Mineral phases and growth conditions of morphologically diverse shelfal ferromanganese concretions

Ferromanganese concretions in the shelf sea regions, such as the Baltic Sea, are of significant interest due to their geochemical properties, economic resource potential, and roles in benthic ecosystems. This study analyses the authigenic and detrital mineral phases and their provenance in the Baltic Sea concretions, as well as their formation mechanisms and diagenetic evolution. These concretions exist in three distinct morphotypes: crust, discoidal, and spheroidal. Using synchrotron-based techniques (µ-XRF and µ-XAS) paired with XRD, stable Pb isotope, and bulk geochemical analyses, we found that discoidal and spheroidal concretions consist of alternating Fe- and Mn-rich layers, whereas crust concretions are predominantly Fe-rich. The Mn phases primarily consist of birnessite-like phyllomanganates with columnar and branched dendritic growth patterns, indicative of microbially-mediated precipitation. In contrast, the Fe phases are represented by poorly crystalline ferrihydrite, the formation of which is influenced by admixing of detrital minerals. The three main components (Fe-rich, Mn-rich and detrital), each exhibit distinct trace element associations. The geochemical composition and morphology of the Baltic Sea concretions resembles other shelfal precipitates, indicating consistency in formation mechanisms across different shelf environments. Slightly negative to intermediate Ce anomaly values and the range in Nd contents in the samples suggest that early diagenetic processes contribute to the formation of all the morphotypes.