(CN) – New research reveals that changes in the Antarctic Ocean led to the rise in atmospheric carbon dioxide levels at the end of the last ice age.
In the report, published Thursday in the journal Science, researchers show that the deep South Pacific was highly stratified during the last ice age, which could have enabled long-term, deep-sea storage of the greenhouse gas.
The findings also suggest that warming at the end of the last ice age coincided with the increased mixing of deep water masses, which released stored CO2 and enhanced global warming.
The Antarctic Ocean plays a critical role in climate events because CO2 can be absorbed by the ocean from the atmosphere. As the amounts of dust deposited into the sea increase, microscopic algae multiply because the iron contained in the dust serves as a fertilizer.
When these single-celled algae die, they sink to the ocean floor, bringing the sequestered greenhouse gas with them. Long-term removal of CO2 from the atmosphere relies on stable deep-water conditions over extended periods of time.
To determine how water masses in the deep South Pacific have evolved over the past 30,000 years, the researchers recovered sediment cores from water depths of nearly 10,000 feet to more than 13,000 feet. Co-lead authors Chandranath Basak and Henning Frollje extracted minute teeth and other skeletal debris of fossil fish from the sediment to analyze the samples’ ratio of isotopes of the rare earth metal neodymium.
“Neodymium is particularly useful for identifying water masses of different origin,” said co-author Katharina Pahnke, the head of the Max Planck Research Group for Marine Isotope Geochemistry in Germany.
The isotope ratios of neodymium vary according to the ocean basin from which the water originates. For example, the coldest and therefore deepest water mass in the South Pacific develops on the Antarctic continental shelf and bears a distinct neodymium signature. Directly above this mass is a layer that combines water from the North Pacific, the South Pacific and the North Atlantic and thus features a different signature.
The researchers used fish debris from deep-sea sediments to trace the variations in neodymium concentrations at different depths over time. They found that at the peak of the last ice age roughly 20,000 years ago, the neodymium signature of samples taken from depths below 13,000 feet was significantly lower than in deeper water.
“The only explanation for such a pronounced difference is that there was no mixing of the water masses at that time,” said Frollje, a biochemist at the University of Bremen in Germany. This led the team to conclude that the deep waters were strongly stratified during the glacial period.
As the climate in the Southern Hemisphere warmed around 18,000 years ago, the stratification of the water masses was broken up neodymium values at different depths intersected.
“There was probably more mixing because the density of the water decreased as a result of the warming,” Pahnke said. This then sparked the release of the CO2 stored in deep waters.
Climate researchers have long since speculated on why variations in atmospheric CO2 levels followed the same pattern as temperatures in the Southern Hemisphere, when conditions in the north at times ran counter to these fluctuations. One theory suggests that certain processes in the Southern Ocean – another name for the Antarctic Ocean – played a key 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 Frank Lamy, a geoscientist at the Alfred Wegener Institute in Germany.
The findings support the theory that the warming of the Southern Hemisphere disrupted stratification in the Antarctic Ocean, leading to the release of CO2 that was stored in these waters.