A transient oxygen increase in the Mesoproterozoic ocean at ∼1.44 Ga: Geochemical evidence from the Tieling Formation, North China Platform

Abstract

Oxygen availability is crucial for the evolution of eukaryotes in geological history, yet detailed Mesoproterozoic oceanic-atmospheric redox conditions remain enigmatic. In contrast to the generally accepted hypothesis of an anoxic mid-Proterozoic ocean and atmosphere, several transient oxygenation events may occur at the Earth’s surface during the Mesoproterozoic, especially for the period around 1.4 Ga. The North China Platform develops one of the most complete and continuous Mesoproterozoic stratigraphic successions globally, preserving key information on the redox state of the surface ocean–atmosphere system during the mid-Proterozoic. In this study, I/(Ca + Mg), rare earth elements (REE) plus Y (REY), and paired stable isotopes (δ34SCAS and δ13Ccarb) data are reported for the carbonate rocks of ∼1.44 Ga Tieling Formation in the Jixian section, North China Platform. The lower Tieling Formation (i.e., the Daizhuangzi Member) exhibits middle-REE-enriched patterns with several relatively high I/(Ca + Mg) values (0.56–0.83 μmol/mol) suggesting a possible oxic or suboxic seawater condition. The upper Tieling Formation (i.e., the Laohuding Member) is characterized by modern seawater-like REY patterns with negative Ce anomalies (0.67–0.91) and variable I/(Ca + Mg) values (0.02–1.77 μmol/mol). The peak I/(Ca + Mg) ratios (up to 1.77 μmol/mol), negative shifts in Ce/Ce* and δ13Ccarb, and previously reported positive Cr isotope fractionation in the middle Laohuding Member indicate a significant oxygen increase in both ocean and atmosphere. A long-term increasing trend in δ34SCAS below the middle Laohuding Member outlines roles of pyrite burial in contributions of rising oxygen levels during this period.