Three-dimensional structure, crystallography, and magnetism of giant magnetofossils

Biomineralization has existed on Earth for billions of years, documenting life evolution history and major geological events. However, determining the origins of putative biogenic minerals remains a persistent challenge. A prime example is the micrometer-sized fossil magnetite crystals (giant magnetofossils) discovered in global marine sediments, dating from the present to 97 Ma ago. Their origin remains unclear because their shape and size are different from any known nanometer-sized magnetite produced by magnetotactic bacteria. Here, we present three-dimensional (3D) reconstructions of giant magnetofossils with nanometer resolution, and report two new morphologies of giant magnetofossils. By identifying diagnostic 3D morphological, crystallographic, and micromagnetic features, our results provide key identification criteria for giant magnetofossils and suggest a potential biologically controlled mineralization origin. This study provides a comprehensive 3D characterisation of giant magnetofossils, aiding the search for ancient microbial activities on Earth and extraterrestrial bodies. Three-dimensional morphological, crystallographic, and micromagnetic diagnosis of giant magnetofossils, provide key identification criteria for micrometer-sized fossil magnetite and suggest a potential biomineralization origin, advancing current knowledge about biosignatures on ancient Earth.