New research has found that 6,000 years ago, the edge of the West Antarctic Ice Sheet may have been grounded as much as 250 kilometers inland from its current location, suggesting a deep retreat of the ice after the end of the last ice age. The discovery further suggested, the researchers say, that the ice sheet gained ground before the modern retreat began.
”The ongoing retreat of the Thwaites Glacier is much faster than we’ve ever seen before, but we see in the geological record that the ice can recover,” said Ryan Venturelli, a paleoglaciologist at the Colorado School of Mines, US. author of the new study.
The grounding line is where a glacier or ice sheet leaves solid ground and begins to float on water as an ice shelf. Today, the Ross Ice Shelf extends hundreds of miles out into the ocean from the grounding line of the West Antarctic Ice Sheet. As ocean water washes the leading edge of the ice, the grounding line may be a zone of rapid melting.
”The concern about grounded ice loss is because ice loss on land contributes to sea level rise,” Venturelli said. ”As the grounding lines retreat further inland, the more vulnerable the ice sheet becomes because it exposes thicker and thicker ice to the warming ocean.
About 20,000 years ago, during the Last Glacial Maximum, the West Antarctic Ice Sheet was so large that it was grounded at the bottom of the ocean, beyond the edge of the continent. Previous observations generally indicate a steady retreat since then, accelerated in the last century by human-induced climate change.
The question for Venturelli was how far inland the ice sheet had retreated after the last ice age. Without knowing this, it was difficult to predict how sensitive the Antarctic ice sheet is and how it will respond to further climate change.
A lake about twice the size of Manhattan (New York City) buried under a kilometer of ice and sealed off from today’s atmosphere held clues to the answer.
Venturelli and her team carefully melted their way in using a hot water “drill” to extract samples of lake water and carbon-laden sediments from the lake floor. They radiocarbon dated the carbon to about 6,000 years.
Because the radioactive carbon (carbon-14) in these sediments must have come from seawater, the find suggested that what is now a lake 150 kilometers from the edge of the modern ice was the ocean floor.
As the ice advanced, it closed the lake and preserved the carbon as part of the lake floor sediments. And based on radiocarbon in water taken from the same lake, the ground line may have been 100 kilometers further inland at the time.
”When we set out to sample this lake, we weren’t sure what we were going to find out about the history of the ice, but the fact that the deglaciation had persisted so far inland was not so wild,” Venturelli said. ”This area of West Antarctica is really flat. There is nothing to slow down the retreat of the earth line. No real topographical doorstops.” “Although the resurgence seen in the geologic record occurs over thousands of years, I like to think of the study of the reversibility process as a small glimmer of hope,” Venturelli said.
”This work highlights that ice sheets are much more dynamic than we previously appreciated, and we need to explore this idea of reversibility – what were the forcing mechanisms that caused the ice sheet to move again to where it is today? – so we can better predict future scenarios,” Venturelli said.
Written by: Vaishali verma
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