluorescence spectroscopy, and a suite of detailed magnetic property investigations. Spheroidal concretions are prevalent in many parts of the coastal Baltic Sea and contain a higher proportion of fine-grained magnetite with evidence of bullet-shaped magnetofossils produced by magnetotactic bacteria. Bullet-shaped magnetofossils are usually produced in eutrophic and less oxic environments, as supported by the possible presence of rhodochrosite, which indicates diagenetic Mn release from surrounding sediments, especially in deeper water settings. In contrast, crust and discoidal concretions in shallower waters contain higher proportions of detrital (including magnetically hard) minerals, which reflects an increased continental influence. Microstructural analysis of the concretions reveals multiple growth stages, with laminated, columnar, and dendritic structures indicating varying hydrodynamic and depositional conditions. In general, spheroidal concretions seem to form in more tranquil settings compared to discoidal and crust concretions.

Our results provide insights into the complex interplay of environmental conditions, biogenic processes, and mineralogical composition that influence ferromanganese concretion growth and magnetic properties in the Baltic Sea. We further argue that biogenic magnetite contributes globally to the remanent magnetization of shallow and deep-sea ferromanganese concretions.  

This work was supported by the Research Council of Finland (Fermaid project, grant 332249).