Subtle Cr isotope signals track the variably anoxic Cryogenian interglacial period with voluminous manganese accumulation and decrease in biodiversity

Abstract

Earth’s atmosphere experienced a step of profound oxygenation during the Neoproterozoic era, accompanied by diversification of animals. However, during the Cryogenian period (720–635 million years ago) Earth experienced its most severe glaciations which likely impacted marine ecosystems and multicellular life in the oceans. In particular, large volumes of Mn and Fe accumulated during the interglacial intervals of the Cryogenian glaciations, indicating large anoxic marine metal reservoirs. Here we present chromium isotope-, rare earth element-, and redox-sensitive trace element data of sedimentary rocks from the interglacial Datangpo Formation deposited between the Sturtian and Marinoan glaciations in South China, in an attempt to investigate the oxidation state of the oceans and atmosphere. Both the Cr isotope and trace element data indicate mainly anoxic water conditions with cryptic oxic surface water incursions after the Sturtian glaciation. Glacial-fed manganese precipitated as manganese carbonate in anoxic basins, and the non-fractionated δ53Cr record of −0.10 ± 0.06‰ identifies anoxic conditions with a cryptic component of slightly fractionated Cr isotope composition in manganese ore, in line with distinctly fractionated Mo isotope composition. Both the manganese carbonate ore and the black shales exhibit very low redox-sensitive element concentrations. Our study demonstrates that the oxygenation of the seawater, and inferably of the atmosphere, at the beginning of the Cryogenian interglacial interval was much subdued. The post-glacial rebound then allowed the Ediacaran biological diversity.

Sedimentary Mn Metallogenesis and Coupling among Major Geo-Environmental Events during the Sturtian Glacial–Interglacial Transition

“…For example, many studies of the redox state of seawater during this period have focused on the Nanhua Rift Basin in the south-eastern Yangtze Platform, where Mn deposits in the Datangpo Formation exist [43][44][45][46][47][48][49][50]. A series of proxies used to reconstruct the redox environment of the Datangpo Formation indicate that the deep water column in the Nanhua Rift Basin might have experienced a redox transition from an anoxic state during the Sturtian glaciation to a temporarily oxygenated state during the Mn metallogenic era (Figure 3).…”

“…Coincidentally, if this redox transition happened to encounter substantial amounts of dissolved Mn(II) stored in the anoxic water column within the relatively enclosed basins, episodic ventilation with cold oxygenated surface water would then have induced large-scale Mn sedimentation. Elemental and isotopic proxies for reconstructing paleoenvironmental oxygenation during deposition of the Sturtian-age Tiesi'ao Formation and the 1 st (Mn ores and black shales overlying the Mn layers) and 2 nd members of the Datangpo Formation (data sources: N isotopes [44]; Fe species [46]; Fe isotopes [48]; Cr isotopes [49]; Cd isotopes (SW: surface water; DW: deep water) [50]). [44]; Fe species [46]; Fe isotopes [48]; Cr isotopes [49]; Cd isotopes (SW: surface water; DW: deep water) [50]).…”

“…Elemental and isotopic proxies for reconstructing paleoenvironmental oxygenation during deposition of the Sturtian-age Tiesi'ao Formation and the 1 st (Mn ores and black shales overlying the Mn layers) and 2 nd members of the Datangpo Formation (data sources: N isotopes [44]; Fe species [46]; Fe isotopes [48]; Cr isotopes [49]; Cd isotopes (SW: surface water; DW: deep water) [50]). [44]; Fe species [46]; Fe isotopes [48]; Cr isotopes [49]; Cd isotopes (SW: surface water; DW: deep water) [50]).…”

Constraining Shallow Seawater Oxygenation for the Yangtze Platform During the Early Cambrian

“…Cr isotopes in ancient marine carbonates and shales are increasingly used to reconstruct the oxygenation history of the atmosphere‐hydrosphere system (Canfield et al., 2018; Cole et al., 2016; Frank et al., 2019; Frei et al., 2011; Gilleaudeau et al., 2016; Holmden et al., 2016; Planavsky et al., 2014; Wei, Frei, et al., 2018; Wei, Frei, et al., 2020; Xu et al., 2019) In a continental setting Cr is most commonly present as Cr(III) in igneous minerals and is characterized by a narrow isotopic range of δ 53 Cr = −0.124 ± 0.101‰ (Schoenberg et al., 2008), with the exception of some altered mafic and ultramafic rocks that are positively fractionated compared to the igneous baseline (Farkaš et al., 2013; Shen et al., 2015). The oxidation of Cr(III) to Cr(VI) is catalyzed by MnO 2 , which is only stable above a threshold value of atmospheric oxygen estimated to be between 0.03% and 1% of PAL based on kinetic constraints (Crowe et al., 2013; Planavsky et al., 2014).…”

Separation of Fe from Mn in the Cryogenian Sedimentary Mn Deposit, South China: Insights from Ore Mineral Chemistry and S Isotopes from the Dawu Deposit

“…The seawater under the ice sheet was in a reduced state, and Mn and Fe could accumulate as ionic compounds (Mn 2+ and Fe 2+ ) in large quantities, which provided a material basis for the subsequent precipitation and enrichment of manganese [13,43,44]. Previous studies have shown that the Nanhua Basin was in a redox stratified environment in the Datangpo stage [45][46][47], and the oxidation of the water in the basin gradually increased, forming the suboxidation-oxidation environment in the shallow parts [48][49][50]. In the process of progressive oxidation in the marine environment after glaciation, Fe 2+ was oxidized and precipitated before Mn 2+ , leading to the first-stage separation of Mn and Fe [51].…”