Ocean deoxygenation after the Sturtian Snowball

Introduction

The Cryogenian Period (c. 717 to 635 million years ago, or Ma) encompasses the Sturtian and Marinoan ‘Snowball Earth’ glaciations¹, together with an intervening non-glacial interlude during which a seemingly ‘normal’ hydrological cycle returned. The period represents a fundamental turning point in Earth-Life evolution, linking the ‘Tonian Transformation’² with the ‘Ediacaran-Cambrian Radiation’³. The nonglacial interlude (c. 661–650 Ma) is of particular interest due to it being a key stage in the rise toward ecosystem dominance by multicellular eukaryotes in the form of both algae and animals (putative sponges), as evidenced by organic biomarkers^4,5. This ecosystem reshaping was accompanied by significant perturbations to the global carbon cycle as indicated by a high carbonate carbon isotope (δ¹³C_carb) baseline punctuated by transient negative δ¹³C_carb excursions. Examination of the Cryogenian carbon cycle has raised the hypothesis that decreased organic carbon remineralisation associated with sulfate-poor deep ocean conditions led to the growth of a large dissolved organic carbon (DOC) pool⁶, but the evidence remains controversial⁷. High δ¹³C_carb values have also been related to ocean-atmosphere oxygenation, possibly caused by increased organic carbon burial and corresponding oxygen release due to a surplus of glacially induced nutrient supply, thus providing a potential link to early animal evolution⁸. Recent studies highlight the role of uninhabitable environments for limiting animal evolution, specifically pinpointing the immediate aftermath of the Sturtian deglaciation as one such key interval^9,10.

Proxy evidence and modelling studies point to enhanced volcanic activity¹⁰, a brief burst of chemical weathering^11,12 and global cooling following a super-greenhouse climate^9,13 in the aftermath of the Sturtian deglaciation. A short-lived euxinic interval has also been inferred from iron speciation data⁹, but the underpinning mechanisms are not fully understood. Moreover, while Mo isotopes from siliciclastic deposits support extensive marine euxinia^14,15, this would appear to contradict U isotope records from carbonate rocks that are interpreted to indicate transient ocean oxygenation¹⁶. These conflicting views hamper our understanding of mechanistic links between climate, ocean dynamics and biological evolution during such a critical interval.

In order to resolve these uncertainties, we revisit the Taishir Formation in Mongolia, which...