Nanoscale Biosignatures Preserved in the Microbially Induced Sedimentary Structures from the ~3.48 Ga Dresser Formation, Western Australia

The preservation of Archaean microfossils is often limited, and individual morphological or isotopic analyses are generally insufficient to establish a complete chain of evidence for biogenicity. Microbially induced sedimentary structures (MISSs) are distinctive products of microbial activity during deposition in both modern and geological settings. Such structures have also been identified in the ~3.48 Ga Dresser Formation cherts of the Pilbara Craton, Western Australia, potentially representing the oldest MISSs in the geological record. This study focuses on potential microfossils preserved as carbonaceous microspheres within these MISSs, and presents a multi-scale morphological and biogeochemical investigation. Optical and scanning electron microscopy (SEM) reveal that these microstructures exhibit a constrained size distribution and frequently occur as bead-like chains or aggregated colony-like networks, with individual microspheres displaying a distinctive concentric, multi-layered ("onion-like") morphology. Raman spectroscopy and SEM-EDS analyses indicate that the microstructures are composed of mature carbonaceous matter hosted within a quartz matrix. Their conformable contact with the surrounding siliceous matrix suggests rapid silica precipitation during early diagenesis, resulting in the physical encapsulation of the organic structures. Subsequent NanoSIMS in situ isotopic imaging demonstrates that carbon (¹²C), nitrogen (¹⁴N), and sulfur (³²S) signals are spatially co-localized within the microstructures, indicating complex biological organic precursors. Additionally, these spherical microstructures are encased by a continuous, massive primary silica envelope that facilitated the retention of their chemical composition and effectively prevented late-stage fluid infiltration and exogenous contamination. Collectively, this work confirms the biogenicity of the micro-scale spherical structures within the Dresser Formation cherts and provides robust evidence for the existence of Archaean microbial mats and benthic ecosystems.