The history of atmospheric oxygen through the Mesoproterozoic Era is uncertain, but may have played a role in the timing of major evolutionary developments among eukaryotes. Previous work using chromium isotopes in sedimentary rocks has suggested that Mesoproterozoic Era atmospheric oxygen levels were too  low in concentration (<0.1% of present-day levels (PAL)) for the expansion of eukaryotic algae and for the evolution of crown-group animals that occurred later in the Neoproterozoic Era. In contrast, our new results on chromium isotopes from Mesoproterozoic-aged sedimentary rocks from the Shennongjia Group from South China is consistent with atmospheric oxygen concentrations of >1% PAL and thus the possibility that a permissive environment existed long before the expansion of various eukaryotic clades.

No abstract

No abstract

Intermediate-deep layers are well-ventilated in most modern oceans (Figure 1a), typically in the North Atlantic and Southern Oceans; this is primarily related to deep-water formation (Marshall & Speer, 2012; Weaver et al., 1999). This process plays a fundamental role in regulating global climate by affecting heat, gas exchange and nutrient cycles (Burls et al., 2017). In contrast, no new deep-water mass are formed in the modern North Pacific due to low-surface salinities and the existence of a strong halocline (Emile-Geay