LIMITS ON ATMOSPHERIC OXYGEN AND ARGON CONCENTRATIONS AT 815 MA

The timing of the Neoproterozoic oxygenation event, which brought atmospheric O₂ to near-modern levels, is not well constrained. Proxy data suggest that oxygenation occurred during early in the Neoproterozoic, but quantification of O₂ concentrations remains difficult. Consequently, the relationship between atmospheric composition, late Proterozoic tectonic evolution, global glaciation events, and the rise of animals is still debated. Fluid inclusions in bedded halites may offer a new perspective because they can trap atmospheric gases at the air-water interface. It is clear from previous work, however, that some samples instead preserve gases at the sediment-water interface, while others still preserve dissolved gases or degradation products. Moreover, microbes can be trapped within halite, recycling elements within fluid inclusions for at least tens of thousands of years. Given this variety of potential complications, evaluating the fidelity of these gas records is vital if one wishes to use them for paleo-atmospheric reconstructions. Here, I re-analyze fluid-inclusion data from 815 ± 15-Ma halites from the Officer Basin (Australia) to test the hypothesis that they preserve the composition of the Neoproterozoic atmosphere. I find that these samples were affected by preservation or post-depositional artifacts; the gases trapped within them are likely contaminated with modern air and air dissolved in brines. Based on the systematics of these contaminations, the Officer Basin samples yield upper limits on atmospheric O₂ concentrations. In this light, the data imply that the 815-Ma atmosphere contained ≤6% present atmospheric levels (PAL) of O₂, constraining the timing of more extensive oxygenation to until after this time. In addition, the analysis reveals a surprisingly low Ar inventory at 815 Ma—≤60% PAL—which, if accurate, challenges our understanding of the solid Earth’s degassing history. The Neoproterozoic may have been a critical time for both O₂ and Ar in Earth's atmosphere.