During Earth's deepest ice ages, something strange happened to the planet's oceans: they lost oxygen. Now, scientists have uncovered exactly how this worked — and the answer surprises even experts who thought they understood climate and oceans.
An international team analyzed more than 27,000 ancient seafloor samples collected from six continents, going back over 2 billion years. The samples came from the Deep-Time Marine Sedimentary Element Database, which holds geological data from North America, South America, Europe, Asia, Africa, and Oceania. That's an enormous amount of data — the researchers say that 500 data points would already be considered excellent for typical studies.
The key discovery involves a metal called manganese. When ocean waters are low in oxygen, manganese dissolves into the water. When oxygen returns, the metal settles into seafloor sediments. By measuring how much manganese sits in sediments from different parts of the world, the scientists could map where oceans had oxygen and where they didn't.
What they found was striking. During ice ages, sediments near the North and South Poles contained far more manganese than sediments near the equator. But during warmer periods without major glaciation, the manganese was distributed more evenly. "When we analyzed the background, nonglacial periods, the gradient was almost flat," said Xubin Wang, a postdoctoral researcher at Syracuse University who led the study. "But during ice ages, it becomes very steep."
The team, which included Syracuse University professor Zunli Lu and Alexandre Pohl from Université de Bourgogne in France, used computer modeling to confirm their results. Pohl's ocean simulations using the cGENIE Earth system model matched the sediment records, showing oxygen-poor surface waters in tropical regions during ice ages.
The strongest manganese patterns appeared during the Proterozoic Snowball Earth episodes — times when ice may have covered nearly the entire planet and the atmosphere contained far less oxygen than today. As atmospheric oxygen gradually increased over hundreds of millions of years, the differences between polar and tropical oceans became less extreme.
The findings, published in the journal Nature Communications, challenge a common assumption. For decades, many scientists thought warming caused ocean oxygen loss. This study shows that cooling can do the same thing — deoxygenation has occurred during dramatically different climate conditions. "For decades, scientists have argued that warming would lead to the expansion of marine dead zones," Lu said. "Mounting evidence points to multiple cases of ocean deoxygenation during cooling related to mass extinctions."
The researchers hope their work helps build a more complete picture of how climate, oxygen, and life on Earth are connected. Understanding these deep-time patterns could help humanity prepare for environmental changes ahead. "I think a more nuanced view of the connections among climate, oxygen and habitability will help our society adapt to future environmental changes," Lu said.