Mesocrystalline Architecture in Hyaline Foraminifer Shells Indicates a Non-Classical Crystallisation Pathway

Calcareous foraminifer shells (tests) represent one of the most important archives for paleoenvironmental and paleoclimatic reconstruction. To develop a mechanistic understanding of the relationship between environmental parameters and proxy signals, knowledge of the fundamental processes operating during foraminiferal biomineralization is essential. Here, we apply microscopic and diffraction-based methods to address the crystallographic and hierarchical structure of the test wall of different hyaline foraminifer species. Our results show that the tests are constructed from micrometer-scale oriented mesocrystals built of nanometer-scale entities. Based on these observations, we propose a mechanistic extension to the biomineralization model for hyaline foraminifers, centered on the formation and assembly of units of metastable carbonate phases to the final mesocrystal via a non-classical particle attachment process, possibly facilitated by organic matter. This implies the presence of metastable precursors such as vaterite or amorphous calcium carbonate, along with phase transitions to calcite, which is relevant for the mechanistic understanding of proxy incorporation in the hyaline foraminifers.