Variations in the abundance of photosynthetic oxygen through Precambrian and Paleozoic time in relation to biotic evolution and mass extinctions: evidence from Mn/Fe ratios

• Variations in O2 abundance through Precambrian and Paleozoic time were investigated.
• Mn/Fe ratios of oxides in non-detrital sediments served as proxies for Eh and [O2].
• Dramatic decreases in O2 indicated Archaean and Ediacaran-Cambrian mass extinctions.
• O2 increased stepwise, showing effects of eukaryote evolution, ca. 1.88 to 0.68 Ga.
• After the Ediacaran-Cambrian crisis, O2 increased logarithmically until the Permian.

This paper reports new information about variations in the abundance of photosynthetic oxygen through Precambrian and Paleozoic time. Non-detrital marine sediments (cherts, limestones, and dolomite) were analysed for NH2OH·HCl/acetic acid-extractable Mn and Fe, and the Mn/Fe ratio (a proxy for the oxidation–reduction potential of the sediment at the time of deposition) was plotted against geologic age. The method has never before been applied to ancient sediments, but previously published data produced independently by other methods confirmed its applicability and underlying assumptions. The Mn/Fe ratio was unexpectedly high ca. 3.416 Ga, implying localised oxidation due to oxygen production by cyanobacteria, but fell dramatically over the interval 3.416–3.298 Ga, suggesting mass mortality or mass extinction of early Archaean cyanobacteria owing to asteroid impacts. However, the ratio increased continuously, though at episodically varying rates, from a minimum at ∼1.8783 Ga to a maximum at ∼0.680 Ga, signifying accumulation of oxygen in the atmosphere and hydrosphere. The rate of increase was relatively high at first but dropped abruptly at some point during the interval 1.8783–1.6 Ga, possibly signalling the appearance of eukaryotic herbivores. The ratio increased exponentially from 1.6 to 0.8 Ga and then rose more rapidly from 0.8 to 0.680 Ga, indicating a late Proterozoic upsurge of oxygen production, whereupon it fell catastrophically to a minimum in the Cambrian, reflecting widespread anoxia due to mass extinction of Ediacaran organisms. The crisis at the Precambrian–Cambrian boundary was followed by a logarithmic increase from the Cambrian to the Permian, indicating a resurgence of photosynthetic activity.