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Ancient glimpse of seas' bleak future
18 June 2005
From New Scientist Print Edition.
Jeff Hecht

Fifty-five million years ago the Earth warmed rapidly, the oceans turned acidic and deep-sea creatures died en masse. A massive release of carbon gases has been the prime suspect, but no one was sure how massive. And it wasn't clear just how long the oceans remained acidic.

Now, sediment cores drilled from the ocean floor have revealed that the emission of nearly 4500 gigatons of carbon into the atmosphere led to the catastrophic global warming, and that the resulting acidification of the oceans lasted more than 100,000 years. It serves as a warning and backs up computer models that have predicted similar long-lasting effects if, as seems likely, humans release comparable amounts of carbon dioxide over the next few hundred years.

The anomalous warm spell, at the end of the Palaeocene epoch and the start of the Eocene, was first recognised more than a decade ago. Sea-surface temperatures rose by 5 °C at the equator and by up to 9 °C near the poles. Although no major extinctions occurred on land, deep-sea creatures suffered their worst extinction in the fossil record.

Earlier analysis of carbon isotopes collected from deep-sea cores had suggested the warming followed the release of more than 2000 gigatons of carbon into the atmosphere. Scientists thought that this carbon came from deposits of methane hydrates on the ocean floor. What triggered the release of methane remains unclear, but the gas would have been rapidly oxidised to CO₂ and eventually taken up by the oceans, turning them acidic.

To understand the effect on the oceans, James Zachos of the University of California at Santa Cruz and his colleagues collected samples of deep-sea sediments from different depths in the South Atlantic. Normally, shells from dead organisms settle out on the seabed, forming thick carbonate-rich deposits. At great enough depth and pressure, however, the water dissolves away the calcium carbonate and leaves only clay. In today's oceans this happens below depths of about 4 kilometres, but in more acidic oceans, the shells would have dissolved in shallower water.

By analysing the clays from different sites and depths, Zachos and his colleagues found that the shells of marine organisms were dissolving at depths of just 2 kilometres. The acidity of the oceans rose to this level in just 10,000 years. Crucially, it took more than 100,000 years for the waters to return to normal, since the neutralisation process depended on the weathering of silicate rocks on land. The exposed silicates reacted with the atmospheric CO₂ and moisture, creating carbonates that eventually entered the oceans and reduced their acidity.

However, in the time it took for oceans to return to normal, deep-sea life with carbonate shells had suffered "by far the largest mass extinction in that group of organisms in Earth's history", says Zachos.

The findings also indicate that the warming event was caused by the release of twice the amount of greenhouse gases as had been suspected, adding close to 4500 gigatons of carbon to the atmosphere - although the source remains something of a mystery. This is not far off what humans could release by burning the known reserves of fossil fuels over the next few hundred years. And natural removal of the extra carbon would take 100,000 years. "It is sobering to think [about] how long it takes to come back [to normal], once we perturb the system," says Zachos.

Computer models have predicted similar scenarios for a human-induced greenhouse Earth, says Gerald Dickens of Rice University in Houston. But no one had tested these models, and now Zachos's results provide some confirmation. "What is extraordinary is to find a natural experiment where we can see the way the world worked," says Dickens. "You're looking at the future of the deep sea."

From issue 2504 of New Scientist magazine, 18 June 2005, page 19

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