Expansion of Antarctic bottom water contributed to the end of the last Ice Age

Around 12,000 years ago, the last Ice Age ended, global temperatures rose and the early Holocene began, during which time human societies became increasingly settled. A new study, published today in Nature Geoscience, shows the important role played by the Southern Ocean surrounding Antarctica in this transition.

Led by Dr Huang Huang of the Laoshan Laboratory in Qingdao, with the involvement of Dr Marcus Gutjahr, a geochemist at GEOMAR, the team reconstructed the spatial extent of Antarctic Bottom Water (AABW) in the Southern Ocean over the past 32,000 years.

“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, and what role it played in the global carbon cycle,” says Huang, who completed his PhD at GEOMAR in 2019 and now works as a scientist in Qingdao, China.

Sediment cores reveal the origin of deep-water masses

To achieve this, the researchers analysed nine sediment cores from the Atlantic and Indian sectors of the Southern Ocean. These cores were taken from depths between 2,200 and 5,000 metres, and from widely spaced locations. By examining the isotopic composition of the trace metal neodymium, which is incorporated into sediments from the surrounding seawater, the researchers were able to reconstruct the extent of Antarctic Bottom Water over tens of thousands of years.

“Dissolved neodymium and its isotopic fingerprint in seawater are excellent indicators of the origin of deep-water masses,” explains 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. Initially, we thought the method was flawed or that there was something wrong with the sediment core. But the real question was: What could generate such a signal? Such an exotic isotopic signature can only develop when deep water remains almost motionless for extended periods. In such circumstances, benthic fluxes – chemical inputs from the seafloor – dominate the isotopic imprint in marine sediments.”

Two phases of expansion and their role in releasing carbon dioxide

During the last Ice Age, the extremely cold, dense deep water that forms around Antarctica today was substantially retracted. Instead, large parts of the deep Southern Ocean were filled with carbon-rich water masses originating from the Pacific – a glacial precursor to today’s Circumpolar Deep Water (CDW). The CDW is described as carbon-rich in the study because it circulates in the deep ocean for long periods with limited ventilation. Consequently, more dissolved carbon remained stored in the ocean, keeping atmospheric CO₂ concentrations low.

As the planet warmed and the ice sheets melted between about 18,000 and 10,000 years ago, the volume of Antarctic Bottom Water expanded in two distinct phases. These phases coincided with known warming events in Antarctica. As vertical mixing in the Southern Ocean increased, the carbon that had been stored in the deep ocean was able to return to the atmosphere.

“The expansion of the AABW is linked to several processes,” explains 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, destabilising the existing water-mass structure and enhancing exchanges between deep and surface waters.”

Until now, many studies have assumed that changes in the North Atlantic, including the formation of the North Atlantic Deep Water (NADW), were the dominant drivers behind shifts in deep-water circulation in the South Atlantic. However, the new data indicate that northern influences were more limited than previously thought. Instead, the displacement of a glacial, carbon-rich deep-water mass by newly formed Antarctic Bottom Water is thought to have played a central role in the rise of atmospheric CO₂ at the end of the last Ice Age.

Southern Ocean heat storage and Antarctic ice loss

“Comparisons with the past are always imperfect,” says Gutjahr, “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.”

Due to its size alone, the Southern Ocean plays a significant role in regulating the Earth’s climate. Over the past five decades, waters deeper than roughly 1,000 metres around Antarctica have warmed significantly faster than most other parts of the global ocean. In order to understand how these changes affect the ocean’s capacity to absorb and release carbon dioxide, physical and biogeochemical processes must be monitored over long periods and integrated into climate models.

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

Palaeoclimate data obtained from sediment cores are indispensable for this, offering insights into past climates that were warmer than today and helping to improve projections of future climate change.