Poster's note : obliquely relevant to OIF and open ocean storage of
supercritical CO2

Scientists verify theory of the role of the South Pacific in natural
atmospheric CO2 fluctuationsFebruary 22, 2018
Alfred Wegener Institute
[image: Scientists verify theory of the role of the South Pacific in
natural atmospheric CO2 fluctuations]
View from RV Polarstern while collecting sediment samples used in the study
by Basak et al. Credit: Dr. Katharina Pahnke

A team led by geochemist Dr. Katharina Pahnke from Oldenburg has discovered
important evidence that the rise in atmospheric carbon dioxide levels at
the end of the last ice age was triggered by changes in the Antarctic
Ocean. The researchers from the University of Oldenburg's Institute for
Chemistry and Biology of the Marine Environment (ICBM), the Max Planck
Institute for Marine Microbiology in Bremen and the Alfred Wegener
Institute, Helmholtz Centre for Polar and Marine Research (AWI) were able
to demonstrate that the deep South Pacific was strongly stratified during
the last ice age, and could thus have facilitated long-term, deep-sea
storage of the greenhouse gas carbon dioxide (CO2). The study, which has
now been published in the academic journal *Science*, also indicates that
in the course of the warming following the end of the last ice age the
mixing of the deep water masses increased, releasing stored CO2 and
enhancing global warming.

The Southern Ocean plays an important 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 fertilizer. 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.

In order 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 3,000 and more than 4,000 metres during an
expedition of the research vessel "Polarstern" to the South Pacific. The
geochemists Dr. Chandranath Basak and Dr. Henning Fröllje of the ICBM, the
two main authors of the study, 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 is particularly useful for identifying water masses of different
origin," said Pahnke, the head of the Max Planck Research Group for Marine
Isotope Geochemistry based at the ICBM and the Max Planck Institute for
Marine Microbiology in Bremen, explaining that each layer of water has its
own characteristic neodymium signature. The isotope ratios of this element
vary depending on which ocean basin the water comes from. For instance, the
coldest and therefore deepest water mass in the Southern Pacific forms on
the continental shelf of Antarctica and carries a distinct neodymium
signature. Overlying this mass is a layer that combines water from the
North Atlantic, the South Pacific and the North Pacific and hence is 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 4,000 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," said Fröllje, who currently works at the
University of Bremen. 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 probably more mixing
because the density of the water decreased as a result of the warming,"
Pahnke explained. 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 is 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 co-author of
the study Dr. Frank Lamy of the AWI in Bremerhaven. The current study
supports the hypothesis that the warming of the southern hemisphere broke
up stable stratification in the Antarctic Ocean, resulting in the release
of the carbon dioxide <>that was
stored in these waters.

*More information:* "Break-up of last glacial deep stratification in the
South Pacific", Chandranath Basak, Henning Fröllje, Frank Lamy, Rainer
Gersonde, Verena Benz, Robert F. Anderson, Mario Molina-Kescher, Katharina
Pahnke, *Science*, 23 February 2018, DOI: 10.1126/science.aao2473

*Provided by:* Alfred Wegener Institute
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