Pulsed oxygenation events at ∼ 1.44–1.43 Ga: Evidence from the Tieling Formation in the North China Platform

Abstract

The unique accumulation of oxygen (O₂) in Earth’s oceans and atmosphere distinguishes it from other rocky planets. The oxygenation of the Mesoproterozoic ocean–atmosphere system was a protracted process, yet it may have included transient oxygenation events, particularly before ∼ 1.4 billion years ago. Despite ongoing research, the detailed nature of redox conditions and the origin of a potential oxygenation event around 1.44–1.43 Ga remain poorly understood. Here, we present a suite of geochemical proxies, including iodine-to-calcium-magnesium [I/(Ca + Mg)], rare earth elements (REE) plus Y (REY), carbonate-associated sulfate sulfur isotopes (δ³⁴S_CAS), Rb/Al, and P/Al ratios for the carbonate rocks of the Tieling Formation (ca. 1.44–1.43 Ga) from two well-preserved sections of the North China Platform. The lower Tieling Formation (Daizhuangzi Member) exhibits middle REE-enriched patterns and elevated I/(Ca + Mg) values (up to 1.32 µmol/mol), suggesting potentially oxic or suboxic seawater conditions. The upper Tieling Formation (Laohuding Member) shows a pronounced increase in I/(Ca + Mg) at the basal interval, followed by a further marked rise in the middle to upper portion of this member. The rising I/(Ca + Mg) ratios, coupled with negative shifts in Ce anomalies, imply at least two pulses of oceanic oxygenation during deposition. Notably, the peak in I/(Ca + Mg) ratios in the middle-upper Laohuding Member corresponds with previously reported positive Cr isotope fractionation, indicating an oxygenation event that extended to the atmosphere. Similar oxygenation signals have also been recognized in the time-equivalent Fengjiawan Formation of southern North China and the Kaltasy Formation of the Russia, suggesting a multi-basin oxygenation process at ∼ 1.44–1.43 Ga. Additionally, elevated P/Al and Rb/Al ratios beneath the Laohuding Member point to intensified terrestrial weathering and nutrient delivery to the ocean that may have accelerated the pyrite burial and contributed to the oxygenated events during this period.