Ocean’s dramatic past

Zeroing in on the evolution of the North Atlantic

By Shalini Srivastava
Published: Tuesday 15 March 2011

oceanTOWARDS the end of the last Ice Age, 10,000-20,000 years ago, the global ocean circulation underwent a lot of changes. This circulation, called meridional overturning circulation (MOC), carries warm, saline surface water north to cooler regions. Now researchers from Cardiff University in the UK have revealed how the MOC in the North Atlantic Ocean was behaving at that time.

This will help understand how the Atlantic may influence climate in the future. In the study, researchers compared carbon isotopes, C-14 and C-12, from marine organisms, known as foraminifera, taken from waters of Iceland at depths 1,200 to 2,300 metres. The samples represent marine conditions that existed between 22,000 and 10,000 years ago. C-14 is produced in the upper atmosphere when cosmic rays interact with nitrogen. It is taken up by the ocean water through air-sea gas exchange with the levels seeking equilibrium.

Over time when the water sinks down to deeper layers, C-14 decays into C-12. The extent of this depletion gives an estimate of the ventilation age, or how long it has been since the water was near the surface and in equilibrium with the atmosphere. The study, published in the January issue of Science, claimed that ventilation ages varied rapidly between young and old. Young ventilation ages, which were around 500 years or shorter, implied good circulation of heat around the globe. Whereas older ventilation ages, which lasted between 3,000 and 5,200 years, led to poor heat circulation.

In general, young ventilation ages are seen during warm intervals. Older ventilation ages are symbolic of stoppage or reduction in deep water convection. It happens when the ocean circulation gets disturbed due to local causes, like melting of glaciers. The study’s finding suggests that the Earth’s climate was warm and cool for centuries at a time. The researchers believe the long ventilation ages were due to the incursion of waters from the Antarctic region, said V Ramaswamy, scientist in geology division of the National Institute of Oceanography, Goa.

The data also suggests that during the Ice Age the ocean waters were cut off from the atmosphere for much longer than they are today, implying that the North Atlantic absorbed and stored a massive amount of atmospheric CO2. The CO2 concentrations in about half of the ocean’s volume were 14 to 28 micromoles per kilogramme higher than today’s concentrations, the study revealed. “The study can pave way for computer models that predict future climate,” said lead researcher David Thornalley.

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