Marine redox fluctuations during the Marinoan glaciation

Abstract

The proliferation of eukaryotes preceding the Cryogenian Marinoan glaciation (650–635 Ma) and the subsequent radiation of early animals imply a conceivable linkage between global glaciation and complex life evolution. However, the marine redox condition remains enigmatic during the Cryogenian, impeding our understanding of the role of O2 on this biological innovation. To fill this gap, we comprehensively reported mineralogy, iron isotope (δ56Fe), and iron speciation in the Cryogenian Nantuo Formation at Liulongshan, South China. This section encompasses glacial marine and non-glacial deposits, where the redbeds situated in the middle of the section, in conjunction with the diamictites adjacent to them, divide the Nantuo Formation into three distinct glacial sedimentary cycles. Micro-particle hematite within the Nantuo Formation indicates that the primary Fe-oxide was authigenic and trapped in water column. Furthermore, the Fe isotopic compositions of these authigenic components (δ56FeHR) calculated by mass balance equation reveal the redox state of the ocean coinciding with dynamic glaciation. The lower diamictite shows an increasing trend of δ56FeHR, Fe/Al, and FeHR/FeT, indicative of gradually more anoxic seawater conditions coupled with ice advance. Although redbeds intervals exhibit higher FeHR/FeT ratios (∼ 0.51), traditionally indicative of anoxic environments, our integration of mineralogical evidence suggested that the increased Feox content in redbeds might represent increased oxidation of Fe(II) at the redox chemocline, rather than the redox conditions of the bottom waters. This interpretation is consistent with the observed decreasing trend in our Fe isotope data, as well as the sedimentological evolution. Subsequently, the positive and increasing δ56FeHR values suggest that the oceanic redox conditions shifted to anoxic as the second glacial episode re-emerged. These findings suggest dynamic fluctuations in the redox condition of the ocean during the Marinoan Ice Age. Specifically, the formation of the redbeds may reflect a locally oxic environment. A modern analogy resembling this situation is blood falls, where the oxidation of iron-rich brine plumes under the Antarctic ice sheets. Such subglacial environments could have potentially provided a habitable “refuge” for early life to survive during the Neoproterozoic glaciation.