Deformed Plankton Reveal Cause of Mass Extinction?

Sometimes palaeopathology can seem a rather arcane subject. It can be argued that it is merely the study of aberrations; diseased or damaged individuals who might be able to tell you something interesting about a local behaviour, or perhaps even a wider ranging epidemic, but little else. This is far from the truth and here we are going to look at an example of palaeopathology which goes way beyond an isolated anomaly and can instead tell us something fundamental about conditions across the whole planet.

The research in question was published earlier this month (August 2015) by a team from Ghent University, Belgium, and it concerned 485 million year old plankton. Plankton is the name given to the tiny marine organisms which form the bottom of almost all aquatic food chains. Most are microscopically small but they form huge colonies forever floating through the waters of the world and because of their central place in the ecosystem they have been studied extensively. The same is true of extinct plankton and we have a pretty good fossil record for these essential organisms. Since their numbers and their species fluctuate according to water temperature, they are an excellent proxy for past oceanic conditions and one that palaeoclimatologists have learned to rely on. Therefore, the discovery of oddly deformed plankton from the early days of Earth’s history hints at something strange.

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Deformed plankton from the fossil sites. Image credit: Ghent University

The age of the plankton, between 485-420 million years is significant as this bridges a time between two great periods of Earth’s history, the Ordovician and the Silurian. This boundary has already attracted a great deal of palaeontological study because it saw a series of extinction pulses which at their worst wiped out nearly 85% of marine life. We already knew from the rock records that this coincided with periods of prolonged glaciation but whether this was enough to cause the mass extinction was unclear. Until now, the actual kill mechanism was unknown. That changed with the discovery of these tiny, malformed plankton. Tests revealed the presence of unusually high levels of several different heavy metals, particularly iron, lead, manganese and arsenic, within the plankton and their host rocks. Metal toxicity is known to cause the kinds of deformities seen in these plankton suggesting that these levels reflect the actual ancient conditions not some later, geological concentration. This, combined with detailed oxygen isotope data from the time, strongly suggests that the oceans were rapidly becoming anoxic. This means the water was depleted of oxygen, making it easier for large volumes of these toxic metals to be dissolved. What is particularly interesting about these deformed fossils is that they appear with every single pulse of anoxic conditions, not just one or two, suggesting this is no coincidence but instead reflects a very real trend. It is all too easy to imagine how the low oxygen conditions alone would have pushed many species to extinction but we now know that this was also linked to dangerous rises in metal pollutants, further stressing vulnerable populations.

This new study adds evidence to the emerging picture of the end-Ordovician mass extinction. Previous models has factored in the changes to the climate, global cooling and habitat loss as possible factors but now the geochemistry of the oceans reveals a harsh, toxic environment which would doubtless have led many species to perish. Although the overall causes of the initial anoxia are still debated the evidence that it did indeed happen is mounting and now evidence from the plankton has revealed a previously unsuspected dimension to the catastrophe. It should almost go without saying that this study also has potential repercussions for our future conservation of the oceans as global warming threatens to alter ocean chemistry of a large scale. Hopefully studies such as this can suggest new ways to manage our own pollutants and if nothing else teach us how to prepare for the worst.

 

Reference: Vandenbroucke, T.R.A., et al. 2015. Metal-induced malformations in early Palaeozoic plankton are harbingers of mass extinction. Nature Communications. 6(7966). DOI: 10.1038/ncomms8966

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