Shallow seawater oxygenation at ca. 1.44 Ga: A reflection of local seafloor oxygen oases or extensive water-column oxygenation?

Abstract It has been suggested that local marine oxygen oases could have formed during the Archean through the oxygenic photosynthesis of microbial mats residing in stromatolites. The ensuing mid-Proterozoic (ca. 1.8–0.8 Ga) was characterized by overall low atmospheric oxygen levels and pervasive oceanic anoxia below a shallow chemocline, but with increasing evidence for several intervals of transient oxygenation. However, since some of the oxygenation intervals have largely been documented by stromatolites, it remains unclear whether these oxygenation signals represent local seafloor oxygen oases or extensive seawater oxygenation. To address this issue, we have conducted a comparative investigation of the mineralogy and geochemistry of stromatolite-rich and stromatolite-poor sections formed during a prominent oxygenation interval in the ca. 1.44 Ga Tieling Formation of North China. Similar iodine concentration and cerium anomaly characteristics in the stromatolite column and inter-column carbonates suggest that no significant difference existed in terms of redox conditions between the stromatolites and the surrounding water column. Furthermore, the stromatolite-rich interval exhibits lower I/(Ca + Mg) values relative to the contemporaneous stromatolite-poor interval. This indicates that the photosynthetic oxygen production capacity of microbial mats developed in the Tieling Formation was limited and insufficient to significantly elevate seafloor oxygen levels. After considering our high-resolution I/(Ca + Mg) and carbonate-carbon isotope data alongside seawater redox investigations of other contemporary basins, we argue that the ca. 1.44 Ga interval of oxygenation may represent an extensive shallow seawater oxygenation event across multiple basins, but with local redox heterogeneity. Our study therefore provides new insight into the nature of oxygenation events recorded by stromatolites during the mid-Proterozoic.