Geography alone decided which ocean ran short of oxygen.
Fifteen million years ago, the Atlantic Ocean suffocated — not from heat, but from the shape of the world itself. Scientists reading chemical signatures in fossil plankton shells have found that an open seaway between the Americas, long since sealed by the rise of Panama, rerouted ocean currents in ways that starved the Atlantic of oxygen while leaving the Pacific comparatively stable. The discovery unsettles a quiet assumption in climate science: that ancient warm periods are reliable previews of warming's future, and that the Pacific's story is the whole story.
- Fossil shells from the Miocene epoch reveal the Atlantic was once blanketed by a vast oxygen-depleted zone — a condition that simply does not exist there today.
- The culprit is not carbon dioxide but continental geometry: an open seaway between the Americas reshuffled ocean currents and nutrient flows, tipping oxygen loss from the Pacific to the Atlantic.
- Computer simulations confirmed the seesaw — every time the seaway opened in the model, the Atlantic's oxygen hole swelled and the Pacific's shrank, regardless of CO2 levels.
- The finding fractures a foundational habit in climate forecasting: using ancient warm spells as direct previews of a warming future, since geography alone can redraw which ocean suffocates.
- Low-oxygen zones govern where fish survive and how much carbon the seafloor buries, meaning the lesson from one ocean basin may be dangerously misleading when applied to another.
Fifteen million years ago, the Atlantic Ocean was oxygen-starved — not because the planet was unusually hot, but because of where the continents sat. Scientists have now recovered the evidence in the form of microscopic fossil shells, recovered from seafloor sediments, that carry a chemical record of the water they once drifted through.
Janet E. Burke of Michigan State University led the team that decoded these shells, the remains of foraminifera — single-celled organisms that absorbed iodine only in oxygen-rich water. Shell after shell from across the Atlantic came back depleted in iodine, tracing a wide band of oxygen-poor water through the western Atlantic and Caribbean. A species of plankton that lives only at the edge of such zones appeared in every Atlantic sample, confirming the pattern. The Pacific, meanwhile, told the opposite story: its low-oxygen zone was narrow and equator-bound during the same warm period, a shadow of its sprawling modern form.
The explanation lies in a waterway that no longer exists. Before the Isthmus of Panama closed, a wide seaway split the Americas and allowed Pacific and Atlantic waters to mingle freely. The team built an Earth system model and tested what happened when that passage stood open. The result was unambiguous: the Pacific's oxygen-poor zone retreated toward the equator while a vast oxygen hole opened across the tropical Atlantic — precisely the seesaw the shells had recorded. Even with carbon dioxide held below today's levels, the open seaway alone was enough to produce the Atlantic's oxygen deficit. Warmth amplified the effect, but geography drove it.
The mechanism appears to work in two directions at once: the open gateway calmed the upwelling that depletes Pacific oxygen while cutting off the deep, oxygen-rich circulation that today refreshes the Atlantic's interior. Nutrients accumulated in the mid-basin Atlantic instead, feeding microbes that consumed what oxygen remained.
The implications reach well beyond one chapter of ocean history. If geography alone can shift low-oxygen zones from one side of the planet to the other, then ancient warm periods are not straightforward previews of a warming future — and conclusions drawn from Pacific records may not hold for the Atlantic. Low-oxygen water shrinks the habitat available to fish and alters how much carbon is buried on the seafloor. Understanding what actually drives these zones, the researchers suggest, means watching more than one lever at a time.
Fifteen million years ago, the Atlantic Ocean was a dead zone. Not because the planet was burning up—though it was warmer then than it is now—but because of where the continents sat and how water moved between them. Scientists drilling into the seafloor have now recovered the evidence: tiny fossil shells, no larger than grains of sand, that tell a story about oxygen and geography that upends what we thought we knew about warming oceans.
Janet E. Burke, a postdoctoral researcher at Michigan State University, led the team that read these shells. They are the remains of foraminifera, single-celled drifters that built themselves chalky armor and sank to the bottom when they died during the Miocene, a warm period when carbon dioxide levels ran higher than they do today. Each shell is a chemical record of the water it lived in. Foraminifera absorb iodine only where oxygen is plentiful, so a shell depleted in this element is a fingerprint of oxygen-starved water. When Burke's team analyzed shells from across the Atlantic basin, nearly every specimen came back low in iodine. The chemistry painted a picture of a vast band of oxygen-poor water stretching hundreds of feet down through the western Atlantic and Caribbean—a region that today stays well oxygenated from surface to seafloor. To confirm the reading was real and not some quirk of biology, the researchers found that a species of plankton that lives only at the edge of oxygen-starved zones turned up in every Atlantic sample. The pattern was unmistakable: the Atlantic had suffocated.
The Pacific told the opposite story. Shells from the equatorial Pacific showed it stayed in oxygen-poor territory throughout the warm period, but the zone was narrow—hemmed in between about 10 and 20 degrees latitude. Today, that low-oxygen zone sprawls from 30 degrees north to 40 degrees south. The Miocene Pacific was a shadow of what it would become, while the Atlantic was drowning in a way it never does now. No one had documented an Atlantic this starved of oxygen during that warm spell. Earlier research had focused almost entirely on the Pacific, leaving the basin across the Americas nearly blank. Filling in that gap redrew the entire picture.
The explanation lies in a waterway that no longer exists. Before the Isthmus of Panama rose and joined the continents, a wide seaway split North and South America, allowing Pacific and Atlantic water to mingle across the tropics. With that passage open, ocean currents ran on a completely different layout. Water that carried nutrients and oxygen followed routes that have since vanished. The team suspected this rearrangement—not the era's heat—was what drove the lopsided map of oxygen depletion. To test the idea, they built a digital ocean, an Earth system model they could rerun under any setting. They dialed carbon dioxide levels and the seaway's width from the modern world back to the Miocene.
The pattern snapped into place. Whenever the seaway stood open, the Pacific's low-oxygen zone pulled back toward the equator while a vast oxygen hole swelled across the tropical Atlantic. This was exactly the seesaw the shells had recorded. What tipped the balance was not the warmth but the gateway itself. Even with carbon dioxide levels lower than today's, the simulations showed the open seaway alone was enough to hand the Atlantic an oxygen hole. More gas only made it worse. The model missed some sharp edges, but the big swap held across every run.
The open gateway appears to have done two things at once. It calmed the process that draws oxygen-poor water up through the Pacific, allowing younger, oxygen-rich water to flood in from across the basin. On the Atlantic side, the oxygen-rich water that today sinks and refreshes the depths never reached mid-basin. Nutrients built up there instead, feeding the microbes that strip oxygen from the water and letting the hole fester. Strip the warmth away entirely and the seesaw still stands. Geography alone, by rerouting where water sinks and where nutrients gather, decided which ocean ran short of oxygen.
The finding reaches far beyond one basin's history. Low-oxygen zones can migrate from one side of the planet to the other on geography alone, with warming doing little more than nudging the dial. The same amount of carbon dioxide, a different map—and the ocean's fate changes. That unsettles a favorite tool in climate science: the habit of reading ancient warm spells as previews of the future. Pacific-only work had hinted that warming might ease ocean oxygen loss. The Miocene's Atlantic tells a murkier story. Low-oxygen water shrinks the space where fish and other animals can survive and governs how much carbon gets buried at the seafloor. Knowing what drives it shows which levers to watch. A lesson from one ocean may not hold for the next.
Notable Quotes
Low-oxygen zones can migrate from one side of the planet to the other on geography alone, with warming doing little more than nudging the dial.— Research findings from the study
The Hearth Conversation Another angle on the story
Why does it matter that the Atlantic was oxygen-starved 15 million years ago? That's ancient history.
Because we're using ancient warm periods as a roadmap for what's coming. If we get the mechanism wrong—if we blame warming when geography is actually the culprit—we'll be watching the wrong thing and missing what's actually changing.
So you're saying the Miocene Atlantic tells us something about our future?
It tells us that the future isn't just about temperature. It's about how water moves. The continents are locked in place now, but ocean currents shift with climate. If we don't understand what drives oxygen loss in each basin, we can't predict which regions will become dead zones and which will stay livable.
The seaway between the Americas—that's gone now. So the Atlantic should be safe?
It should be, based on geography alone. But we're adding carbon dioxide at a rate the planet hasn't seen in millions of years. The model showed that even without warming, the open seaway created an oxygen hole. Now we have both: a fixed geography and rising heat. We're running an experiment we've never run before.
What happens to the fish?
They leave, or they die. Low-oxygen water is a shrinking habitat. It's not just about the animals that live there—it's about the whole system. Carbon gets buried differently. Food webs collapse. One ocean's oxygen crisis becomes everyone's problem eventually.
So the lesson is: don't assume warming explains everything?
Exactly. Geography can shuffle where the damage happens. Warming can make it worse. But they're not the same thing, and treating them as if they are means we'll be surprised when the crisis shows up somewhere we didn't expect.