FIG. 2.

Summary of geochemical proxy records. (A) Carbon isotope data from marine carbonates (data from many sources); (B) chromium isotope data from marine ironstones and shales (Cole et al., 2016; Canfield et al., 2018); (C) iodine concentrations from shallow marine carbonates (Hardisty et al., 2017; Lu et al., 2018); (D) molybdenum (Scott et al., 2008; Reinhard et al., 2013) and (E) uranium concentrations (Partin et al., 2013) from marine shales deposited under euxinic and anoxic conditions, respectively; (F) zinc isotope data from marine sedimentary sulfides (Isson et al., 2018); and (G) timeline of evolutionary milestones. Carbon isotope data are interpreted at the first order to reflect the balance of organic-C to carbonate-C burial through time, with heavier values potentially reflecting higher organic-C burial and commensurate release of O₂. Chromium isotope data are interpreted to be a direct measure of atmospheric O₂ because large fractionations, like those beginning ∼800 Ma, require oxidative Cr-cycling in the presence of Mn-oxides during weathering and pO₂ > 1% PAL (Planavsky et al., 2014; Cole et al., 2016). Iodine concentrations in carbonate rocks scale proportionally with concentration of iodate (the oxidized species of I) in seawater; higher concentrations beginning ∼800 Ma are interpreted to reflect increase in stability of well-oxygenated surface ocean conditions and concomitant deepening of the chemocline (Hardisty et al., 2017). The concentrations of Mo and U in marine shales scale proportionally with the concentrations of these elements in overlying seawater. The reservoir sizes of these redox-sensitive elements are modulated at first order by the spatial extent of reducing bottom-water conditions (and hence the size of the global removal flux), with expansion of oceanic euxinia and anoxia drawing down concentrations of Mo and U, respectively. The increase in Mo at ∼800 Ma is therefore interpreted to represent a decrease in the prevalence of euxinia. A corresponding increase in U concentrations has not been observed, suggesting that the event recorded in other proxies at ∼800 Ma could have been restricted to the atmosphere, shallow oceans, and intermediate depths along continental margins where euxinia prevailed—while the deep ocean remained stably anoxic until later times. A delay in the rise of phosphorus and rhenium, like that observed for uranium, could also be explained by later deep-water oxygenation (Reinhard et al., 2017a; Sheen et al., 2018). Supporting details are provided in the supplementary material.