29 July 2010, by Adele Rackley

Most scientists think that fluctuations in ocean circulation are linked to changes in climate. Now they've found evidence that links those fluctuations to the kind of temperature increases that can end an ice age.

Ocean circulation in the North Atlantic

Ocean circulation in the North Atlantic.

A team of researchers, led by Dr Stephen Barker from Cardiff University, has investigated how changes in a key component of global ocean circulation are related to significant changes in temperature, which have taken place in the relatively recent geological past.

The team focused on the Atlantic meridional overturning circulation, or AMOC. This carries tropical surface waters northwards, and cold deep water from the North Atlantic (NADW) southwards to fill the Atlantic basin. There it mixes with deep waters that originated in the Antarctic region.

When ocean circulation is strong, heat is moved efficiently from the tropics to the poles. When circulation is weak the poles become colder.

'The fact that [AMOC] can react in such a way is really exciting.'

Dr Steve Barker, Cardiff University

Scientists think that during particularly cold periods in the last ice age (so-called Heinrich Stadial events) the AMOC weakened significantly. A stronger AMOC is associated with warmer phases.

But the researchers believe the link between the AMOC and deglaciations over the last half a million years is too strong to be a coincidence. It looks like deglaciation may only happen when the AMOC shifts from weak to strong.

But why should it have such a strong influence?

Models predict that when the AMOC strengthens after an interval of weak circulation, it doesn't just return to its 'normal' extent but it gets stronger than before – it 'overshoots'.

'When the circulation kicks back in, it comes back with a vengeance,' says Barker.

And these changes can have extreme effects. During the Bølling-Allerød (B–A) warm phase, 14,600 years ago, temperatures rose by 9 degrees Celsius over the course of just a few decades.

The theory is that this was due to an overshoot in the AMOC, but until now no one has been able to show any real evidence that it happened.

Foraminifera shells

Enhanced preservation of foraminiferal shells in the deep South Atlantic. Shells are up to ~0.5mm in diameter.

To find the link, the scientists analysed the contents of a sediment core from the deep South Atlantic Ocean, and related changes in the core to the abrupt temperature changes observed in the surface ocean and in ice cores from Greenland. Their results are published in Nature Geoscience.

By looking at the radiocarbon content and preservation of carbonate shells in the sediments, they identified the origin of the waters over the sample site at different times.

This is because 'old' waters, such as those in the Southern Ocean, are only briefly in contact with the surface, so they are poorly ventilated and don't preserve carbonate well. In contrast, water like the NADW has had relatively recent contact with the atmosphere, so it's well ventilated and gives better preservation as a result.

Under present-day 'normal' AMOC conditions the sample site would be under poorly ventilated Southern Ocean waters.

But the core sediments from the B–A period are well preserved and radiocarbon ages are relatively young. This suggested that during this time the sample site was indeed covered by well-ventilated water – like NADW.

'The good preservation was really anomalous for 5000m depth in the South Atlantic,' says Barker. 'It's very strong evidence for better-ventilated water being there at that time.'

So it looks like the AMOC did overshoot during the B–A, bringing NADW much deeper than normal and pushing the older southern waters out of the way.

These results are particularly significant because they show the AMOC overshooting to well beyond its present-day state. And when overshoots occur, the effects on surface temperature are extreme.

'We think of the modern state of the AMOC as healthy,' says Barker, 'but the fact that it can react in such a way is really exciting.'

And such extreme changes aren't just geological phenomena. 'Humans were around in north-west Europe when some of these events happened,' Barker adds. 'I'd love to know what they made of such massive climate change.'