Around 12,000 years ago, the last Ice Age drew to a close. Global temperatures rose, the early Holocene began, and human communities gradually shifted toward more permanent settlements. A new study published in Nature Geoscience highlights how the Southern Ocean around Antarctica helped drive this major climate transition.
The research team, led by Dr. Huang Huang of the Laoshan Laboratory in Qingdao and including geochemist Dr. Marcus Gutjahr from GEOMAR, set out to reconstruct how far Antarctic Bottom Water (AABW) extended through 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 and chemical fingerprints in the deep sea
To tackle this question, the scientists examined nine sediment cores collected from the Atlantic and Indian sectors of the Southern Ocean. The cores came from water depths between about 2,200 and 5,000 meters and from locations spread widely across the region. By analyzing the isotopic composition of the trace metal neodymium preserved in the sediments, which reflects the chemistry of the surrounding seawater, they could reconstruct how Antarctic Bottom Water changed through time on the scale of 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."
Stagnant deep waters, carbon storage and the last Ice Age
During the last Ice Age, the cold and very dense deep water that currently forms around Antarctica did not spread as widely as it does today. Instead, much of the deep Southern Ocean was filled with carbon-rich waters that originated in the Pacific, a glacial precursor to today's Circumpolar Deep Water (CDW). In the study, CDW is described as carbon-rich because it circulates in the deep ocean for long periods with limited contact with the surface. This isolation allowed large amounts of dissolved carbon to remain locked in the deep ocean, helping to keep atmospheric CO2 levels relatively low.
As Earth warmed and ice sheets retreated between roughly 18,000 and 10,000 years ago, the volume of Antarctic Bottom Water increased in two clear phases. These expansion phases occurred at the same time as known warming events in Antarctica. With more vertical mixing in the Southern Ocean, deep waters that had stored carbon for long periods were brought closer to the surface, allowing that carbon to escape into 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, destabilizing the existing water-mass structure and enhancing exchanges between deep and surface waters."
Previously, many scientists assumed that changes in the North Atlantic, particularly the formation of North Atlantic Deep Water (NADW), were the main drivers of shifts in deep-water circulation in the South Atlantic. The new results suggest that this northern influence was more restricted than earlier thought. Instead, the replacement of a glacial, carbon-rich deep-water mass by newly formed Antarctic Bottom Water appears to have been crucial for the rise in atmospheric CO2 toward the end of the last Ice Age.
Southern Ocean heat, Antarctic ice loss and today's climate
"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."
Because of its vast size and unique circulation, the Southern Ocean plays a major role in controlling the global climate. Over the past 50 years, waters deeper than about 1,000 meters around Antarctica have warmed significantly faster than much of the rest of the world's oceans. To work out how this rapid deep-ocean warming affects the ability of the ocean to absorb and release carbon dioxide, scientists must track physical and biogeochemical changes over long timescales and incorporate them 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."
Paleoclimate 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.