About 1.5 million years ago, the mid-depth waters of the tropical Pacific Ocean suddenly grew cooler. The change came rapidly, and it spread across thousands of kilometers.
Before then, Earth’s climate had been relatively stable. Cycles of ice ages and interglacial periods had already begun, but they were shorter, and the tropical Pacific remained warm. Its surface temperature barely changed even as polar ice advanced and retreated.
So how did the waters suddenly become cold?
“We’re usually interested in the mid-depth waters—not the surface, not the deep ocean—because that’s where the music is.”
A new study published in Communications Earth and Environment suggests that the cold water came from the Southern Ocean and traveled northward through ocean tunnels into the tropics. An ocean tunnel, the research explains, describes a “channel for water masses that connects different oceanic and consequently, atmospheric regions.”
“We’re usually interested in the mid-depth waters—not the surface, not the deep ocean—because that’s where the music is,” said Jacek Raddatz, a climate scientist at GEOMAR Helmholtz Centre for Ocean Research Kiel, in Germany, and first author of the study.
“The Pacific is the largest ocean and important for global circulation and climate,” he continued. “That’s why we focused our study on the tropical Pacific.”
Tunnels of Colder, Fresher Water
The researchers analyzed tests of planktonic foraminifera (forams) recovered from a sediment core drilled from the Manihiki Plateau, a submerged ridge in the tropical South Pacific. The plateau is located at the eastern edge of the Western Pacific Warm Pool, the region with the highest ocean temperatures in the world.
The team measured magnesium-to-calcium ratios and oxygen isotope values in tests of two species of forams. One species lived near the surface, and the other lived about 400 meters down. With those values, the scientists reconstructed past temperatures and salinity spanning a period from about 2.5 million to 1 million years ago.
“At the Manihiki Plateau, we see that around 1.5 million years ago, there’s a drop in both temperature and salinity,” said Raddatz.
This timing matched a major growth of Antarctic ice.
The new research indicates cold Antarctic water traveled northward through the Pacific’s mid-depths as a pulse, a process known as ocean tunneling.
“Cold water forms off places like Chile, Peru, and California and slowly sinks. It moves toward the equator beneath the surface,” explained Matt Luongo, a climate scientist and postdoctoral researcher at the University of Washington, in Seattle. He was not involved in the study. “If that water becomes cooler or fresher, then maybe…10 to 20 years later, the equator ends up bringing up cooler water too. That’s basically how ocean tunneling connects distant parts of the ocean.”
Raddatz and his fellow researchers also examined how the cooling was related to Earth’s orbital cycles: eccentricity, or the shape of Earth’s orbit; obliquity, or the angle at which Earth’s axis is tilted with respect to its orbital plane; and precession, or the direction Earth’s axis is pointed.
They found a consistent pattern. “We see the same increase in obliquity-related signals in Antarctic ice volume, in midlatitude temperature reconstructions, and in our salinity record,” Raddatz said. “That led us to conclude that they’re all connected through the same mechanism.”
Raddatz and his colleagues think the cooling may have been an early step toward the period when Earth’s ice ages grew longer and more intense. “We think this might be a first step that led, maybe, to the Mid-Pleistocene Transition.”
Interesting, but Not Definitive
Luongo agreed the study shows that South Pacific waters did chill out and freshen up about 1.5 million years ago and that the source of these changes came from the Southern Hemisphere. The research is “interesting, because it helps explain why the thermocline in the equatorial Pacific is so cold and fresh,” he said.
But he also cautioned against directly linking the changes to Antarctic ice growth. “It’s suggestive of the ice sheet, the wiggles match, but it also could be something else,” said Luongo. “There are a lot of things that can cause freshening of the waters.”
Raddatz and his colleagues plan to extend their work to other ocean basins. They want to add nutrient and carbon measurements to build a fuller picture of how mid-depth waters evolve. “If we understand these variations,” he said, “we can also explain the growth and decline of biodiversity hot spots in the deep sea.”
—Larissa G. Capella (@CapellaLarissa), Science Writer
Citation: Capella, L. G. (2025), Ocean tunneling may have set off an ancient Pacific cooldown,
Eos, 106, https://doi.org/10.1029/2025EO250428. Published on 18 November 2025.
Text © 2025. The authors.
CC BY-NC-ND 3.0Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.
