Quantifying Trace Metals in Gunflint Microfossils by 3D Correlative X-ray Nanoimaging

Probing elemental and structural heterogeneities is essential for understanding how materials evolve and degrade, yet correlating chemical and structural information across mesoscale specimens at the nanoscale remains analytically challenging. Here, we introduce a nondestructive correlative workflow that combines three-dimensional data sets from nanoscale X-ray ptychographic and fluorescence tomographies acquired on the same sample at dedicated synchrotron instruments. This workflow was developed and applied to quantify metal traces within individual low-Z compartments of a whole 1.88-billion-year-old Gunflint microfossil exhibiting a distinctive nanoscale architecture characteristic of thick-walled Huroniospora. Beyond the thick wall containing pluri-micrometer-sized quartz crystals with nanoinclusions, our analyses reveal an extracellular shell composed of submicrometer-sized quartz and Fe²⁺-rich greenalite embedded within an Fe³⁺-rich chert matrix. The specimen is unfractured and contains organic matter comparable to that of modern bacterial cells of similar volume. The trace sensitivity achieved in the multimetal three-dimensional maps, combined with precise volumetric quantification at nanometer scales, enabled accurate determination of metal concentrations in fossil compartments. Iron and manganese levels comparable to those of extant photosynthetic microorganisms, together with manganese-calcium-enriched iron silicate cores, suggest a cyanobacterial lineage and provide new insights into early microbial metabolisms. This correlative workflow, enabling high-sensitivity multielement trace analysis of low-Z nanostructured mesoscopic systems without physical subsampling, is broadly transferable to applications ranging from biominerals and geobiological materials to advanced functional materials such as catalysts, batteries, and nanocomposites.