Oxygenation of a Cryogenian ocean (Nanhua Basin, South China) revealed by pyrite Fe isotope compositions

- A pyrite δ56Fe record was obtained for Cryogenian sedimentary rocks, South China.
- Pyrite δ56Fe records increased oxygenation in the interglacial Nanhua Basin.
- Increased oxygenation in the Nanhua Basin may reflect local or global controls.A possible local control is a decline in the seafloor hydrothermal iron flux.
- The increased ocean oxygenation may also reflect increased atmospheric O2 levels.

The nature of ocean redox chemistry between the Cryogenian Sturtian and Marinoan glaciations (ca. 663-654 Ma) is important for understanding the relationship between environmental conditions and the subsequent emergence and expansion of early animals. The Cryogenian-to-Ediacaran stratigraphic succession of the Nanhua Basin in South China provides a nearly complete sedimentary record of the Cryogenian, including a continuous record of interglacial sedimentation. Here, we present a high-resolution pyrite Fe isotope record for a ∼120-m-long drill-core (ZK105) through Sturtian glacial diamictites and the overlying interglacial sediments in the Nanhua Basin to explore changes in marine chemistry during the late Cryogenian. Our pyrite Fe isotope profile exhibits significant stratigraphic variation: Interval I, comprising middle to upper Tiesi'ao diamictites (correlative with the Sturtian glaciation), is characterized by light, modern seawater-like Fe isotope compositions; Interval II, comprising uppermost Tiesi'ao diamictites and the basal organic-rich Datangpo Formation, is characterized by an abrupt shift to heavier Fe isotope compositions; and Interval III, comprising organic-poor grey shales in the middle Datangpo Formation, is characterized by the return of lighter, seawater-like Fe isotope compositions. We infer that Interval I pyrite was deposited in a predominantly anoxic glacial Nanhua Basin through reaction of dissolved Fe2+ and H2S mediated by microbial sulfate reduction (MSR). The shift to heavier pyrite Fe isotope values in Interval II is interpreted as partial oxidation of ferrous iron to ferric iron and subsequent near-quantitative reduction and transformation of Fe-oxyhydroxides to pyrite through coupling with oxidation of organic matter in the local diagenetic environment. In Interval III, near-quantitative oxidation of ferrous iron to Fe-oxyhydroxides followed by near-quantitative reduction and conversion to pyrite in the local diagenetic environment was likely responsible for the return of lighter, seawater-like Fe isotope compositions in pyrite. Our pyrite Fe isotope profile thus records increased oxygenation in the Nanhua Basin between the Sturtian and Marinoan glaciations.The increased oxygenation of Nanhua Basin seawater deduced from pyrite Fe isotopes could have resulted from either local or global controls. Further work will be needed to determine whether this increasing oxygenation extended to the global scale.