Bugs or gunk? Nanoscale methods for assessing the biogenicity of ancient microfossils and organic matter

Ancient microfossils and stromatolites represent some of our best evidence of early living organisms on Earth. However, due to increasingly critical evaluation of such biomarkers in the last decade, assessing the biogenicity of preserved microbe-like features and stromatolitic structures is far from trivial. Carbonaceous matter associated with bona fide microfossils and stromatolites should contain a general microstructure consistent with that of kerogen, the biogenic organic matter common in oil, gas, and coal source rocks. Although kerogen structure and composition can vary, there are consistent characteristics that may be used to identify kerogen-like carbonaceous material. Fischer-Trospch-type (FTT) reactions and other organic synthesis mechanisms may be a significant source of abiotic organic matter on the early Earth. Many minerals associated with hydrothermal activity, which was prevalent on the early Earth, have been shown to catalyze FTT synthesis. However, significant research into the maturation and preservation of FTT-derived carbonaceous matter is necessary before a robust distinction can be made from biogenic kerogen. Controversial microfossils from the 3.458-3.465 Ga Apex Basalt and stromatolites from the 3.350-3.458 Ga Strelley Pool Chert are associated with hydrothermal activity, leading to the suggestion that the carbonaceous matter comprising these features is derived from abiotic FTT synthesis rather than biological processes. Carbon-rich black chert dike samples were obtained from the Apex and Strelley Pool localities for comparison with kerogen from the 1.9 Ga Gunflint Formation and laboratory-derived FTT carbonaceous matter. In situ scanning-transmission X-ray microscopy (STXM) and transmission electron microscopy (TEM) reveal the presence of ∼100 nm carbonaceous films along quartz grain boundaries in the chert samples, which may be separated from the quartz grains in ultramicrotomed sections. Electron energy-loss near-edge structure spectroscopy (ELNES) and X-ray absorption near-edge structure spectroscopy (XANES) indicate Apex and Strelley carbonaceous matter contains a complex microstructure with abundant aromatic domains and oxygenated organic functional groups similar to that observed in Gunflint kerogen. FTT samples were chemically distinct, containing abundant carboxyl functional groups and lacking polyaromatic domains. Although it is possible that FTT material could mature into an abiotic kerogen-like material with similar characteristics, it is more likely that the Apex and Strelley Pool carbonaceous matter formed from biological processes. Biogenic material could be preserved within an ancient hydrothermal system either through gravitational influx of microbial material from the surface or transport of carbonaceous matter from a deeper sediment.