No evidence for high atmospheric oxygen levels 1,400 million years ago
Planavsky et al
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 much less than 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).
Zhang et al's reply:
Reply to Planavsky et al.: Strong evidence for high atmospheric oxygen levels 1,400 million years ago
Zhang et al
Planavsky et al. (1) argue that variability in the V/Al of soils compromises our ability to detect V depletions and thus oxygenated bottom waters in the Xiamaling Formation. Indeed, because of such variability, we explored trace metal chemistry through several units of the Xiamaling Formation to establish V/Al background values and trace metal behavior. Unit 4 lacks trace metal enrichments, with V/Al values distributed around the crustal average (CA) (Fig. 1A), which we take to represent unaltered particles entering the basin. In contrast, unit 3 was enriched in Mo and U, with V/Al either depleted or similar to CA (Fig. 1A). These trace metal patterns are, in the modern ocean, uniquely found in organic-rich sediments depositing in oxygenated water (2).