Marine redox fluctuation during the Marinoan Snowball Earth: evidence from red beds of the Nantuo Formation in South China

Abstract

The Marinoan Snowball Earth event marks a critical time in Earth’s history, with significant climate change characterized by low-latitude glaciation. The ocean is hypothesized to be anoxic during the Marinoan glaciation (ca. 649 to 635 Ma). However, this hypothesis is contradicted with global marine red beds (MRB) deposited in the Marinoan glaciation and diversification of complex eukaryotes after the termination of the Marinoan. To better comprehend the redox conditions of the late Cryogenian seawater, we present systematic sedimentological, mineralogical (SEM), and Fe geochemical analyses (Fe isotope and Fe speciation) of the MRB from the Nantuo Formation in South China.Two continuous ice advance-retreat cycles in the Nantuo Formation are separated by a MRB sequence, indicating an interglacial period with limited influence from glaciation. The results of Fe speciation show that the predominant phase of FeHR (highly reactive Fe) is Fe-oxide with extremely low content of Fe-pyrite and Fe-carbonate of the Nantuo Formation. Furthermore, well-preserved and nanometer-sized hematite (Fe-oxide) particles randomly dispersed in the matrix, suggesting that Fe(II) oxidation occurred in the water column rather than in the sediments. High FeHR/FeT ratios and near-zero δ56FeHR values of the MRB suggest scavenging of dissolved Fe(II) by quantitative oxidation at the basal of the Nantuo Formation. A positive δ56FeHR shift above the MRB further reflects the partial oxidation of Fe(II) to Fe(III) in the water column. These suggest sufficient oxygen derived from meltwater and/or atmosphere during the inter-glacial episode, facilitating the deposition of MRB. The oceanic redox conditions switch to anoxic in response to the re-appearance of the second glacial episode. Our findings suggest that the dynamic redox state of seawater was likely controlled by ice-sheet advancing-retreating cycles and provide new insight into redox conditions during the end-Cryogenian Marinoan glaciation.