No evidence for high atmospheric oxygen levels 1,400 million years ago

Zhang et al. (1) recently proposed atmospheric oxygen levels of ∼4% present atmospheric levels (PAL) based on modeling a paleoenvironment reconstructed from trace metal and biomarker data from the 1,400 Ma Xiamaling Formation in China. Intriguingly, this pO2 level is above the threshold oxygen requirements of basal animals and clashes with evidence for atmospheric oxygen levels <<1% PAL in the mid-Proterozoic (2). However, there are fundamental problems with the inorganic and organic geochemical work presented by Zhang et al. (1).