Ancient dust from the ocean’s depths may have helped keep the last ice age cool

The ocean floor of the South Pacific contains traces of ancient dust that may have changed Earth’s very climate, and new research suggests it came all the way from beneath ice-age glaciers of what is now Argentina. 

Whipped up by strong westerly winds some 20,000 years ago, these microscopic minerals would have circumnavigated nearly the entire globe before finally coming to rest in the middle latitudes of the Pacific.

Importantly, they carried a nutrient that could explain a period of global cooling. That ingredient was iron.

Iron is a vital nutrient for microscopic algae in our oceans, known as phytoplankton, and these creatures are in turn a fundamental part of Earth’s climate.

That’s because phytoplankton absorb carbon during photosynthesis, thereby storing atmospheric CO2 in our oceans and driving global cooling. They might even represent “the largest biological carbon sequestration mechanism on the planet“.

Today, iron still helps fertilise our oceans, but during the peak of Earth’s last ice age, a lot more iron-containing dust was unearthed during seasonal glacier melt, and it was blown into the ocean at a much higher rate.

All this extra iron fed phytoplankton that then lowered CO2 levels in the atmosphere and could help to explain “how the Earth could have become so cold at all at that time”, says Torben Struve, a geoscientist at the University of Oldenburg in Germany.

As such, some scientists think iron fertilisation might be a useful way to increase the carbon sink of our oceans and help cool our planet down in the future.

But geoengineering of this sort is a risky and controversial strategy, and the results of this new study just go to show how much dust would be needed to have a big enough impact.

Today, human emissions have caused CO2 levels to increase from around 280 to around 415 ppm (parts per million) since the industrial revolution – a surge that is far above natural levels.

During the last ice age, however, previous models have confirmed iron-bearing dust was responsible for drawing down atmospheric CO2 by some 40 ppm.

That’s roughly half the natural variation between that ice age and the following interglacial period, and not even a quarter of our own emissions.

Nevertheless, scientists are determined to learn more about this complex feedback system in the hopes that it could one day improve our climate models or help us capture more atmospheric carbon.

Analysing 18 sediment cores from the South Pacific Ocean between Antarctica, New Zealand, and Chile, the new study has compared the chemical fingerprints of ancient dust to geological data from several different continents. 

In the end, the findings suggest up to 80 percent of iron-containing dust came from what is now north-west Argentina – and it probably blew there the long way, travelling roughly 20,000 kilometres (12,400 miles) on powerful westerly winds during the last major ice age. 

That’s a unique and interesting discovery, because today, dust input from Australia’s rivers and lakes dominates the entire study area.

Even in the past, Patagonia is usually considered the major source of far-travelled, ancient dust, not regions further north in Central South America.

“[W]e were surprised to find that the sources and transport routes of the dust were completely different from today and also different from what we would have expected,” says Struve.

“Global warming has changed the winds and environmental conditions in the source regions.”

Even something as small as dust can have global repercussions. Thirty years after we first discovered its impact on the climate system, we are still learning more about these microscopic minerals, including where they came from.

The study was published in Nature Communications.

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