Scientists have found compelling evidence that a water channel flowed through the West Antarctic ice sheet just 125,000 years ago. The discovery of the prehistoric seaway sheds new light on the stability of the third largest ice mass on the planet which will have a huge impact on global sea levels if it melts.

The research, published in the journal Global Change Biology is the most comprehensive study of its kind. Scientists from the British Antarctic Survey (BAS) pieced together all available data on bryozoan abundance and diversity across Antarctica from the mid nineteenth century to the present. The researchers, part of the Census of Antarctic Marine Life (CAML), initially set out to investigate the origins of the diverse marine fauna living in Western Antarctica. But when comparing two isolated seas they found something puzzling. “When we sampled the Weddell and Ross seas we found incredibly similar fauna,” says David Barnes, marine ecologist at the British Antarctic Survey who led the research team. The two seas are only 1,500 miles apart. But they are separated by the West Antarctic ice sheet, a vast impenetrable block of ice two kilometres deep in its thickest point.

So how could such related fauna develop in these isolated conditions?

Barnes and colleagues set about on a quest to find out. They first turned to biology. “Bryozoans are fixed on the seabed with a very short larval stage. So dispersal from the deep-sea is not the answer,” says Barnes. Another theory was that the fauna had independently evolved similar traits in a process known as convergent evolution. “But the seafloor fauna living in Western Antarctica today is relatively young,” Barnes adds. “During each glaciation event the ice sheets grind the seabed and bulldoze everything in its path. Only pockets of fauna would survive and most would have recolonised during warmer periods after the ice retreated.” For colonization of the same fauna to take place, the researchers concluded that both seas were connected by a trans-Atlantic seaway. “And this would have happened in the recent past perhaps during the last interglacial,” he says.

And the findings may also help interpret how modern ice sheets will change with global warming. “It is vital we understand how stable the West Antarctic ice sheet is and how the ice mass has changed in the past. If part of the ice sheet collapsed during the last interglacial then a substantial amount of ice could have been lost very quickly, contributing to the rise in sea level, which was 5 metres higher than today’s levels,” says Barnes. And with global temperatures currently rising, we should follow with careful interest any indication from the past or present of how the West Antarctic ice sheet may respond and influence sea level.

Image: Kymella polaris, a cheilostome bryozoan and common Antarctic species / British Antarctic Survey