Testing the cellular nature of large (>10µm) spheroids in the ~3.4 Ga Strelley Pool Formation

The oldest traces of life on Earth are still highly debated and both biotic and abiotic arguments are proposed for isotopic, mineral and morphological biosignatures1,2. The ca. 3.4 Strelley Pool Formation (SPF), in Western Australia, is well-known for its stromatolites and for its diverse assemblages of carbonaceous microstructures. Some of these microstructures have been claimed as the oldest microfossils2,3,. The SPF lies in greenstone belts of the Pilbara craton, and comprises (volcano)clastic material, silicified sediments, dolomite (including stromatolites) and precipitated cherts (including stromatolites). Metamorphism grades from to greenschist to amphibolite facies and it is cut across by siliceous and siliceous-carbonaceous veins4. Such veins may have transported carbonaceous matter into hydrothermally-influenced sediments2,3. For example, microstructures in silicified volcanic clasts of the SPF exhibit morphologies resembling mineral textures associated with organic matter5, such as simple spheres or lenses. Here we rigorously describe unusual spherical microstructures which differ from the abundant, smaller (≤10 µm) spheres previously reported in cherts of the SPF3. These microstructures are imaged and analyzed through a combination of in situ techniques, including optical microscopy and confocal laser scanning microscopy (CLSM), electron backscattered diffraction (EBSD), and transmitted electron microscopy (TEM) on ultrathin (focused ion beam) sections (FIB). This approach yields an effective visualization of carbonaceous microstructures and their relationships with quartz crystal grains down to the nanoscale. It revealed a wealth of textural features that allow us to classify the spheroids into three new types (+sub-types) and to discuss possible cellular origin versus mineral morphogenesis in presence of organic compounds.