This suggests that ice retreated deep into the continent after the end of the last ice age and re-advanced before modern retreat began, concludes the study, published in AGU Advances . It presents the first geological constraint for the location and the movement of the ice sheet since the last ice age.
"In the last few thousand years before we started looking, the ice in some parts of Antarctica retreated and advanced again over a much larger area than we previously appreciated," Ryan Venturelli, a paleoglaciologist at the University of Antarctica, said in a statement. Colorado School of Mines and lead author of the new study. "The ongoing retreat of the Thwaites Glacier is much faster than we have seen before, but in the geological record, we see that the ice can recover," he noted.
The grounding line is where a glacier or ice sheet leaves land and begins to float on water as an ice shelf. Today, the Ross Ice Shelf extends hundreds of kilometers over the ocean from the grounding line of the West Antarctic Ice Sheet. Because ocean water washes against the leading edge of the ice, the grounding line can be an area of rapid melting.
"The concern about land ice loss is because land ice loss is what contributes to sea level rise," Venturelli said. "As grounding lines retreat inland, the more vulnerable the ice sheet becomes, as it exposes increasingly thicker ice to ocean warming."
During the Last Glacial Maximum, about 20,000 years ago, the West Antarctic ice sheet was so large that it sat on the ocean floor, beyond the edge of the continent. Previous observations generally indicate a steady retreat since then, accelerating in the last century due to human-caused climate change.
The question for Venturelli was how far inland the ice sheet had retreated after the last ice age. Without knowing that, it's difficult to predict how sensitive the Antarctic ice sheet is and how it will respond to further change. climate.
A lake about twice the size of Manhattan buried under a kilometer of ice and isolated from today's atmosphere held clues to the answer. To reach it, Venturelli and his team carefully made their way with a hot water "drill." that had access, they took samples of lake water and carbon-filled sediments from the lake bed. Using radiocarbon dating, they found that the carbon was about 6,000 years old.
Because the radiocarbon (carbon-14) in these sediments must have come from seawater, the find suggests that what is now a lake 150 kilometers from the edge of the modern ice was the ocean floor. When the ice advanced, capped the lake, preserving the carbon as part of the lake bottom sediments. And based on the radiocarbon in the water sample, the grounding line could have been 100 kilometers even further inland at that time.
"When we set out to sample this lake, we weren't sure what we would find about the history of the ice, but the fact that deglaciation persisted this far inland was not such a far-fetched possibility," Venturelli said. "This area of West Antarctica is really flat. There is nothing to stop the grounding line from receding.“There are no real topographical boundaries.”
The new evidence of the Antarctic ice's ability to re-advance was good news for Venturelli.
"Sometimes it can be a pain to study ice loss in Antarctica," he said .
