Nanoscale petrographic insights into the morphogenesis of carbonaceous microstructures from the ca. 3.4 Ga Strelley Pool Formation

Advanced characterization techniques are commonly used to assess the biotic or abiotic origin of organic microstructures. Although morphological, isotopic and geochemical features are used to support a biotic and/or an abiotic morphogenesis for questionable microfossils, strong debates persist regarding the candidates for the oldest microfossils on Earth.
Stromatolites from the ca. 3.4 Ga Strelley Pool Formation (SPF) in Western Australia yield the oldest undisputed biosignatures of microbial communities1. Moreover, the non-stromatolitic chert of the SPF preserves carbonaceous microstructures interpreted as biogenic organic matter2 or even microfossils3. This formation is part of the Pilbara Craton greenstone belt, which experienced metamorphism up to greenschist facies. These possible fossils are found in sedimentary cherts, silicified deposits common in hydrothermal vents environments, usually cut across by multiple veins of chert (some being carbonaceous). These secondary infilling transport additional, possibly abiotic organic matter. Through aggregation, possibly using mineral templates, sedimentary and exogenous organic matter may form carbonaceous microstructures mimicking microfossils in sedimentary beds submitted to hydrothermal activity, diagenesis, and/or metamorphism1.
Here we report a thorough investigation of carbonaceous spheroids and lenses considered as the oldest assemblage of microfossils, using advanced petrographic techniques to decipher morphological and ultrastructure features and to assess their biogenicity. Optical microscopy (Z stacking images) and Confocal Laser Scanning Microscopy (CLSM) permitted the observation of the organic matter textures and 3D reconstructions, while Electron Back-Scattered Diffraction coupled to photonic microscopy allowed the analysis of the quartz crystals’ arrangement, sizes, shapes, and relationships with organic matter. At the nano-scale, Scanning Transmitted Electron Microscopy (STEM) and nano-diffraction mapping were performed. This combined methodology enables the nanoscale characterization of the links between the organic matter textures and the quartz matrix in cherts. The observation of organic matter coalescence patterns associated with discontinuous films partly embedded in quartz crystals supports an abiotic morphogenesis for at least three types of spheroids. In contrast, some clustered spheroids, large spheroids and lenticular microstructures bear structural features that remain consistent with possible microfossils