Evidence for elevated and variable atmospheric oxygen in the Precambrian

Abstract

Oxygen concentration in the Precambrian atmosphere-ocean system is currently estimated from a series of geochemical proxies that depend on the concentration of redox-sensitive trace elements or their isotopic ratios measured in organic-bearing marine shales. This research has indicated that atmospheric oxygen content throughout the Precambrian was very low, <0.01% of the present atmospheric level (PAL; <0.002% O2). However, a rise in oxygen, known as the Great Oxygenation Event (GOE), is thought to have occurred around 2400 to 2300 Ma. Here we combine the results from two independent methods to estimate atmospheric oxygen concentrations during the Precambrian. Firstly, measurement of oxygen concentrations in sedimentary halite and ooids at four times – around 800, 1440, 2000 and 2660 Ma. Secondly, measurement of the Se/Co ratio in 2,037 sedimentary pyrite grains from 310 black shale samples spread throughout the Precambrian. By combining these two approaches we have derived the following relationship: AtmosphericO2%=30×P/1+PWhereP=100.89×logSe/Co+0.07,r2;r2=0.85The oxygen curve revealed by this relationship suggests a broad first-order trend of increasing oxygen of ~0.25 to 27% from 3500 to 1850 Ma, followed by a general decrease to around ~1.0% at ~1000 Ma, with a spike around 1400 Ma. Oxygen then rose gradually through the Ediacaran to reach a maximum of 20 to 29% in the early Cambrian. Detailed analysis of sedimentary pyrite in sixteen separate black shale formations, spaced throughout the Precambrian, demonstrates consistent results using the marine pyrite proxy, with standard deviations of the predicted oxygen for each formation of between 0.4 and 7%. In seven formations, we show that each formation demonstrates an increase in oxygen passing up stratigraphy from the base of the black shale facies toward the top.