The glaciers of the West Antarctic Ice Sheet are melting at an increasing rate as a result of climate change. A look at the formation time of the ice sheet 35 million years ago now provides important information that can help to better estimate future developments: Using sediment analyzes researchers have shown that even then warm ocean currents played an important role and the spread of the ice despite falling global temperatures. Today they could accelerate the melting of glaciers.
The kilometer-thick Antarctic ice sheet comprises the largest mass of ice on earth – its potential influence on future sea level developments is correspondingly large. A particular focus of climate research is on the West Antarctic Ice Sheet, which stretches from the Antarctic mainland to the Amundsen Sea. The glaciers in this region are among the fastest declining in the world – with serious consequences for sea levels. Should the West Antarctic Ice Sheet collapse completely, sea levels are projected to rise by more than three meters.
Better forecasts thanks to knowledge of the past
“The stability of the West Antarctic ice sheet is of crucial importance for the future development of global sea levels,” says Gabriele Uenzelmann-Neben from the Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research in Bremerhaven. “That’s why researchers all over the world are working on using computer models to predict the future behavior of the ice in a warmer world. The more one knows about the past of the West Antarctic Ice Sheet, the more accurate these are. Its recent history is well documented. However, the early period – especially the educational phase – is largely unknown.”
Together with her team, Uenzelmann-Neben therefore took a look at the past of this ice sheet. To do this, the researchers examined the sediments in the area of the Pine Island trough. This channel-like incision begins in the shallow area of the Amundsen Sea on the sea floor and runs north to south directly towards the West Antarctic coast. This is where Pine Island Glacier meets the sea, transporting more ice into the sea every year than any other glacier in the world. Its touchdown line – i.e. the boundary at which the ice is still in contact with the sea floor – retreats more and more towards land and larger and larger parts are undermined by water.
Uneven sediment
The researchers used artificially generated seismic waves to explore the layers of seabed that have formed in this area over millions of years. Sent out by a research ship, these spread out to below the sea floor. They are reflected at certain layer boundaries, such as the boundary between sediment and rock. Measuring devices register the travel time of the seismic waves and, on this basis, enable conclusions to be drawn about the inner structure of the seabed.
The result: A large body of sediment is present on the eastern flank of the Pine Island trough that is missing on the opposite western side. From the researchers’ point of view, the only plausible explanation for this asymmetric deposition is that at the time the ice sheet formed, there was a deep water current moving from north to south towards the coast. The Coriolis force caused by the rotation of the earth ensured that the sediment was only deposited on one side. “This is only possible if the ocean circulation at the time of deposition was similar to that of today,” says Uenzelmann-Neben. “And similar to today, the deep water rising through the trough must have been relatively warm back then.”
Great influence of ocean currents
In addition, the researchers used sediment cores from the same area and examined the pollen contained therein. From these analyses, they determined the age of the sedimentary base to be between 34 and 36 million years. During this time – at the border of Eocene and Oligocene – the Drake Passage between South America and Antarctica was created by continental drift and thus cleared the way for the Antarctic Circumpolar Current. This ring current, which once circled Antarctica, is still characteristic of the conditions in the Southern Ocean today. At the same time, temperatures dropped worldwide and the Antarctic continent froze. Despite these climate changes, however, the deep water current off West Antarctica delayed the formation of the ice sheet there.
“Our study is a strong indication that warm deep water in the area of the Amundsen Sea shelf was upwelling at the time of the great glaciation and delayed the advance of the West Antarctic ice sheet onto the sea,” explains Uenzelmann-Neben. “This important and surprising finding underscores the enormous importance that ocean currents had in the formation phase of the West Antarctic Ice Sheet and still have today. With the additional knowledge about the early phase of the ice sheet, the forecasts of its future stability and ice retreat can now also be improved.”
Source: Gabriele Uenzelmann-Neben (Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research, Bremerhaven) et al., Communications Earth & Environment, doi: 10.1038/s43247-022-00369-x