Severe selenium depletion in the oceans in the Phanerozoic as a plausible factor in three major mass extinction events

Many trace elements (TEs) are essential for life. Laser Ablation-Inductively Coupled Plasma Mass Spectrometry (LAICPMS) techniques can measure such elements with accuracy down to single parts per billion in pyrite from marine black shale samples. This new dataset of TEs throughout the past 3 billion years reveals trends in TEs in the oceans. Se is an essential TE for all animal life as it is utilised in the formation of anti-oxidising enzymes and selenoproteins, with aquatic organisms generally having larger and more Se-dependent selenoproteomes. Known environmental and tolerance levels of the TE selenium (Se) for extant organisms can be utilised to infer how periods of severe Se depletion offer a potential causal factor in certain mass extinction events. Our data shows that extreme Se and other vital TE deficiency in the oceans fell below estimated thresholds for life in modern oceans, and these periods of TE deficiency preceded three major mass extinction events at the end Ordovician, end Devonian and end Triassic. Oceanic anoxia has been invoked for these events as a cause for marine extinctions, but TE depletion begins before peak oxygen lows based on current O₂curves, and persist longer. The end Devonian extinctions comprise 4 subsequent events (Taghanic, Frasne, Kellwasser, Hagenberg events) over 15 million years (385–359 Ma) in which rapid decline of Se, Co, Mb, Zn and Cu in the oceans formed a ‘perfect storm’ of severe TE depletions. Such depletions of essential TEs would have stressed much of the marine biota and represent an important but hitherto underexplored correlate of major mass extinctions in the marine realm. Fluctuations in other trace elements do not correlate as closely with these extinction events, nor is there any abnormal TE trend associated with end Permian or end Cretaceous extinctions. We suggest these major extinction-related cycles were broken either by