By Abbas Nazil
Marine geoscientists have uncovered evidence that a massive reservoir of salty water trapped deep in the ocean played a key role in past global warming, offering new insight into how Earth’s climate system regulates carbon dioxide.
The research shows that the end of the last ice age around 18,000 years ago coincided with the release of this ancient “salty blob” from the deep ocean, a process that contributed to rapid planetary warming.
The study, published in Nature Geoscience, was led by Ryan H. Glaubke, now a postdoctoral researcher at the University of Arizona, alongside Elisabeth Sikes, a professor at Rutgers University.
Their findings highlight how variations in deep ocean salinity can influence the storage and release of carbon dioxide, one of the most important greenhouse gases driving climate change.
Scientists have long known that the oceans act as Earth’s largest carbon sink, absorbing vast amounts of carbon dioxide from the atmosphere.
Much of this carbon is taken up by microscopic marine organisms at the ocean surface during photosynthesis.
When these organisms die, they sink to the deep ocean, where their remains decompose and release carbon dioxide into deeper waters.
Differences in salinity between ocean layers help maintain density barriers that trap carbon dioxide in the depths for long periods.
According to Sikes, researchers have speculated for decades that changes in deep ocean salinity were linked to atmospheric carbon dioxide fluctuations across ice age cycles.
The new study provides direct evidence supporting that theory.
During colder periods, ocean circulation slows, allowing dense, salty water to sink and store more carbon dioxide in the deep ocean.
This process helps cool the planet over long timescales.
During warmer periods, ocean circulation speeds up, reducing the ocean’s ability to trap carbon dioxide and allowing more of the gas to return to the atmosphere.
The researchers focused on sediments collected from the boundary between the Indian Ocean and the Southern Ocean, off the coast of western Australia.
They analyzed microscopic fossil shells known as foraminifera, which preserve chemical signatures of the water in which they formed.
These microfossils revealed that shallow waters in the upper Indian Ocean became unusually salty at the onset of the last deglaciation.
This spike in salinity lasted several thousand years and matched other chemical fingerprints showing the salt originated from deep ocean waters.
The findings suggest that the release of the deep ocean’s salty water weakened the barrier that had been trapping carbon dioxide.
As a result, large amounts of stored carbon dioxide were released into the atmosphere, accelerating global warming.
The study underscores the critical role of the Southern Ocean, one of the few regions where deep waters rise to the surface and release carbon dioxide back into the air.
Sikes emphasized that understanding modern climate change requires close attention to processes occurring in the Southern Hemisphere.
Today’s oceans have absorbed roughly one-third of all carbon emissions produced by human activity.
However, the researchers warn that without a strong deep-ocean salinity structure like the ancient “salty blob,” the ocean’s capacity to store carbon may be limited.
Glaubke described the ocean as humanity’s strongest ally against climate change, but one with finite ability to buffer rising emissions.
The study suggests that as the planet continues to warm, the ocean may become less effective at holding carbon dioxide, potentially accelerating future climate change.
