(CN) — Scientists recently discovered that the East Antarctic ice sheet — the world’s largest reservoir of freshwater — retreated 435 miles during a warming period in which it was previously believed to have been stable.
A study published Wednesday in the journal Nature examines the substantial ice loss that occurred in the East Antarctic ice sheet around 400,000 years ago, during an interglacial period known as the Marine Isotopic Stage 11. During that time, the average global temperature hovered between 1.8 degrees and 3.6 degrees Fahrenheit warmer than today, causing sea levels to rise 19 to 43 feet above their current levels.
The researchers studied the Wilkes Basin — a bowl-shaped region at the edge of an ice sheet considered particularly vulnerable to melting in our own time because it sits below sea level.
The Wilkes Basin holds enough ice to cause sea levels to rise between 10 to 13 feet were it to melt. To put that into perspective, a 10-foot rise in sea levels would wipe out nearly 29,000 square miles of coastline in the United States alone and destroy the homes of more than 12 million people. The largest affected U.S. population centers would be in New York, New Orleans and Miami.
The team led by Terrence Blackburn, assistant professor of Earth and planetary sciences at University of California, Santa Cruz, employed a new technique to measure uranium isotope levels in mineral deposits, providing a glimpse into how water flowed beneath Antarctica’s glaciers and formed these ancient ice sheets.
The accumulation of uranium-234 in subglacial fluids from the surrounding rock is influenced by several factors, including sediment grain size, uranium content and porosity. U-234 typically gets diluted in large bodies of water, however, water beneath an ice sheet is forced to creep along slowly, allowing the isotope to concentrate there over time.
Blackburn explained that the ice sheet insulates the layer nearest the ground, allowing Earth’s heat to melt it, while the thinner ice around the periphery creates a frozen border.
“Water flowing beneath the ice starts refreezing at the edges, which concentrates all the dissolved minerals until it becomes supersaturated and the minerals precipitate out to form deposits of opal or calcite,” Blackburn said in a statement. “Those deposits trap uranium-234, so we can date the deposits and measure their composition, and we can track that through time to get a deep history of the composition of water under the ice sheet.”
Researchers discovered a change in ocean water composition caused when a uranium-enriched reservoir within the Laurentide ice sheet was flushed into the ocean 400,000 years ago in advance of large-scale glacial melting. That flushing reset the concentration of U-234 in the nearby ice, which began accumulating again shortly after the ice resumed its advance.
Blackburn said a modern-day analog of U-234 accumulation in subglacial fluids is found in the McMurdo Dry Valleys, the largest ice-free region of Antarctica, where highly concentrated brines exist today. Because the glaciers terminate onto land, the subglacial brines flow out onto the surface and pool into lakes. The researchers believe the brines of the McMurdo Dry Valleys formed long after the Miocene period ended 2.6 million years ago and originated when subglacial fluids within the Wilkes Basin were concentrated by subglacial freezing.
“The isotopic compositions of those brines are comparable to the precipitates that we’ve dated from a range of locations, and they all share the characteristic U-234 enrichment,” Blackburn said. “The brines are what’s left when the subglacial fluids get all the way to the edge of the ice sheet.”
Data collected by the authors show the grounding line in the Wilkes Basin retreated 435 miles inland during the previous interglacial period, which is comparable to the results expected from modern-day levels of greenhouse gas emissions.
A grounding line is the point where glacial ice loses contact with the ground and begins to float, which shifts around based on the rate of ice loss. Greenland and West Antarctica together accounted for over 30 feet of the sea level rise during that period.
“We’ve opened the freezer door, but that block of ice is still cold and it’s not going anywhere in the short term,” said Blackburn. “To understand what will happen over longer time scales, we need to see what happened under comparable conditions in the past.”