Pyrite morphology and sulfur isotopes refine taphonomic models for the 2.1 Ga Francevillian biota, Gabon.

Pyritization is a key taphonomic process that preserves some of Earth's oldest fossils. It is influenced by various factors such as organic matter type, the availability of iron and sulfur, and sedimentation rates. In this study, we analyzed pyritized biotic and abiotic structures from 2.1 Ga deposits in Gabon's Francevillian Basin, to reconstruct their taphonomic pathway at the micron scale. Using secondary ion mass spectrometry and scanning electron microscopy, we examine sulfur isotope compositions, pyrite morphology and grain size within individual fossils and compare them to abiotic pyritic concretions from the same stratigraphic level. Our results reveal differences in pyrite grain size and sulfur isotope composition between fossils and concretions. More importantly, chemical and morphological variations are observed within individual fossils, likely due to distinct reactive environments for pyrite mineralization, linked to organic matter, sulfate and iron availability during early diagenesis. This remarkable variation in pyrite morphology and δS values in the fossilized specimens, indicates that they were compositionally more complex than the substrate that formed the homogeneously pyritized concretions. This well-preserved ecological window represents an exceptional record of the earliest multicellular life forms on Earth.