Searching for the Dust That Cooled the Planet
A new study uncovers the origins of iron-rich dust that contributed to the last ice age’s coldest temperatures.
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Eleven years ago, in the Southern Ocean off the coast of Antarctica, Katharina Pahnke stood on the deck of a large research vessel. Rocking back and forth on the ocean waves, Pahnke watched as a piston corer, a long, heavy tube, was lowered off the side of the ship and plunged through the frigid water to pierce the soft seabed below. Six hours later, the tube—and the long cylinder of mud contained within—was hauled back up to the surface.
On this 2010 research expedition, Pahnke and others scoured the Southern Ocean between Antarctica, New Zealand, and Chile—collecting sediment cores over an area roughly the size of Russia. These tubes of mud have since been used for a wide range of scientific studies. For Pahnke, a geochemist who studies the ocean’s history, the dust trapped within these sediment cores would eventually allow her to answer questions about the coldest period of the last ice age, which happened around 20,000 years ago.
In 1990, 20 years before Pahnke trekked out to the Southern Ocean, oceanographer John Martin proposed that iron-rich dust greatly contributed to the last ice age. Known as the iron hypothesis, Martin argued that flurries of this dust traveled on the wind from cold, barren landscapes to the Southern Ocean. The dust fertilized the marine ecosystem, triggering massive blooms of phytoplankton. These photosynthesizing marine algae sucked carbon dioxide out of the air to produce sugar and oxygen. Once the phytoplankton died, they fell to the bottom of the ocean, taking the sequestered carbon with them, which dramatically cooled the planet.
Today, it is widely accepted that there was an increase in iron fertilization and dust in the Southern Ocean during the last ice age. Yet where this iron-rich dust came from has remained unknown—until now. Using the same sediment cores collected years before, Pahnke and other scientists from the University of Oldenburg in Germany discovered that the dust captured in the tubes traveled up to 20,000 kilometers east from northwestern Argentina—all the way around the world—to where it was deposited in the Southern Ocean.
By knowing the dust’s origin, scientists can fill in the gaps about what caused carbon dioxide levels to change so drastically in the past, which may offer clues to how we can manage the climate in the future.
Climate models, says Zanna Chase, a paleoceanographer at the University of Tasmania in Australia who was not involved with the study, are based around proper estimations of how much dust there was, which is dependent on where it came from. “This study gives us a more realistic picture as to what was going on in the glacial ocean, and more accurate picture of the role of iron fertilization,” says Chase. “It’s a big piece of the puzzle.”
After identifying the sediment layer from around 20,000 years ago within each Southern Ocean core, the team isolated the dust and analyzed its key chemical signatures, including its concentrations of rare earth elements and the isotopic compositions of neodymium, strontium, and lead. They then compared this chemical fingerprint to that of sediments from several other places, including South America, Australia, New Zealand, South Africa, and Antarctica.
They found that up to 80 percent of the dust found in the sediment cores came from the arid Andes Mountains in northwestern Argentina. The dust was borne by the westerly winds, the prevailing winds that travel from west to east, which were likely stronger during the last ice age. Surprisingly, while Australia is a dominant source of dust in the Southern Ocean today, only a small proportion of the ancient dust originated there.
“It’s not common that you get completely stunned and surprised by something that is in the data,” says Gisela Winckler, a geochemist from New York’s Columbia University and an author on the study. “This was some of the clearest data I have ever seen.”
Today, the level of carbon dioxide in the atmosphere is higher than it has been at any point over the past 800,000 years, and reducing this greenhouse gas is more vital than ever. Past attempts to combat climate change by fertilizing the Southern Ocean with iron have caused extreme controversy. But with this new study, scientists can create more accurate climate models that simulate how iron contributed to the last ice age, and therefore more rigorously assess its potential role in fighting climate change today.