Dynamic redox fluctuations and transient oxygenation during deposition of the ~1.56 Ga Gaoyuzhuang Formation, North China Craton

There is increasing evidence that the redox state of the Mesoproterozoic (1.6–1.0 Ga) ocean was much more dynamic than previously recognized, although the trajectory of Earth surface oxygenation over this immense interval of time remains poorly understood. In order to further constrain oceanic oxygenation dynamics, we present a high-resolution, multi-proxy geochemical study of carbonate-rich rocks from the Mesoproterozoic Gaoyuzhuang Formation, North China Craton, which represents a critical Mesoproterozoic section due to its association with the earliest known, decimetre-scale eukaryotes. Combined with Fe-speciation analyses, our approach utilizes sequential leaching to target the primary carbonate-phase, and utilizes an improved enrichment factor calculation for application of U (UEF*) and Mo (MoEF*) systematics to carbonate-rich rocks. Rather than recording a series of discrete oxygenation pulses, our data suggest a prolonged interval of enhanced oxygenation, from the upper part of Member I to the lower part of Member III in the Gaoyuzhuang Formation. However, this only oxygenated surface waters to shallow subtidal depths. Subsequently, a more distinct oxygenation pulse occurred below the Gaoyuzhuang fossil horizon of Member III (∼750 m), which oxygenated the water column to around storm wave base. However, this enhanced oxygenation appears to have ceased during deposition of Member IV in the Gaoyuzhuang Formation, with a decline in surface water oxygenation, at least on a local or regional scale, raising uncertainty over the broader scale trajectory of Earth surface oxygenation. Based on the oceanic redox evolution recorded by the whole formation, we speculate that the prolonged stable oxygenation of shallower waters, rather than a discrete oxygenation event, may have permitted the evolution of the large-scale Gaoyuzhuang eukaryotes.