How warming climate 'churned' the Southern Ocean

The last time the climate went through a major warming cycle, the Southern Ocean kicked up a substantial amount of carbon as the ice sheets retreated. Photo / Richard Robinson

The last time the planet's climate went through a major warming cycle, something dramatic happened in the Southern Ocean.

In a new study, scientists have revealed how at the end of the last ice age, some 18,000 years ago, the vast ocean fired out an enormous amount of carbon as Antarctic ice sheets retreated.

The international team of researchers behind the study say this happened because, at that point, the ocean was layered with carbon-rich waters at its bottom, and these were effectively churned to the surface as the climate warmed, releasing carbon into the atmosphere.

The study helps to answer long-standing questions around why CO2 fluctuations in the Southern Hemisphere were in step with temperatures in the Southern Ocean – a pattern that didn't play out on the other side of the planet.

Published in the major journal Science, the study also showed how global warming was enhanced when the mixing of those deep water masses increased, releasing the stored CO2.

The Southern Ocean plays a crucial role in climate events because CO2 can be absorbed from the atmosphere into the ocean.

When increased amounts of dust are deposited in the seawater, microscopic algae multiply because the iron contained in the dust acts as a fertiliser.

When these single-celled algae die, they sink to the ocean floor, taking the sequestered carbon dioxide with them.

To ensure long-term removal of the CO2 from the atmosphere, however, it must be stored in stable conditions in deep water over long periods of time.

To find out how water masses in the deep South Pacific have developed over the last 30,000 years, the team recovered sediment cores from water depths of between 3000 and more than 4000 metres during an expedition of the research vessel Polarstern to the South Pacific

Geochemists Dr Chandranath Basak and Dr Henning Frollje, of the Institute for Chemistry and Biology of the Marine Environment at the University of Oldenburg in Germany, extracted tiny teeth and other skeletal debris of fossil fish from the sediment to analyse their content of isotopes of the rare earth metal neodymium.

Neodymium was particularly useful for identifying water masses of different origin, as each layer of water had its own characteristic neodymium signature.

The isotope ratios of this element varied depending on which ocean basin the water comes from.

For instance, the coldest and therefore deepest water mass in the Southern Pacific formed on the continental shelf of Antarctica, carrying a distinct neodymium signature.

Overlying this mass was a layer that combines water from the North Atlantic, the South Pacific and the North Pacific and hence was marked by a different signature.

Using fish debris in deep-sea sediments, the researchers were able to trace the variations in neodymium concentrations at different depths over the course of time.

The result: at the peak of the last ice age approximately 20,000 years ago, the neodymium signature of samples taken from depths below 4000 metres was significantly lower than at lower depths.

"The only explanation for such a pronounced difference is that there was no mixing of the water masses at that time," Frollje said.

He and his colleagues concluded from this that the deep waters were strongly stratified during the glacial period.

As the climate in the southern hemisphere grew warmer towards the end of the last ice age around 18,000 years ago, the stratification of the water masses was broken up and neodymium values at different depths converged.

There was likely more mixing, because the density of the water decreased as a result of the warming, and this then led to the release of the carbon dioxide stored in deep waters.

For some time now climate researchers have been speculating on why fluctuations in atmospheric CO2 levels followed the same pattern as temperature in the southern hemisphere, whereas the temperature in the north at times ran counter to these fluctuations.

One theory had been that certain processes in the Southern Ocean played an important role.

"With our analyses we have for the first time provided concrete evidence supporting the theory that there is a connection between the CO2 fluctuations and stratification in the Southern Ocean," said study co-author Dr Frank Lamy, of Germany's Helmholtz Centre for Polar and Marine Research.

The new findings come as a new, million-dollar Otago University-led study seeks to understand the implications of disturbing the long-term cycle of carbon through our atmosphere, through what scientists call negative feedback mechanisms.

The Earth was now in the midst of a climate crisis - with a carbon cycle disturbance comparable to those that drove biological turnover, and even mass-extinction events, previously in geological history.

It wasn't known exactly when, or how, the natural climate system might return to "normal", and it was even possible that our activity could have delayed the next natural glaciation cycle.

The new study, supported by the Marsden Fund, would investigate what these man-made disruptions to the planet's climate might mean for its future.