A transient swing to higher oxygen levels in the atmosphere and oceans at ~1.4 Ga

Abstract

The mid-Proterozoic (1.8–0.8 Ga) may have witnessed persistent and predominant anoxia at the Earth’s surface. However, recent studies suggest that there was a period around ~1.4 Ga where oxygen levels might have been transiently elevated, both in the atmosphere and oceans. In order to shed light on this debated topic, we analyzed rare earth elements plus Y (REY) and Cr isotope profiles of the carbonate rocks from the ~1.44-Gyr-old Tieling Formation consisting of the lower Daizhuangzi (DZZ) and upper Laohuding (LHD) Members at the Jixian section, North China. The DZZ samples are characterized by middle REE enriched, hump-shaped REY distribution patterns, while those of the LHD Member display seawater-like REY patterns with slightly negative Ce anomalies (0.67 < Ce/Ce* < 0.87). Although differing greatly, the REY profiles both in the DZZ and LHD Members indicate the presence of Mn oxyhydroxides and thus an oxygenation at least locally in the shallow seawater of the North China Platform at ~1.44 Ga. The middle LHD Member records positive authigenic δ53Cr up to 0.48‰, indicating an enhanced continental oxidative weathering transiently occurred at ~1.44 Ga with pO2 higher than 1% of the present atmospheric level, and extending the mid-Proterozoic sedimentary record of positive Cr isotope fractionation to as early as ~1.44 Ga. Combining the investigations in this study with other published geochemical evidence from the North China Platform and elsewhere, we suggest that there was a transient and global swing to higher oxygen levels in the atmosphere and oceans at ~1.4 Ga.