Climate change has many causes, and the ocean is not usually the first one that comes to mind. People tend to think about smokestacks, tailpipes, and farmland.
Yet far below the waves, another force has been shaping Earth’s climate for thousands of years. It is a body of unusually salty water sitting deep in the ocean, largely unseen and easy to dismiss.
That deep ocean salt played a quiet but powerful role in controlling how much carbon dioxide stayed out of the atmosphere. When it shifted long ago, the climate shifted too.
New research that tracks ancient seawater shows how this hidden process matched up with a major warming event near the end of the last ice age, about 18,000 years ago.
A hidden storehouse in the ocean
The ocean is not one big bathtub. It is layered, with waters that differ in temperature and saltiness. Those differences matter.
Saltier water is heavier, so it tends to sink and stay put. When layers do not mix much, gases trapped in the deep ocean remain there for a long time.
Carbon dioxide moves into the ocean in huge amounts. At the surface, marine life uses it during photosynthesis.
When those organisms die, they sink. As they break down, carbon dioxide is released into deep water. If the layers stay separated, the gas stays trapped.
Over Earth’s history, warming and cooling have followed a cycle. That cycle is tied to how fast the ocean circulates through a process known as “the global ocean conveyor belt.”
Faster circulation during warm times keeps deep water from holding as much carbon dioxide. Slower circulation during cold times allows more of it to build up far below the surface.
What changed after the ice age
During the last ice age, which peaked about 20,000 years ago, the deep ocean was a more efficient sink of carbon dioxide than it is today.
That helps explain why global temperatures were more than 5 degrees Celsius lower back then.
There’s one thing scientists have known for a long time: when the planet started to warm up again, the deep ocean released a massive amount of carbon dioxide into the atmosphere.
What remained unclear was the role of salt. If salty deep water helped lock carbon dioxide away, where did that salt go when the warming began?
The answer came from tiny fossils no bigger than grains of sand. They come from single-celled organisms called foraminifera.
When they formed, the fossils recorded details about the water around them, including its salinity.
Clues written in ancient fossils
By studying these microfossils found in marine sediments near the boundary of the Indian and Southern oceans, researchers rebuilt a history of past ocean conditions.
The samples came from waters off the coast of western Australia, a key location for deep ocean circulation.
The data showed a sudden jump in salinity in the upper Indian Ocean at the start of the last deglaciation. That spike lasted several thousand years.
Other chemical signals matched it, pointing to a clear source. The salt had risen from the deep ocean.
This finding helped solve a long-standing puzzle. It showed that the deep ocean’s salt did not vanish. It moved. And when it did, the barrier keeping carbon dioxide trapped weakened.
The puzzle of deep ocean salt
“In today’s oceans there are different major water masses, and each has a distinctive salinity,” said Elisabeth Sikes, a professor at Rutgers-New Brunswick.
“Researchers have long speculated that deep ocean salinity levels were linked to changes in atmospheric carbon dioxide across ice age cycles. Our paper proves it.”
Study lead author Ryan H. Glaubke is a postdoctoral research associate at the University of Arizona.
“The exact mechanism, the actual physical explanation for why that happens, is something researchers have been trying to resolve,” said Glaubke.
Why the Southern Ocean matters
One reason this salt shift had such power is location. The Southern Ocean is one of the few places where truly deep water reaches the surface. When it does, it releases stored carbon dioxide back into the air.
“This paper supports the idea that it’s the salinity of deep ocean water – the ‘salty blob’ – that keeps carbon dioxide locked away for long periods of time,” Glaubke said.
Changes in this region ripple across the planet. When salty deep water stays put, carbon dioxide remains trapped. When it rises, the gas escapes, adding heat to the atmosphere.
Ocean salt and today’s climate
The ocean has already absorbed about a third of all carbon emissions from human activity. That has slowed the pace of climate change.
But the ocean’s ability to store carbon depends on its structure, including deep salinity patterns that formed under colder conditions.
“In a way, the ocean has been our greatest champion in the fight against climate change,” Glaubke said. “But without a pronounced ‘salty blob’ like the ancient glacial ocean had, it can’t hold on to our carbon emissions forever.”
As modern warming continues, this research highlights a simple truth. What happens in the deepest parts of the ocean does not stay there. Over time, it reaches the surface and shapes the climate we live in every day.
The full study was published in the journal Nature Geoscience.
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