Planet Earth Online

Isotopes track ancient mass extinction

26 March 2010, by Sara Coelho

Although not as famous as the disaster that spelled the end of the dinosaurs, the end-Ordovician event was one of the world's biggest mass extinctions.

'At the end of the Ordovician period, England was part of the tropical continent of Avalonia-Baltica, whereas Scotland was linked to North America on the other side of the Iapetus Ocean,' says Alexander Finlay, a PhD geochemistry student based at the University of Durham.

The extinction was caused by a sudden decrease in temperature and the onset of the so-called Hirnantian glaciation, 445 million years ago. As the icecaps grew inland, the sea level fell exposing the coastal habitats which were home to many species of trilobites, brachiopods and other marine creatures. As a result of the fall in temperature and sea level, about 85 per cent of them died out.

Finlay, his supervisor Dr David Selby and Durham colleague Dr Darren Gröcke used osmium chemistry to find out more about this devastating event.

This is how it works: the old rocks that make up the bulk of the continents have more light osmium isotopes (¹⁸⁷Os) than the heavier variety (¹⁸⁸Os). As the continents are eroded by the weather, the osmium finds its way to the oceans in very small, but measurable amounts. This osmium isotope signature is then recorded in rocks bearing organic material forming in the ocean.

The ratio between the two osmium isotopes 'is a gauge for the weathering rate of rocks,' explains Finlay. The more 187-osmium in seawater, the greater the erosion of continental rocks is.

Finlay and colleagues were not able to measure osmium composition in 445 million year old sea water, so they sampled black and grey shale rocks from the end of the Ordovician period in Scotland.

'Shales are deposited in the deep sea and the preserved organic material within them records the chemical composition of ancient seawater,' says Finlay, who published the results in Earth and Planetary Science Letters.

They found that at the beginning of the Hirnantian glaciation, the amount of 187-osmium decreased dramatically. This suggests a decrease in the rate of erosion, possibly because continental rocks were protected from the elements by icecaps.

At the same time there is an increase of organic carbon isotopes in the shales. The carbon spike is related to sea level: as the seawater retreated, the reefs near the shore were exposed to erosion and more heavy carbon isotopes were washed into the oceans.

The shale's chemical signature - with the increase in organic carbon and drop in 187-osmium - paints a gloomy picture, with continents covered by ice and reefs left to dry by the receding coastal line.

'At the end of the glaciation we see a rapid increase in radiogenic [187-]osmium, which coincides with a drop in organic carbon isotopes,' says Finlay. The recovery happened very fast in less than 100,000 years - just a blink of an eye in geological time.

'Osmium has been used to track climate changes and recent glaciations,' he says, but 'this is the first time the method is used to study a very old glaciation event.'

'We also have demonstrated that osmium isotopes are useful to track very old weathering of rocks,' Finlay says.

Keywords: Environmental change, Europe, Geology,

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