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A New Cause for Three Global Mass Extinction Events

By John Long

Dangerously lows levels of trace elements might be implicated in three mass extinctions.

Throughout the past 600 million years there have been five major mass extinction events and a host of smaller biotic crises, each of which saw the demise of large swaths of life on Earth and in the oceans. While some of these events are very well studied, such as the killer asteroid that wiped out the dinosaurs 66 million years ago, others are more enigmatic and entertain a variety of possible causes.

The first three extinction events took place near the end of the Ordovician Period (c. 445 Ma), end of the Devonian Period (385–359 Ma) and at the end of the Permian Period (252 Ma). The most devastating of these events was the end of Permian event, which wiped out 96% of all marine species and about 70% of life on Earth. The likely causes are from massive volcanic eruptions on a scale never before seen, with other effects that include a runaway greenhouse effect triggered by methane release from the seafloor. Because the devastation was so severe, recovery took around 10 million years.

The big extinction event at the end of the Ordovician is linked to glaciation and widespread anoxia, or loss of oxygen in the seas. About 57% of all genera were wiped out in the oceans, making it the second largest of all extinction events. The reliability of interpreting the past oxygen levels of the Earth is the key to understanding this event – the anoxia may have been brought on by another cause.

Prof Ross Large’s team at The University of Tasmania has been charting the abundance of trace elements in the past oceans, and now has more than 2000 samples spanning 3.5 billion years. His Laser Ablation-Inductively Coupled Mass Plasma Spectrometry Lab is one of the few in the world to be capable of measuring very small amounts of trace elements from sea-floor muds containing pyrite. Two new papers published this year in Gondwana Research show that dangerously lows levels of certain trace elements, like selenium, might be implicated as a new factor in three major mass extinction events.

Most life on Earth needs selenium in very specific amounts. Too much or too little is toxic. Today’s oceans contain about 155 ppt of selenium in their surface waters, changing slightly with depth. Tolerance levels of selenium for phytoplankton, molluscs, fish and many terrestrial plants and animals are very well-known due to aquaculture requirements.

If the oceans rapidly dropped their levels of selenium by two orders of magnitude, would food chains likely be affected? This is precisely what happened at the end of the Ordovician, Devonian and Triassic Periods.

The real question is whether selenium, which seems to be a good proxy for past oxygen levels, was the prime driver for the loss of oxygen in the oceans that caused the extinctions, or was itself the main cause of extinctions.

The cycle of nutrients in the oceans is driven by plate tectonics. The uplift of mountains at plate margins causes erosion of crustal surfaces, enabling trace elements to wash into rivers and end up in the seas. When erosion is prolonged and tectonics is slow, the supply of these nutrients slows down, and the depletion of certain elements begins. Only activation of the Earth’s engine, to drive more mountain-building, seems to set the cycle back to normal again.

Yet mass extinctions have their bright side. Without them, new life couldn’t emerge. Mammals rose to prominence after the decline of the dinosaurs, and thus we are here today, thanks mainly to a mass extinction event 66 million year ago.

John Long is Strategic Professor in Palaeontology at Flinders University, and current President of the Society of Vertebrate Paleontology.