Stagnation in the deep South Pacific

Why did the CO₂ content of the atmosphere suddenly rise after the end of the last ice age? Processes at the bottom of the Southern Ocean could be responsible, report researchers led by geochemist Dr Katharina Pahnke in the journal Science.

An eternal cycle that is extremely important for the climate takes place off the coasts of the Antarctic. Microscopic algae play the main role in this cycle: As long as they are alive, the single-celled organisms absorb the greenhouse gas carbon dioxide (CO₂) from the atmosphere and bind it in the form of organic compounds. After their death, the microbes sink into the deep sea - and take the CO₂ with them. In this way, the greenhouse gas can disappear into the sea for many millennia. Experts call the process a "biological pump".

"However, in order to remove CO₂ from the atmosphere in the long term, it must be ensured that it is stored stably at depth," reports geochemist Dr Katharina Pahnke, head of the Max Planck Research Group Marine Isotope Geochemistry, which is based at the ICBM and the Max Planck Institute for Marine Microbiology in Bremen. A team led by the Oldenburg researcher has now found an important indication that the deep South Pacific was strongly stratified during the last ice age and could therefore have contributed to the long-term storage of carbon dioxide in the deep sea.

CO₂ storage in the deep sea

The study by researchers from the Institute of Chemistry and Biology of the Marine Environment (ICBM) at the University of Oldenburg and the Max Planck Institute for Marine Microbiology in Bremen and the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research in Bremerhaven (AWI) and other partners, which has now been published in the journal Science, also indicates that the water masses mixed more intensively in the course of warming in the southern hemisphere after the end of the ice age. This allowed the stored CO₂ to escape from the depths and intensify global warming. The research results thus support a theory according to which processes in the deep Southern Ocean have contributed significantly to the natural CO₂ fluctuations.

In order to find out how the water masses there have developed over the last 30,000 years, the team collected sediment cores at water depths of between 3,000 and more than 4,000 metres during a voyage of the research vessel Polarstern in the South Pacific. Geochemists Dr Chandranath Basak and Dr Henning Fröllje from the ICBM - the two lead authors of the study - extracted tiny teeth and other skeletal fragments of fossil fish from the sediment in order to analyse these remains for isotopes of the rare earth metal neodymium.

Conspicuous neodymium signatures in fish fragments

"Neodymium is particularly suitable for identifying water masses of different origins," says Pahnke. This is because each layer has a characteristic neodymium signature. The ratio of different heavy variants of the element depends on which ocean basin the water comes from. The coldest and therefore deepest water mass in the South Pacific, for example, is formed at the continental margin of the Antarctic and has a specific neodymium signature. Above this is a layer in which water from the North Atlantic, South Pacific and North Pacific mixes and therefore has a different signature.

Using the fish remains from the deep-sea sediments, which archive the neodymium signal of the deep waters, the researchers were able to trace how the neodymium values have developed in the past at different water depths. The result: during the peak of the last ice age around 20,000 years ago, the neodymium signature from samples below a water depth of 4,000 metres was significantly lower than in shallower water depths. "Such a pronounced difference can only be explained by the fact that the water masses did not mix at that time," says Fröllje, who now works at the University of Bremen. He and his colleagues conclude that the water was stably stratified during the cold period.

Stronger mixing 18,000 years ago

When the climate in the southern hemisphere warmed up at the end of the last ice age around 18,000 years ago, the stratification broke up and the neodymium values in the different water depths equalised. "There was probably more mixing because the density of the water decreased due to the warming," explains Pahnke. This allowed the carbon stored in the depths to be released.

Climate researchers have been puzzling for some time as to why the CO2 content of the atmosphere fluctuated in parallel with temperatures in the southern hemisphere in the past, while temperatures in the north sometimes developed in the opposite direction. One theory is that processes in the Southern Ocean played an important role in this. "For the first time, our investigations provide solid evidence for the theory that there was a connection between the CO2 fluctuations and the stratification in the Southern Ocean," says Dr Frank Lamy from the AWI, one of the co-authors. The current study supports the assumption that the warming of the Southern Hemisphere broke up the stable stratification in the Southern Ocean and thus led to the outgassing of the stored carbon.