How the Southern Ocean helped end the last Ice Age

Around 12,000 years ago, the last Ice Age faded, global temperatures increased, and the early Holocene epoch began. Ice sheets retreated, sea level rose, and human groups started to settle down instead of roaming from place to place.

That shift in climate did not come only from the air above our heads. It also came from water miles below the ocean surface.

A new study points straight at the Southern Ocean around Antarctica and shows how the deep sea helped move carbon dioxide from ocean to atmosphere during this big climate transition.

Deep water locks away carbon

Far to the south, cold, salty water forms around Antarctica, sinks, and spreads along the seafloor. This Antarctic Bottom Water is the coldest and densest water mass in the global ocean. It is a major engine of the deep “conveyor belt” that moves heat and carbon around the planet.

When deep water sits isolated from the surface, it can store dissolved carbon for a very long time. This means less carbon dioxide in the air.

A research team led by Dr. Huang Huang of the Laoshan Laboratory in Qingdao, with geochemist Marcus Gutjahr of GEOMAR in Germany, set out to see how this bottom water behaved during the last major warming.

“We wanted to understand how the influence of Antarctic Bottom Water, the coldest and densest water mass in the global ocean, changed during the last deglaciation,” said Huang.

The researchers also investigated the Antarctic Bottom Water’s role in the global carbon cycle.

Ancient clues from the seafloor

To follow water that vanished thousands of years ago, the team turned to mud. They analyzed nine sediment cores from the Atlantic and Indian sectors of the Southern Ocean.

The researchers collected these from the seafloor at depths of between 7,200 and 16,400 feet (2,195 and 5,000 meters), and from locations spread over a large area.

Layer by layer, those cores preserve chemical traces of the seawater that once bathed the ocean bottom.

One clue the team relied on came from the rare metal neodymium, which settles into seafloor mud along with other particles.

“Dissolved neodymium and its isotopic fingerprint in seawater are excellent indicators of the origin of deep-water masses,” explained Dr. Marcus Gutjahr.

“In earlier studies, we noticed that the neodymium signature in the deep South Atlantic only reached its modern composition around 12,000 years ago. However, sediments from the last Ice Age showed values that are not found anywhere in the Southern Ocean today,” he said.

A mysterious isotopic fingerprint

Initially, the scientists thought their method was flawed or that there was something wrong with the sediment core. They pondered what could generate such an unusual isotopic fingerprint.

“Such an exotic isotopic signature can only develop when deep water remains almost motionless for extended periods,” explained Dr. Gutjahr.

“In such circumstances, benthic fluxes – chemical inputs from the seafloor – dominate the isotopic imprint in marine sediments.”

Water in the deep Southern Ocean

The pattern in the cores points to a Southern Ocean that looked very different during the last Ice Age. The extremely cold, dense deep water that forms around Antarctica today was substantially pulled back.

Carbon-rich water masses filled large parts of the deep Southern Ocean. These flowed in from the Pacific, and were a glacial forerunner of what scientists today call Circumpolar Deep Water.

That deep water is described as carbon-rich in the study. It circulates in the depths for long periods with limited ventilation. This means that dissolved carbon stays locked away, which helps to keep atmospheric CO₂ concentrations low.

Two clear phases of change

As the planet warmed and ice sheets melted, between about 18,000 and 10,000 years ago, the volume of Antarctic Bottom Water grew. This occurred in two clear phases that lined up with known warming events in Antarctica.

Stronger vertical mixing then brought carbon stored in the deep Southern Ocean back to the surface and released it into the air.

“The expansion of the AABW is linked to several processes,” explained Dr. Gutjahr.

“Warming around Antarctica reduced sea-ice cover, resulting in more meltwater entering the Southern Ocean. The Antarctic Bottom Water formed during this transitional climate period had a lower density due to reduced salinity.”

“This late-glacial AABW was able to spread further through the Southern Ocean, destabilizing the existing water-mass structure and enhancing exchanges between deep and surface waters.”

Shifts in deep-water circulation

For many years, a lot of attention in climate research has gone to the North Atlantic. In this region, cold, salty water also sinks and forms North Atlantic Deep Water.

Many studies treated changes in that northern branch as the main driver of shifts in deep-water circulation in the South Atlantic.

The new data point in a different direction and suggest that northern influences were more limited than often assumed.

Instead, the study argues that the replacement of a glacial, carbon-rich deep-water mass by newly formed Antarctic Bottom Water in the south played a central part in the rise of atmospheric CO₂ at the end of the last Ice Age.

Changes in Antarctic Bottom Water do not just affect carbon. When this deep water grows or shrinks, it also reshapes how oxygen and nutrients spread through the deep sea. This creates knock-on effects for ecosystems and the chemistry of the ocean interior.

A warming Southern Ocean today

The Southern Ocean wraps around Antarctica and, simply because of its size, plays a major role in regulating Earth’s climate.

Over the past five decades, waters deeper than roughly 3,300 feet (1,000 meters) around Antarctica have warmed significantly faster than most other parts of the global ocean.

“Comparisons with the past are always imperfect, but ultimately it comes down to how much energy is in the system. If we understand how the ocean responded to warming in the past, we can better grasp what is happening today as Antarctic ice shelves continue to melt,” said Dr. Gutjahr

To see how this modern warming will affect the ocean’s ability to absorb and release carbon dioxide, scientists need measurements that span many years.

Scientists will need to monitor physical and biogeochemical processes in the Southern Ocean over long periods and incorporate them into climate models to ensure that projections of future carbon uptake and sea-level rise rest on solid ground.

Predicting future changes

The team’s work depends on tying present-day observations to what is stored in old mud on the seafloor.

“I want to properly understand the modern ocean in order to interpret signals from the past,” said Dr. Gutjahr.

“If we can trace how Antarctic Bottom Water has changed over the last few thousand years, we can assess more accurately how rapidly the Antarctic Ice Sheet may continue to lose mass in the future.”

Paleoclimate data from sediment cores are central to that effort. They offer a window into past climates that were warmer than today. They also help sharpen projections of how the climate system, Antarctic ice, and the global ocean may change in the years ahead.

The full study was published in the journal Nature Geoscience.

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