Questions about the stability of the East Antarctic Ice Sheet are a major source of uncertainty when it comes to estimating sea-level rise as the Earth continues to warm. For decades, scientists have believed that the East Antarctic ice sheet has remained stable for millions of years, but recent research has begun to question this idea. Now researchers from the University of California, Santa Cruz have reported new evidence of significant ice loss in East Antarctica during the interglacial warm period about 400,000 years ago. Details of the study are published in the journal Nature.
Scientists’ attention was drawn to the Wilkes Basin, one of several bowl-shaped basins at the edges of the ice sheet believed to be vulnerable to melting. Here ice lies on land below sea level. The Wilkes Basin currently has enough ice to raise the sea level by 3-4 meters.
Ice flows slowly through the basins from the interior of the continent to floating ice shelves on the outskirts. The loss of ice causes the ground line – the point at which ice loses contact with the ground and begins to float – displaces.
Scientists’ data show that the Wilkes Basin ground line retreated 700 kilometers during one of the last warm interglacial periods when global temperatures were 1-2 ° C higher than now. This probably contributed to the sea level rise by 3-4 meters, and the melting of Greenland and West Antarctica together gave another 10 meters.
In other words, a period of global warming, comparable to what is expected in current scenarios of man-made greenhouse gas emissions, has resulted in a sea-level rise of about 13 meters. Of course, this will not happen right away – it takes time for the ice to melt.
We opened the freezer door, but this block of ice is still cold and isn’t going anywhere in the short term. To understand what will happen on longer timescales, we need to see what has happened under comparable conditions in the past.
Terrence Blackburn is an assistant professor in the Department of Earth and Planetary Sciences at the University of California, Santa Cruz.
The problem with studying interglacial periods in the Pleistocene is that they all ended in another ice age when the ice sheet advanced again and hid evidence. For the new study, Blackburn and his colleagues used a new technique based on isotope measurements in mineral deposits that record past changes in subglacial fluids.
Uranium-234 (U-234) is an isotope of uranium that accumulates very slowly in water that is in contact with rocks due to the high-energy decay of U-238. This happens everywhere, but in most places, hydrological processes carry water away from the sources of enrichment, and U-234 dissolves in large bodies of water. In Antarctica, however, water is held at the bottom of the ice sheet and moves very slowly while the ice is stable, allowing U-234 to rise to very high levels over long periods of time.
Blackburn explained that the ice sheet acts as an insulating blanket so that heat from within the Earth causes it to melt at the base. But temperatures are colder where ice is thinner at the edges of the ice sheet, causing subglacial water to freeze.
Water flowing under the ice begins to freeze at the edges, which concentrates all dissolved minerals until it becomes supersaturated and the minerals precipitate, forming opal or calcite deposits. These deposits also contain U-234, so scientists can date and measure their composition. This gives researchers a more complete history of the composition of the water under the ice sheet.
Eventually, U-234 in the subglacial water in the Wilkes Basin was washed away during the interglacial period 400,000 years ago, when the ice melted and the ground line receded. This drove the concentration of U-234 to low levels, and then the accumulation resumed when the ice increased again.