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.

The lateral distribution and morphology of concretions are influenced by local hydrodynamic conditions, sedimentation dynamics, and redox fluctuations. An important factor is the periodic cover of a very organic-rich “fluffy” mud layer, which is driven by near-bottom currents, imports detrital minerals and modifies redox conditions, impacting microbial activity within the concretions. The higher occurrence of detrital minerals in Fe-rich concretions, particularly in the crust morphotype, suggests formation under stronger terrigenous influence in high-energy sedimentation conditions as opposed to more Mn containing concretions (mainly discoidal and spheroidal) forming in a relatively tranquil depositional setting and deeper water. The maturity of the detrital mineral fraction generally increases from crust to discoidal to spheroidal concretions. The Fe-rich concretions contain greater proportion of micas, clay minerals and K-feldspar to plagioclase, while the more Mn-containing concretions have proportionally high quartz contents. The detrital minerals likely act as nucleation sites promoting Fe precipitation and are redistributed diagenetically toward the interfaces dominated by Fe phases, which are slightly more tolerant to reductive dissolution than Mn phases. The preferential reductive dissolution of Mn phases results in thick Fe-rich growth layers and relative enrichment of the detrital mineral fraction. Stable Pb isotope data show distinct regional signatures, indicating that the composition of concretions is affected by bedrock erosion in the catchment area. Our findings highlight contrasts in mineral phases, geochemistry, formation environments, and diagenetic evolution across sites in Fe-rich crust concretions compared to more Mn containing discoidal and spheroidal concretions. These insights are relevant to ferromanganese concretions in shelf areas worldwide, and their potential use in paleoenvironmental reconstructions and resource exploration.