A pulse of oxygen increase in the early Mesoproterozoic ocean at ca. 1.57–1.56 Ga

Abstract

The relationship between oxygen and evolution of early eukaryotes including algae and primitive animals in geological history has been debated, partly due to the varying estimates of oxygen levels in the mid-Proterozoic (ca. 1.8–0.8 Ga) ocean and atmosphere. The upper part of the Gaoyuzhuang Formation (ca. 1.60–1.54 Ga) in North China hosts decimeter-scale multicellular eukaryotic fossils and is documented with a decrease in cerium anomaly indicative of ocean oxygenation. However, the atmospheric oxygen level across this interval and its subsequent oxidation state require further investigation using additional redox proxies. Here we report I/(Ca+Mg) ratios, carbonate/organic carbon isotopes (δ13Ccarb and δ13Corg), and phosphorous (P) contents across the ca. 1.57–1.56 Ga fossil-bearing interval in the North China Platform. High I/(Ca+Mg) ratios (≥2.6 μmol/mol; up to 3.8 μmol/mol) from shallow-water carbonates of the Gaoyuzhuang Formation suggest an episode of significant oxygen increase up to ≥4% PAL (present atmospheric level). The I/(Ca+Mg) ratios return back to ≤0.5 μmol/mol shortly after the peak values without evidence for increasing water depth or diagenetic alteration, implying a short-lived oxidation event. The increase of I/(Ca+Mg) ratios is associated with a −3.5‰ negative δ13Ccarb and δ13Corg anomaly and an increase in P/Al ratios that are best explained by oxidation of dissolved organic carbon (DOC) in the ocean. Oxygen consumption through oxidation of DOC may have quickly lowered marine and atmospheric O2 levels to the early mid-Proterozoic (1.8–1.4 Ga) background oxygen concentration of ≤0.1–1% PAL. Short-lived oxidation events in an overall anoxic mid-Proterozoic ocean and atmosphere best explain the existing geochemical data and evolutionary stasis of eukaryotes during the “Boring Billion”.