Polynyas are like a window through which the ocean and the atmosphere can interact. A look into the past shows that today's changes in the Antarctic could prevent these from forming.
Hole in the sea ice thickened the Antarctic ice sheet in the past
AWI researchers reconstruct the subsurface ocean temperature and salinity in the Southern Ocean off East Antarctica at high resolution during the last ice age for the first time and show that present-day processes could prevent the formation
The Southern Ocean, which surrounds the Antarctic continent, protects the sea ice from heat of the other oceans by keeping it in its depths. Under certain conditions, however, this heat can reach the surface, where it breaks open large holes in the sea ice. These so-called polynyas are like a window through which the ocean and the atmosphere can interact. They are also hotspots for organisms such as phytoplankton and zooplankton. Researchers at the Alfred Wegener Institute have now been able to reconstruct the subsurface ocean temperature and salinity in the Southern Ocean offshore East Antarctica at high temporal resolution during the last ice age for the first time using a marine sediment core. Their results show that there were repeated polynyas during this period, which were very likely larger than Germany and had a massive impact on the Antarctic Ice Sheet. The study was published in the scientific journal Nature Communications.
The Southern Ocean is layered like a sandwich: At the top, colder, less salty water flows in from melted ice masses, further down, from 200 meters, warm, salty deep-water flows in from the world's oceans and at depths of 1,500 meters and below, the water is cold and salty. Due to their different temperature and salinity, the water masses remain stably stratified and could be mixed by processes such as strong winds or changes in density structure, specifically, an increase in surface-ocean density combined with a decrease in subsurface density (i.e., weakened upper-ocean stratification). Consequently, variations in upper-ocean stratification govern the degree of mixing between surface and subsurface waters. If the warm water from the middle layer makes it to the top, it melts the sea ice from below, creating a hole in the sea ice – so-called polynyas. These can be of huge dimensions: In the 1970s, the Great Weddell polynya (at Maud Rise) left a gaping hole in the Antarctic ice sheet that was the size of Germany.
“Polynyas change the way the ocean and atmosphere interact with each other,” says Tainã Pinho, lead author from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI). “They release heat and moisture into the air and influence the sea ice, ocean circulation and snowfall in the Antarctic.” In his study, the paleoclimate researcher analyzed sediment cores from East Antarctica and found evidence that polynyas formed repeatedly during the last ice age (between 75,000 and 19,000 years ago), which were even larger than those from the 1970s. “Although they cause holes in the sea ice, polynyas may have contributed to the thickening of the edge of the East Antarctic ice sheet in the past through their effect on the climate.”
This is because such an enormous, recurring area of open water in the Southern Ocean, which was otherwise covered with ice, releases large amounts of heat and moisture into the atmosphere. In combination with colder atmospheric temperatures in Antarctica at the time, more snow fell, which accumulated at the edge of the continental shelf, thickening the advancing ice sheet. “What surprised us was that the subsurface ocean was warming while the atmosphere above Antarctica was cooling,” says Tainã Pinho. “We were able to find a possible reason for this antiphase pattern in our sediment core.” With these, the researchers were able to reconstruct the temperature and salinity of the Southern Ocean just a few kilometers away from Dronning Maud Land (East Antarctica) during the last ice age for the first time. “Our record represents both the highest temporal resolution and the southernmost subsurface ocean temperature reconstruction for the last glacial period published to date."
This was made possible by tiny organisms (planktonic foraminifera) and their calcareous shells: These are geochemical archives that contain information about the temperature and salinity of the water in which they lived, as well as about algal blooms at the time. “Our investigations suggest that in the past there were fluctuations in the stratification between the cold surface water and the warmer deep water,” explains Tainã Pinho. This could either have led to an increase in stratification between the upper water masses or to the formation of polynyas. “The fact that the ocean warmed subsurface while the salt and nutrient content increased indicates that the stratification collapsed, warm water reached the surface and caused a polynya.”
This assumption is confirmed by the extraordinary abundance and preservation of planktonic foraminifera, as such favorable conditions along the Antarctic continental margin during an ice age are very rare. “Increased biological productivity indicates that more light was able to enter the ocean at these locations because there was no ice cover.”
A look into the past also shows what could happen in Antarctica today: The mechanisms that allowed the Southern Ocean to release heat and thus thickening the ice sheet during the last ice age are being massively disrupted in modern times by the increasing input of freshwater from the melting ice. This leads to even stronger stratification, which suppresses mixing at depth. As a result, heat is retained underground instead of being released to the surface and into the atmosphere. This water can then intensify the melting of the ice shelf and thus contribute to rising sea levels. “The future extinction of the Weddell Polynya is projected and may locally increase the subsurface marine heat reservoir near East Antarctica.”
“With our study, we are closing a crucial gap in our understanding of the past dynamics of the Southern Ocean,” says Tainã Pinho. “This is important to understand the feedbacks between ice, ocean and the atmosphere, as well as their effects over human and geological time scales.”
Further Information
Behind the Paper by Tainã Pinho
Original publication
Pinho, T.M.L., Nürnberg, D., Nele Meckler, A. et al. Millennial-to-orbital-scale subsurface ocean warming and Polynya formation off Dronning Maud Land during the last glacial.Nat Commun 17, 2440 (2026). https://doi.org/10.1038/s41467-026-70498-w
