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CORVALLIS, Ore. – During the last ice age, atmospheric carbon dioxide was about one-third lower than it is today and scientists are uncertain why the distribution of global carbon was different than today.

A newly published modeling study suggests that the carbon was stored in the deep ocean – likely as a result of an increased amount of dust coming off the continents and a weakened Atlantic Meridional Overturning Circulation, or AMOC.

Dust contains the micronutrient iron, which acts like a fertilizer and can enhance oceanic primary productivity, the researchers say. They found an increase in nitrogen-15 isotopes in sediment cores, which supports the hypothesis. They also looked at ocean circulation by using two other isotopes, radiocarbon and carbon-13, whose ratios suggest that ocean circulation patterns were weaker than today.

Results of the study have been published in the journal Earth and Planetary Science Letters.

“This has been one of those mysteries that has never really been resolved and although this study doesn’t have a ‘smoking gun’ it does provide a rational explanation for why the carbon cycle was different during the last glacial maximum,” said Andreas Schmittner, an Oregon State University paleoclimatologist and co-author on the study.

The abundance of the three isotopes in the sediment cores could best be explained by the increase in dust and a weak and shallow AMOC, according to lead author Juan Muglia, a postdoctoral researcher in Oregon State’s College of Earth, Ocean, and Atmospheric Sciences.

“By themselves, the isotopes cannot tell the entire story,” Muglia said. “But when you bundle all three together – and combine them with a model – it provides a compelling explanation for why the carbon would be sequestered in the deep ocean during the height of the last ice age.”

Schmittner said the increase in dust likely was a result of the climate being colder and drier, and the ice sheets and glaciers “bulldozing” materials off the continent, which was picked up by the wind and delivered into the ocean.

“If you take the carbon out of the atmosphere, it has to be somewhere,” Schmittner said, “and scientists have long suspected, if not known, that it was in the deep ocean. Juan’s model of dust increase and a weak, shallow AMOC provides a strong hypothesis for how it got there.”