Fig. 2: Geochemical and isotopic profiles.
From: An expansive global oxygenation of Earth’s surface environments 1.4 billion years ago
a Dashed lines on FeHR/FeT profiles represent calibrated thresholds indicative of oxic (<0.22) and anoxic (>0.38) water column conditions9. Black circles in the FeHR/FeT profiles represent an extra supply of hydrothermal iron that affects interpretation of Fe speciation data (see text for detailed explanation). The dashed lines on the Fepy/FeHR profiles represent the lower (0.6) and upper (0.8) thresholds for euxinia9. Dashed lines on I/(Ca + Mg) profiles represent the baseline (~0.5) for Precambrian carbonates24. Shading indicates the approximate position of cycles from ferruginous (white) to oxic (gray) to suboxic (transition color) conditions. b Coeval with Unit 3 Mn-rich shale, other Mn-rich sedimentary rocks can be subdivided into two ore belts of primary Mn-oxide and diagenetic Mn-carbonates, respectively38. Dashed lines on elemental ratio profiles represent the average upper continental crust (UCC)50. The high Mo/Al contents and high Mn-Fe contents exclude the influence of detrital Mo. Oxygenated conditions during deposition of these Mn-rich rocks are verified by uniformly negative δ98Mo and δ56Fe values and a decrease in redox-sensitive trace-element (RSTE) enrichment factors. The error bars for δ98Mo and δ56Fe are smaller than symbol sizes. TOC Total Organic Carbon Content.
