Climate scientists have modeled a weakening Atlantic current for years, and the results have repeatedly pointed in the same direction.
Deep waters near Antarctica should respond as well, but predictions are not the same thing as proof.
A team analyzing sediment cores off the coast of Chile has now found the evidence.
Chemical signals locked inside shells smaller than a grain of sand show that the predicted response unfolded rapidly 39,400 years ago.
Clues from old shells
Pushpak Martin John Nadar, a paleoclimate researcher at the University of Bergen (UiB) in Norway, led the team.
They read chemical fingerprints locked into tiny shells of seafloor microorganisms called foraminifera, built from the water around them.
Carbon and oxygen isotopes are slightly different chemical forms of each element.
Those isotopes remain locked within the shells and let researchers trace deep water from tens of thousands of years ago.
Their core sits off the coast of Chile, where Southern Ocean flows push north into the Pacific.
The window they zoomed in on falls around 39,400 years ago. Greenland ice cores show that the North Atlantic had grown unusually cold during that period and had likely weakened.
How the Atlantic stalls
Scientists call this loop the Atlantic Meridional Overturning Circulation (AMOC).
Warm, salty surface water moves north, releases heat into the atmosphere, sinks, and then returns south at depth. Without it, the planet’s heat budget would look different.
A slowing loop affects more than the North Atlantic. Models predict that a weaker AMOC pushes heat and nutrients around the global ocean in unexpected ways.
Recent observations confirm a decline is already underway. Nadar’s team wanted to know if old sediments hold physical traces of that change reaching far from the Atlantic.
If they do, today’s slowing AMOC could already be reshaping ocean chemistry far beyond the Atlantic.
A signal in the Pacific
Inside the cores was a remarkable story. Around 39,400 years ago, bottom waters off Chile warmed quickly, oxygen levels climbed sharply, and carbon isotopes rose.
All three changes lined up at once. Such alignment is rare in paleoclimate records, where changes usually unfold gradually.
Together those signals describe water that had recently been present at the surface.
Ocean researchers call this enhanced ventilation, meaning the water had recently contacted the atmosphere, picking up oxygen and shedding nutrients.
The water mass is the Antarctic Intermediate Water, which forms near Antarctica and spreads toward the tropics about half a mile below the surface.
Until this study, no one had tied its response to an Atlantic slowdown so precisely.
The chemical response
A sharp oxygen jump stood out. That kind of rise happens when recently surfaced water arrives at depth.
Carbon isotopes confirmed it. The chemical signature matched freshly circulated water rather than the depleted signal left by stagnant deep water.
The slight bottom-water warming matched what climate models predict when AMOC slows.
Heat that would normally travel north may instead have accumulated in the Southern Hemisphere, nudging warmth into the intermediate layer.
Nutrients in this layer also dropped. That decline likely signals a wider redistribution of biological fuel through middle waters, feeding surface ecosystems far away.
That interpretation fits earlier work describing the Southern Ocean as the deep ocean’s largest oxygen source.
Deeper, warmer layers
In model after model, a weaker AMOC pushes the thermocline, the boundary between warm surface water and cold depths, downward in the Pacific.
Northward heat transport stalls and warmth accumulates south of the equator instead. Nadar’s results match that prediction with real-world evidence.
The chemical signals in the Chilean cores look exactly like what would happen if the thermocline deepened and the layer just below the surface pulled in younger, surface-influenced water.
Such corroboration is rare in paleoclimate work. Models predict one outcome, but real sediment records rarely confirm it.
Matching the models against the sediment record clarifies how the global ocean responds when part of its circulation weakens.
Carbon’s missing piece
Ice cores show that carbon dioxide rose during these old Atlantic slowdowns, and scientists have argued for decades about why.
Some proposals lean on deep Pacific waters, and others focus on a Southern Ocean upwelling near Antarctica.
The new findings narrow the field. If the intermediate water was venting more vigorously then, stored carbon could have escaped to shallower layers, where it exchanges with the atmosphere.
Earlier research on Antarctic bottom waters suggests a related role. That doesn’t settle the CO2 puzzle on its own.
But it removes some candidate explanations and leaves a smaller set that future studies can test against more sediment records and more climate simulations.
The implications ahead
The AMOC is weakening now, and the same physical levers from 39,400 years ago are at work again.
If Antarctic intermediate waters react today as they did then, oxygen, carbon, and nutrient patterns across much of the ocean could shift substantially.
For fisheries, those shifts could affect productive zones off South America and Africa, where intermediate-layer nutrients feed surface life.
For forecasting, models that ignore the Southern Hemisphere’s response to Atlantic changes will miss part of the picture.
The Bergen team has shown the linkage is real, rapid, and chemically detectable. Researchers no longer need to assume the Antarctic-to-Atlantic feedback exists from models alone.
The sediment record confirms that the process occurred before, and that it could occur again.
The study is published in Communications Earth & Environment.
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