Four cycles of oxygenation in the phanerozoic

Sedimentary pyrite captures trace elements (TE) from theoceans and tracks variations in their seawater concentrationsthrough time [1]. LA-ICPMS analysis of sedimentary pyrites,based on newly developed standards, has enabled thedevelopment of temporal ocean concentration curves for 22TE [2]. Our results show that TE variations over the last 700million years of ocean history have been strongly cyclical. Weinterpret these cycles to indicate that the Late Neoproterozoicto Phanerozoic oceans went through dramatic changes in meanoxygen content. Four major cycles are recognised: LateCryogenian to Late Ordovician, Early Silurian to lateDevonian, Early Carboniferous to Late Permian and Triassicto Quaternary. Oxygen maxima, indicated by Se, U and Moproxies, occur at 540, 390, 310 and 0 Ma, supporting previousmodels [3, 4]. Oxygen minima, indicated by trace elementdrawdown, occur at 700, 455, 365 and 200 Ma. Extendedperiods of low oxygen in the oceans have led to extremedeficiency of some elements that are critical for life. Theperiods of extreme Se depletion coincide with the massextinction events at end Ordovician, Late Devonian and theTriassic-Jurassic boundary, suggesting that Se-deficiency inthe oceans may be a contributing cause of marine massextinctions.