Saturday, 14 June 2014

A Volcanic Origin For The Mid Cambrian Extinction

Evidence of the explosion of life can be found at the
Burgess Shale in Canada's Rocky Mountains
The Cambrian period is often thought of as a time of unbounded diversification. The explosion of life, recorded in the rocks of the Burgess Shale, the Doushantuo Formation and Kootenay National Park, just to name a few, show that the number of species was on the rise.

All the major animal phyla originated during the Cambrian, surviving in some form or another even during the most intense upheavals in planetary history. Yet it was not a complete time of advancement. Life suffered setbacks as well.

The middle of the Cambrian was punctuated by an extinction event. Climate change is a major cause of extinction as it affects habitats. Yet the trigger for this climate change, however, is a debated subject. 'It has been well-documented that this extinction, which eradicated 50 per cent of species, was related to climatic changes and depletion of oxygen in the oceans, but the exact mechanism causing these changes was not known, until now,' said Dr Fred Jourdan from the Curtin's Department of Applied Geology.

Using radiometric dating Dr Jourdan, and a team composed of members from several Australian research institutes, discovered that a two million square kilometre stretch of basalt in the Northern Territory and Western Australia, known as the Kalkarindji volcanic province, formed during the mid Cambrian some 510 to 511 million years ago. This date coincides exactly with the mid Cambrian extinction. The researchers hypothesised that the eruption products were responsible for the climate change driving the loss of biodiversity.

Australia's Glass House national park. The rocks here formed 510 to 511
million years ago in a massive volcanic event now thought to be
responsible for the mid Cambrian extinction event.
'Not only were we able to demonstrate that the Kalkarindji volcanic province was emplaced at the exact same time as the Cambrian extinction, but were also able to measure a depletion of sulphur dioxide from the province's volcanic rocks -- which indicates sulphur was released into the atmosphere during the eruptions,' continued Dr Jourdan.

'As a modern comparison, when the small volcano Pinatubo erupted in 1991, the resulting discharge of sulphur dioxide decreased the average global temperatures by a few tenths of a degree for a few years following the eruption. If relatively small eruptions like Pinatubo can affect the climate just imagine what a volcanic province with an area equivalent to the size of the state of Western Australia can do.'

The mixture of carbon dioxide, sulphur dioxide and ash would have produced rapid temperature fluctuations, resulting in changes in sea level and weather patterns, wreaking havoc in ecosystems across the globe. 'We calculated a near perfect chronological correlation between large volcanic province eruptions, climate shifts and mass extinctions over the history of life during the last 550 million years, with only one chance over 20 billion that this correlation is just a coincidence,' concluded Dr Jourdan.

What is interesting is that a similar volcanic event operating on a similar scale is thought to be responsible for the Permian mass extinction 250 million years ago when a staggering 83% of all species on Earth became extinct - significantly higher than the mid Cambrian event. The discrepancy is difficult to explain, highlighting the fact that we still do not fully understand how life responds to changes in climate. In order to understand the consequences of future volcanic events it is vital that we are able to accurately interpret the underlying causes of past extinctions in the Earth's history.