About 15 million years ago, the eastern tropical Pacific was a very different place. Where massive low-oxygen zones now threaten marine life off the coast of Peru, the waters were teeming with oxygen. Meanwhile, the Atlantic Ocean hosted vast oxygen-poor regions in places where life thrives today. This complete reversal of ocean chemistry, now documented for the first time by Michigan State University researchers, is reshaping how scientists understand our seas — and what they might look like in a warming world.
The discovery, published in Nature Communications, emerged from a happy accident. Janet Burke, then a graduate student and now MSU's fixed-term assistant professor, was studying ancient Pacific sediments to confirm what she expected: that low-oxygen zones existed there millions of years ago, just as they do now. Her Atlantic samples were meant purely as a control group. When the results came back flipped, she assumed something had gone wrong.
"My first thought was that something was wrong," Burke said. "I thought I might have to disregard the samples."
Instead, she brought the puzzling data to Associate Professor Dalton Hardisty, who enlisted graduate student Keyi Cheng to build a computer model recreating ancient ocean conditions. Together, they found the explanation: a watery channel once separated North and South America, allowing ocean currents to mingle freely between the Atlantic and Pacific. This ancient gateway reshaped circulation patterns and relocated where low-oxygen waters formed.
The implications stretch far beyond paleontology. About 15 million years ago, Earth was warmer with higher carbon dioxide levels — conditions that serve as a natural preview of potential future warming. Scientists have long known that warming reduces ocean oxygen overall, but the MSU team found that circulation and continental positions matter just as much as temperature.
"When we talk about climate change and global impacts, we say oxygen in the ocean is going to go up or down, but this study has brought to light that that's not necessarily true everywhere," Burke said. "It's going to change the climate system, ocean circulation and atmospheric circulation in ways that might increase oxygen levels in some places and decrease them in other places. The impacts are regionally variable."
To reconstruct ancient oceans, the researchers turned to Foraminifera — microscopic plankton whose shells act as chemical time capsules. When these organisms die, their shells drift to the ocean floor, building up in layers that record millions of years of ocean history. Burke obtained sediment samples through the International Ocean Discovery Program, then spent hours extracting individual plankton shells under a microscope before analyzing their chemical composition.
The findings add crucial nuance to climate projections. Rather than a simple global decline in ocean oxygen, the research suggests that warming's effects will vary dramatically by region — knowledge that could help communities and ecosystems prepare for the changes ahead.