Depositional age and geochemistry of the 2.44–2.32 Ga Granular Iron Formation in the Songshan Group, North China Craton: Tracing the effects of atmospheric oxygenation on continental weathering and seawater environment

Abstract The responses of oceanic chemistry and redox state to the oxygenation of atmosphere at the period (2.45–2.10 Ga) of the global Great Oxidation Event remain poorly constrained due to the lacking of coeval marine chemical precipitates, e.g., Iron Formation. Here, we report a Granular Iron Formation (GIF) in the Songshan Group of the North China Craton and its depositional age was constrained at 2.44–2.32 Ga at the first time. In the GIF, hematite is the primary iron oxide and its occurrence indicates that the shallow seawater at 2.44–2.32 Ga was oxygenated, which is in agreement with the positive δ13C excursion of the coeval carbonates up and below the Songshan GIF. No significant correlations between redox-sensitive trace elements (Cr, V, Mo and U) and Al2O3 contents exclude terrigenous clastic origin, suggesting that these elements are enriched during the deposition and have been adsorbed by the Fe-rich chemical precipitates. The high Cr (up to 2000 ppm) and B (up to 300 ppm) contents of the GIF and the authigenic hematites further indicate an increase of chemical weathering and subsequent input into the seawater. Combined with high V (up to 1500 ppm) and P (up to 7000 ppm) contents in the GIF, it can be inferred that continental oxidative weathering was intensive during 2.44–2.32 Ga. The enrichments of Cr contents and Cr/Ti ratios of the Songshan GIF and its authigenic hematites suggest that Cr was transported to the oceans as soluble Cr (VI) species by rivers because of the oxidative Cr(III) in Cr-rich accessory minerals in soil or crust. The enrichment processes of Cr and V in the hematites are likely to be that soluble Cr (VI) and V (V) supplying by the downwelling surface waters transform into Cr (III) and V (III) in the suboxic-anoxic bottom waters and then removed from seawater via authigenic burial of hematite because trivalent cations can substitute in the hematite structure. Based on above, we therefore suggest a new depositional scenario for the Songshan GIF, in which precipitation occurred due to upwelling of deep, anoxic, reduced Cr-V-rich ferruginous waters into an oxygenated, high productivity shallow-water setting.