Plate Tectonics May Have Driven “Cambrian Explosion”

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The Cambrian follows the Vendian period, during which time the continents had been joined in a single supercontinent called Rodinia. As the Cambrian began, Rodinia began to fragment into smaller continents, which did not always correspond to the ones we see today.

 

While a number of theories have been put forward to explain this landmark period, the most credible is that it was fueled by a significant rise in oxygen levels which allowed a wide variety of animals to thrive.

The new study suggests that such a rise in oxygen levels was the result of extraordinary changes in global plate tectonics.
 

 

The reconstruction below shows the rifting of Rodinia durring the Tommotian. Green represents land above water at this time, red indicates mountains, light blue indicates shallow seas of the continental shelves, and dark blue denotes the deep ocean basins. (For clarity, the outlines of present-day continents have been superimposed on the map.)

Credit: University of California Berkeley

Tthe Cambrian world was concentrated in the southern hemisphere. The largest landmass (lower right) was Gondwana (a collection of today’s southern continents). The second largest continent, Laurentia is just left of center in the map, and includes most of North America (the southeastern US can be seen wedged between Africa and South America as part of Gondwana). Between Gondwana and Laurentia lie Siberia (just south of the equator) and Baltica (Scandinavia, eastern Europe, and European Russia). The rest of Europe and much of what is today Asia lay in fragments along the north coast of Gondwana.

During the formation of the supercontinent ‘Gondwana’, there was a major increase in continental arc volcanism – chains of volcanoes often thousands of miles long formed where continental and oceanic tectonic plates collided. This in turn led to increased ‘degassing’ of CO2 from ancient, subducted sedimentary rocks.

This, the team calculated, led to an increase in atmospheric CO2 and warming of the planet, which in turn amplified the weathering of continental rocks, which supplied the nutrient phosphorus to the ocean to drive photosynthesis and oxygen production.

The study was led by Josh Williams, who began the research as an MSc student at the University of Exeter and is now studying for a PhD at the University of Edinburgh. During his MSc project he used a sophisticated biogeochemical model to make the first quantification of changes in atmospheric oxygen levels just prior to this explosion of life.

Co-author and project supervisor Professor Tim Lenton, from the University of Exeter’s Global Systems Institute said: “One of the great dilemmas originally recognised by Darwin is why complex life, in the form of fossil animals, appeared so abruptly in what is now known as the Cambrian explosion.

“Many studies have suggested this was linked to a rise in oxygen levels – but without a clear cause for such a rise, or any attempt to quantify it.”

Not only did the model predict a marked rise in oxygen levels due to changes in plate tectonic activity, but that rise in oxygen – to about a quarter of the level in today’s atmosphere – crossed the critical levels estimated to be needed by the animals seen in the Cambrian explosion.

Williams added: “What is particularly compelling about this research is that not only does the model predict a rise in oxygen to levels estimated to be necessary to support the large, mobile, predatory animal life of the Cambrian, but the model predictions also show strong agreement with existing geochemical evidence.”

“It is remarkable to think that our oldest animal ancestors – and therefore all of us – may owe our existence, in part, to an unusual episode of plate tectonics over half a billion years ago” said Professor Lenton.

A tectonically driven Ediacaran oxygenation event by Joshua Williams, Benjamin Mills and Tim Lenton is published in Nature Communications on Wednesday, June 19th 2019

Contacts and sources:
University of Exeter

Citation: A tectonically driven Ediacaran oxygenation event.
Joshua J. Williams, Benjamin J. W. Mills, Timothy M. Lenton. Nature Communications, 2019; 10 (1) DOI: 10.1038/s41467-019-10286-x