SIMS and NanoSIMS analyses of well-preserved microfossils imply oxygen-producing photosynthesis in the Mesoproterozoic anoxic ocean

Abstract

Well-preserved microfossils in the stromatolitic cherts from the Gaoyuzhuang Formation (~1500Ma), which have one of the best state of preservation in the Proterozoic Era, may play key roles in systematizing information about the evolution of early life in the Proterozoic Ocean. Here, a combination of light microscopy (LM), scanning electron microscopy (SEM), nano-scale secondary ion mass spectrometry (NanoSIMS), secondary ion mass spectrometry (SIMS) and geochemical techniques was employed to characterize the morphology, elemental distributions and carbon isotope values of individual Gaoyuzhuang microfossils, and their growing environment. Light microscopy analyses show that abundant filamentous and coccoid microfossils, classified as oscillatoriacean and chroococcacean cyanobacteria, respectively, are exceptionally well preserved in chert. NanoSIMS analyses show that metabolically important elements, such as 12C−, 13C−, 12C14N−, 32S−, and 34S−, are concentrated in these microfossils and that the variations in the concentrations of these elements are similar to each other, establishing the elemental patterns in undoubtedly biogenic microstructures. The carbon isotope (δ13C) values of individual microfossils range from −31.7‰±0.9‰ to −23.1‰±1.0‰ (weighted mean=−28.5‰±0.1‰), consistent not only with a Calvin Cycle-based photosynthesis, but typically for chroococcacean cyanobacteria as well. Our results, for the first time, provide the element distributions and cell specific carbon isotope values on convincing Mesoproterozoic cyanobacterial fossils, supporting sustained oxygen-producing photosynthesis in the Proterozoic Ocean. The geochemical data indicate these fossil microorganisms may grow in an anoxic seawater, potentially supporting the delayed oxygenation on the Earth. In this regard, this integrated approach may be a step towards a more comprehensive picture of the evolution of early life on the Earth.