240 / 2025-04-15 18:26:46

An euxinic snapshot of unfrozen sea driven by storm-induced upwelling during the Sturtian Snowball glaciation

Sturtian glaciation,cyclone,upwelling,redox conditions

Theme 4: Microbial and Biogeochemical Evolution through Time > Session 4c: Reconstructing Biogeochemical Cycles in Deep Time: Assumptions, Bias, Constraints, and Directions

The Cryogenian ocean is no longer thought of as rigid anoxic ferruginous conditions. It is generally accepted that there is a close relationship between marine redox fluctuations and ice sheet advancing-retreating cycles. Ice volume changes reflect climate changes, and climate forcing drives continental chemical weathering to import nutrients and redox sensitive substances into oceans, thereby altering marine redox conditions. This internal connection is particularly obvious in the shallow offshore area. Here we report a marine redox step-change from a ~678 Ma storm-prone shelf, South China. Redox indicators uniformly show changes from euxinic to ferruginous to oxic conditions. It is notable that the initial euxinic snapshot is coupled with a high authigenic francolite content. The negative correlation between francolite content and chemical index of alteration suggests that elevated seawater phosphorus levels are mainly attributed to upwellings. The fully consumed oxic iron speciation and organic matter indicate that a sufficient nutrients cycling stimulates the recovery of marine primary productivity, thereby promoting the microbial sulfate reduction. According to the data compilation, high phosphorus content and marine euxinia in seawater are rare in the Cryogenian. We consider storms are directly controlled by the climate change. The cyclone, triggered by the high sea surface temperature, stirred an unfrozen sea. As mixing proceeds, the cold eutrophic deep-water consumes energy. Weakening storms are reflected in decreases of storm erosion surface and hummocky/swaley cross-bedding structures; the stagnation of upwellings is reflected in the decrease of francolite. Finally, the oxic unfrozen sea without storms indicates the recognized shallow marine redox conditions. The storm-prone shelf in South China suggests marine redox conditions are controlled not only by weathering but directly climate-induced variation. Given the possibility of vast unfrozen areas during the Sturtian glaciation, the Earth’s surface temperature difference could be easily eliminated through our explanation for sustaining the longest glaciation.