Unravelling biomineralization processes in modern brachiopods: integrating structural analyses with high-resolution elemental mapping and ecology

Brachiopod shells provide a unique biomineral archive of Phanerozoic environmental and climatic changes owing to their exceptional preservation potential (low-Mg calcite) and widespread presence in the fossil record. However, significant uncertainties remain in the interpretation of their geochemical signals, as biomineralization processes and the influence of vital effects–including the debated relationship between shell fabric and geochemistry–remain poorly understood. Addressing these uncertainties requires an interdisciplinary approach integrating structural analyses, with high-precision geochemical techniques, and detailed ecological and biological understanding. Applied to modern brachiopods, this integrated approach offers potential to disentangle the influence of vital/growth effects from environmental controls. Here, we combine mesostructural characterization (SEM) with high-resolution elemental analyses (LA-ICP-MS and TOF-MS) to investigate biomineralization processes in modern two- and three-layered brachiopod species from different geographic and bathymetric settings. Our results show that shell microstructure is not the primary driver of intra-shell geochemical variability, contrary to previous interpretations. Consistent geochemical differences between fibrous and columnar fabrics were observed only in Liothyrella neozelanica. Statistical analyses of elemental compositions across species and fabrics reveal a significant Sr–Li–Na–B cluster, suggesting paired incorporation pathway associated with active biomineralization and rapid shell growth. Conversely, Mg incorporation appears paired with Mn, with both elements apparently enriched in specific shell regions, potentially reflecting reduced exclusion efficiency during late ontogenetic stages or physiological stress. These findings reveal previously unknown pairings of trace elements during incorporation and point towards at least two different incorporation pathways that vary with growth stage.