Atmospheric carbon dioxide (CO2) levels were significantly lower during the Earth’s past ice ages, prompting speculation that this decrease was partly responsible for the glacial cycles in which the massive, continent-spanning ice sheets grew and shrunk. But the reason for CO2 drops remained unknown. 

“The cause of the ice ages is one of the great unsolved problems in the geosciences,” said Daniel Sigman, the Dusenbury Professor of Geological and Geophysical Sciences. “Explaining this dominant climate phenomenon will improve our ability to predict future climate change.”

Now, in a new study, scientists from Princeton University and the Max Planck Institute for Chemistry have found that a change in ocean currents in the Antarctic Ocean kept more CO2 in the deep ocean during the ice ages.

The scientists measured nitrogen isotopes in a fossilized algae species, Fragilariopsis kerguelensis, collected from deep sea cores. The algae proliferate in Antarctic waters, and, upon death, their shells accumulate in deep sea sediment. These data allowed the researchers to trace nitrogen concentrations in Antarctic surface waters over the past 150,000 years, covering two ice ages and two warm interglacial periods.

“Deep water has high concentrations of the nitrogen that algae rely on,” explained Ellen Ai, first author of the study. When ocean currents raise that deep water toward the ocean surface – a process known as "upwelling" – the nitrogen concentration in the surface water increases. “Our results allowed us to reconstruct Antarctic upwelling changes.”