Title

Extremely low oxygen concentration in mid-Proterozoic shallow seawaters

Document Type

Article

Abstract

The mid-Proterozoic (1.8-0.8 Ga) witnessed the first appearance but unusually low diversification of eukaryotes. The stagnant biotic evolution during this billion-year-long period (commonly referred to as the "Boring Billion") was arguably ascribed to low oxygen levels in atmosphere and ocean. However, evidence supporting low oxygen in shallow-marine environments where early eukaryotes first evolved is generally lacking or insufficient. Here we report Rare Earth Element and yttrium (REE + Y) data, particularly cerium (Ce) anomalies, from a suite of mid-Proterozoic sedimentary rocks of the North China platform. The new data from North China, in combination with available Ce anomaly data from other Proterozoic successions, demonstrate that during mid-Proterozoic, negative Ce anomalies did not occur until ~1.54 Ga and after ~1.54 Ga, only episodic negative Ce anomalies were present in shallow-water carbonates. Trace element enrichments (UEF, VEF, and MoEF) remained at the average continental crust level before ~1.54 Ga but showed a sudden increase at ~1.54 Ga. The data suggest that oxygen concentration in shallow-marine environments of the mid-Proterozoic ocean was extremely low, probably <0.2 μM prior to ~1.54 Ga (based on minimal oxygen concentration requirement for Ce(III) oxidation) and fluctuating around 0.2 μM afterwards. The low oxygen concentration (~0.2 μM) in shallow waters of the mid-Proterozoic ocean accounts for only ~0.1% of the modern surface ocean oxygen level (~280 μM) and may help explain the evolutionary stasis of eukaryotes during the mid-Proterozoic. © 2016 Elsevier B.V.