SIMS and NanoSIMS analyses of Mesoproterozoic individual microfossils indicating continuous oxygen-producing photosynthesis in Proterozoic Ocean

Abstract:

Well-preserved microfossils in the stromatolites from the Gaoyuzhuang Formation (~1500Ma), which is younger than the Gunflint Formation (~1880Ma) and older than the Bitter Springs Formation (~850Ma), may play key roles in systematizing information about the evolution of early life and environmental changes in the Proterozoic Ocean. Here, a combination of light microscopy (LM), scanning electron microscopy (SEM), focused ion beam (FIB), nano-scale secondary ion mass spectrometry (NanoSIMS) and secondary ion mass spectrometry (SIMS) were employed to characterize the morphology, elemental distributions and carbon isotope values of individual microfossils in the stromatolites from the Gaoyuahzuang Formation. Light microscopy analyses show that abundant filamentous and coccoid microfossils 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, establishing the elemental distributions in incontestably biogenic microstructures. Carbon isotope (δ¹³C) values of individual microfossils range from -32.2‰ ± 0.9‰ to -23.3‰ ± 1.0‰ (weighted mean= -28.9‰ ± 0.1‰), consistent with carbon fixation via the Calvin cycle. The elevated δ¹³C values of the microfossils from Early-, Meso- to Late Proterozoic Era, possibly indicate decreasing CO₂ and increasing O₂ concentrations in the Proterozoic atmosphere. Our results, for the first time, provided the element distributions and cell specific carbon isotope values on convincing Mesoproterozoic cyanobacterial fossils, supporting continuous oxygen-producing photosynthesis in the Proterozoic Ocean.