The Antarctic Ocean from the last ice age reveals how a critical CO₂ storage process could slow down again

Off the coast of Antarctica, sea ice retreated toward the southernmost continent, and like the cap on a soda bottle being removed, this reduced pressure slowed a critical carbon dioxide capture process, dramatically accelerating global warming.

But all this happened thousands of years ago, sounding the end of the last ice age.

And yet our era’s sea ice is also retreating, so it is essential that we understand these ocean processes that have such a profound effect on the planet.

An ocean seesaw

We have long known that the warming of the Antarctic Ocean contributed to the end of the last ice age, but the traditional hypothesis held that the abyssal waters around Antarctica and the deep waters of the North Atlantic warmed in a “see-saw” pattern “that suggested that as one weakened, the other strengthened,” says Chengfei He, climatologist at Northeastern University.

He, an assistant professor of marine and environmental sciences at Northeastern, discovered something that could give rise to a radical reinterpretation.

Instead of the waters of these two oceans oscillating as temperatures rise, he and his co-researchers, using radiocarbon dating of deep-sea seafloor sediments, observed that the aquatic formations on the bottom were “weakening simultaneously,” he says. Their study is published in the journal Natural communications.

Deep water storage

Antarctic bottom waters – or AABW, in marine scientist parlance – “form when extremely cold, salty water flows near Antarctica due to the formation of sea ice,” he says.

“This dense water then flows northward along the ocean floor, eventually rising to the surface,” he continues. How much and how quickly these deep waters rise is called the overturning rate, a process that connects all the oceans together in a cycle called thermohaline circulation.

The AABW formation sequesters massive amounts of “atmospheric CO”₂ in the depths of the ocean for centuries,” he says. If its overturning rate increases, that is to say if the waters trapping carbon in the depths of the ocean rise to the surface and release this CO₂ faster – this could represent “a critical tipping point that could significantly alter regional climates, disrupt global weather patterns and reduce the oceans’ capacity to absorb carbon dioxide.”

Radiocarbon dating…water?

Carbon dating as a concept (or even just the phrase) is familiar to many, but how do you carbon date an ocean, let alone its movements?

He says that radiocarbon – a particular isotope of carbon, “radio” because it is radioactive and therefore decays at a predictable rate – “acts like a natural clock in seawater”.

When seawater is on the surface, it acquires the contemporary radiocarbon isotope. As it sinks (perhaps becoming part of the AABW formation), the radiocarbon then decays at a known rate and eventually settles on the seafloor, where scientists can collect their cores.

But, with the help of a “state-of-the-art Earth system model, in which we track both water movement and radiocarbon over time,” he says, they made a surprising observation: “What looked like faster-moving deep water in the Southern Ocean 17,000 years ago was actually slower-moving water that had just started with younger radiocarbon ages on the surface.” »

At the start of the deglaciation period, between 15,000 and 17,000 years ago, sea ice retreated and the formation of Antarctic bottom waters weakened, that is, the process that creates AABW slowed down, capturing less CO.₂ over time.

A climate indicator

According to the researchers’ model, this suggests that two background aquatic formations, the North Atlantic and Antarctica, have weakened simultaneously, like two enormous storage units that are no longer accepting CO2.₂.

“All of a sudden”, here, is of the order of about 2 millennia, but represents half of the total CO₂ increase throughout the complete deglaciation of 8,000 years, according to the paper.

“We see similar trends today,” he writes. “Recent observations show that the AABW is weakening as the Southern Ocean warms.

“Understanding how these massive water masses behaved during past climate transitions helps us better predict future changes,” he explains.

As the oceans continue to warm, carbon sequestration in the ocean depths will increasingly slow, “potentially affecting global heat distribution, carbon sequestration and regional climate patterns.”

This story is republished courtesy of Northeastern Global News news.northeastern.edu.