End and beginning linked: The sensitive reaction of West Antarctica to even slight warming has deep-rooted origins. This part of Antarctica did not freeze 34 million years ago, together with East Antarctica, but only about seven million years later, according to analyses of Antarctic sediment drill cores. Accordingly, even back then, West Antarctica reacted more sensitively to elevated CO2 levels, similar to today. This also reveals a lot about its future behavior.
A kilometers-thick ice sheet covers Antarctica today, but it wasn’t always like this. Until the end of the Cretaceous period, there were dinosaurs and early birds in Antarctica, and a species-rich rainforest grew at the South Pole. It wasn’t until about 34 million years ago that the climate cooled significantly, and glaciers and ice masses gradually began to cover the previously diverse landscape with fertile valleys, gorges, mountains, and volcanic areas.
First drill cores from the bottom of the Amundsen Sea
However, where and when exactly the glaciation of Antarctica began was pure speculation until now. A research team led by Johann Klages from the Alfred Wegener Institute for Polar and Marine Research (AWI) has now gained more clarity. The team succeeded for the first time in extracting a drill core from the highly compacted, dense seabed of the West Antarctic Amundsen Sea during an expedition with the research vessel Polarstern. Using a special drilling device, they took two drill cores from two depressions in front of the two large glaciers, Pine Island and Thwaites.
The analyses revealed surprising results: Contrary to expectations, the approximately 34-million-year-old sediment layers of the drill core showed no signs of glaciation. Although global and regional temperatures reached a low point at this time, West Antarctica apparently remained ice-free. Instead, the landscape there was still covered by dense deciduous forests and had a cool-temperate climate, as Klages and his team determined.
West Antarctica Initially Remained Ice-Free
“This completely overturns our knowledge of the first Antarctic glaciation,” says co-author Gerrit Lohmann from AWI. To better understand the geological data and their climatic backgrounds, the researchers conducted a complementary computer simulation in which they reconstructed the climatic conditions during the Oligocene. The researchers determined the temperatures and CO2 levels that could have led to glaciation in various areas of Antarctica.
The results of the simulation confirmed the geological data: “Our reconstructed climate data for the glacial maximum in the early Oligocene reveals a clear pattern of warmer conditions in the southwestern Pacific and along the coast of Wilkes Land,” the researchers report. Accordingly, even during this first glacial maximum, West Antarctica remained ice-free. It wasn’t until about seven million years after East Antarctica had already glaciated that the western edge of the southern continent also froze over.
Where Was the Nucleus of Antarctic Glaciation?
The reconstruction also revealed where the glaciation of Antarctica once began: contrary to what was thought, the “nucleus” of the ice sheet was not in the now particularly cold interior of the continent. Instead, the team identified the coastal region of northern Victoria Land in East Antarctica as the glaciation’s origin. “In a nearby location in the Ross Sea, there is geological evidence of ice reaching all the way to the bottom as early as 33 million years ago,” Klages and his colleagues report.
The favorable local climate is the reason for this surprising coastal glaciation birthplace. In this area of East Antarctica, moist air masses coming from the sea encountered the already 1,500 to 2,000-meter-high Transantarctic Mountains. This forced the air masses to rise; they cooled down and produced heavy snowfall. The globally falling temperatures then caused this snow to remain and form a growing ice cap, rather than melting.
From the northeast coast, the ice sheet then spread further inland, while West Antarctica initially remained ice-free. “It wasn’t until seven million years later that conditions prevailed here under which an ice sheet could form,” explains co-author Hanna Knahl from AWI. “Our results make it clear how cold it had to get to bring the ice advance into West Antarctica, which was already below sea level in many parts.”
Indication of Future Development
However, these new findings also provide valuable insights into the current and future behavior of the West Antarctic Ice Sheet: “Our model shows an ice-free West Antarctica not only for CO2 levels three times higher than pre-industrial values, but also for just twice as high,” Klages and his team report. This could explain why the glaciers and ice masses of West Antarctica are also reacting more sensitively to climate change today than the ice sheet of East Antarctica.
“A slight warming is enough to make the ice of West Antarctica melt again, and that’s exactly where we are right now,” says Klages. “Our multi-proxy data and climate-ice sheet simulation show a highly asymmetric behavior of the Antarctic ice sheet,” the team writes. “This provides essential insights into its response to past and future climate changes.”