New research reveals the Pacific Ocean’s circulation system fundamentally flipped its structure during the last ice age, a discovery that reshapes scientific understanding of how Earth’s climate cools on a global scale. Two independent studies using evidence from ancient deep-sea fossils found that this radical reorganization of water flow in the planet’s largest ocean basin played a crucial role in drawing carbon dioxide from the atmosphere and sequestering it in the deep sea, amplifying the glacial period.
For decades, scientists have largely focused on the Atlantic Ocean’s circulation as the primary driver of climate changes related to ice ages, but the Pacific’s enormous volume suggested it had to be a significant factor. These findings provide the first comprehensive evidence that the Pacific was not merely a passive participant but an active and powerful agent of global cooling. This revised view of oceanic circulation dynamics during glacial periods offers critical insights for refining climate models used to project future environmental changes, highlighting previously unknown mechanisms by which oceans regulate atmospheric carbon dioxide.
A Radically Different Ocean Circulation
The established model of modern ocean circulation involves warm surface water flowing north through the Atlantic, where it cools, sinks, and returns south at depth in a system known as the Atlantic Meridional Overturning Circulation. The Pacific, in its current state, has a much weaker and shallower overturning cell. However, recent studies indicate that during the last glacial period, which peaked around 20,000 years ago, this configuration was reversed. The evidence now suggests that the Pacific developed a powerful, deep circulation system that was fundamentally different from today’s.
According to researchers at the University of California, Irvine, the glacial-era Pacific saw surface waters sink to depths of approximately 2,000 meters in the northern part of the basin, subsequently spreading southwards. This process effectively “flipped” the overturning circulation, creating a new pathway for atmospheric gases to be transported into the abyss. John Southon, a co-author of one of the studies, emphasized that this was not just a minor adjustment. “The payoff is that for the first time there are sufficient data to show clear evidence that the glacial ocean circulation was not just a slower version of today’s but radically different,” he stated.
Unlocking a Prehistoric Climate Puzzle
A central question in paleoclimatology is how atmospheric carbon dioxide levels dropped so significantly during ice ages. The geologic record, preserved in ice cores, shows a dramatic reduction in this greenhouse gas, but the specific processes responsible for removing it from the atmosphere have been debated. The sheer size of the Pacific Ocean—it holds more than double the volume of the Atlantic—made it a prime suspect, but concrete evidence was scarce.
Patrick Rafter, an assistant researcher at UCI and lead author of a study in Science Advances, noted the long-standing challenge. “It’s intuitive to think that the Pacific would play a major role in climate regulation during the last glacial period,” he explained. “But we didn’t have a lot of data to say that previously.” The new research fills this critical gap by establishing a robust connection between changes in Pacific circulation and the planet’s capacity for carbon storage, demonstrating that the ocean was a significant driver of lower greenhouse gas concentrations.
Evidence from the Deep Sea Floor
Reading Radiocarbon Signatures
To reconstruct the ancient Pacific’s behavior, the UCI-led team undertook a massive data synthesis effort, analyzing thousands of fossil sediment samples from across the globe. Their work focused on measuring traces of carbon-14, a naturally occurring radioactive isotope of carbon, in the shells of tiny marine organisms called foraminifera. Because carbon-14 is created in the atmosphere and decays at a known rate, its concentration in deep-sea fossils reveals how long the surrounding water has been isolated from the surface.
By compiling a global benchmark of these measurements dating back 25,000 years, the researchers could compare the “age” of deep water between the Atlantic and Pacific basins. Their results showed that during the last ice age, the deep North Pacific held younger, more ventilated water, while the deep Atlantic held older, more stagnant water—a complete reversal of the modern arrangement. This provided clear evidence of a vigorous overturning circulation in the Pacific that was actively drawing carbon from the atmosphere.
Clues from Ancient Corals
In a separate study published in Nature Communications, an international team led by scientists from the University of Oldenburg found converging evidence in a different type of marine archive: fossilized deep-sea corals. The researchers analyzed 62 specimens of the stony coral Desmophyllum dianthus collected from depths of 1,400 to 1,700 meters in the Tasman Sea, located between Australia and New Zealand. These corals, which lived between 10,000 and 70,000 years ago, preserve the chemical signature of the water masses they grew in.
The team focused on isotopes of the element neodymium, which act as a reliable tracer for the origin of different ocean water masses. Their analysis revealed that during the peak of the ice age, water originating from the North Pacific flowed through the depths of the Tasman Sea. According to lead author Dr. Torben Struve, these cold-water corals serve as a particularly good archive for studying past ocean currents. The findings confirmed that upper layers of the Pacific were more mixed and dynamic than they are today.
The Pacific’s Carbon Sequestration Engine
The combined evidence from these studies paints a clear picture of how the glacial Pacific acted as a powerful carbon sink. The sinking of dense, cold surface water in the North Pacific transported dissolved carbon dioxide from the atmosphere directly into the deep ocean. Once there, the carbon-rich water was isolated from the surface for centuries, preventing the gas from returning to the atmosphere and thereby contributing to the planet’s cooling climate.
This process was likely enhanced by another key finding from the coral study: while the upper-to-mid-level waters of the Pacific were more mixed, the deepest layers of the ocean became more strongly stratified and isolated. This deep-ocean stratification would have effectively trapped vast quantities of older, carbon-rich water, further enhancing the ocean’s overall capacity for carbon storage and amplifying the global cooling trend.
Rethinking Global Climate Models
These discoveries have significant implications for modern climate science. The finding that the world’s largest ocean can fundamentally reorganize its circulation system highlights a level of variability that is not yet fully captured in many climate models. Understanding the dynamics of this “flipped” state is essential for improving projections of how the ocean will respond to and influence future climate change.
The studies underscore that the global conveyor belt of ocean currents is a complex and adaptable system. The new data provides a clearer picture of how different basins interact and how water masses are exchanged under different climate conditions. “Our study contributes to a better understanding of the dynamics of this global ocean circulation system under changing climatic conditions,” said Struve. This improved understanding is vital for predicting the ocean’s future role in absorbing anthropogenic carbon emissions.
An Overlooked Convection Hotspot
A surprising element of the research was the identification of the Tasman Sea as a critical but previously neglected component of the ice-age ocean conveyor belt. The coral data demonstrated that this region was a key conduit through which Pacific deep water flowed on its journey toward the Indian Ocean, ultimately joining and reinforcing the global circulation network.
While the Tasman Sea is involved in today’s water mass exchange, its role during the last glacial period appears to have been much more profound and to have occurred at greater depths. Recognizing the importance of such marginal seas provides a more nuanced view of the global circulatory system. It suggests that climate models need to incorporate these smaller, geographically constrained areas to accurately simulate large-scale climate transitions, both in the past and in the future.
