West Antarctica Ice Sheet retreated far inland after end of last ice age, regained since

New Delhi: New research has found that 6,000 years ago, the grounded edge of the West Antarctic Ice Sheet may have been as far as 250 kilometres inland from its current location, suggesting a deep retreating of the ice after end of last ice age.

The discovery further suggested, say scientists, that the ice sheet gained before modern retreat began.

"The ongoing retreat of Thwaites Glacier is much faster than we've ever seen before, but in the geologic record, we see the ice can recover," said Ryan Venturelli, a paleoglaciologist at Colorado School of Mines, US, and lead 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 over the ocean from the grounding line of the West Antarctic Ice Sheet. Because ocean water washes up against the leading edge of the ice, the grounding line can be a zone of rapid melting.

"The concern of grounded ice loss is because the loss of ice on land is what contributes to sea level rise," Venturelli said. "As grounding lines retreat farther inland, the more vulnerable the ice sheet becomes as 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 on the ocean floor, beyond the edge of the continent. Previous observations generally indicate a steady retreat since then, accelerated in the last century by human-caused climate change.

The question for Venturelli was just how far inland the ice sheet had retreated after the last ice age. Without knowing that, it was hard 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 with a hot water "drill" to pull up samples of lake water and carbon-filled sediments from the lake bed. They radiocarbon dated the carbon to be about 6,000 years old.

Because radiocarbon (carbon-14) in these sediments must have come from seawater, the finding suggested that what is now a lake 150 kilometers from the modern ice edge was the floor of the ocean.

When the ice advanced, it capped off the lake, preserving the carbon as part of the lake bottom's sediments. And based on radiocarbon in water sampled from the same lake, the grounding line could have been 100 kilometers even farther inland at that time.

"When we set out to sample this lake, we weren't sure what we would find out about ice history, but the fact that deglaciation persisted this far inland was not that wild of a possibility," Venturelli said. "This area of West Antarctica is really flat. There is nothing to put brakes on the retreat of the grounding line. No real topographical doorstops." "Although the re-advance identified in the geologic record happens over thousands of years, I like to think of studying the process of reversibility as a little shred of hope," said Venturelli.

"This work highlights that the ice sheets are much more dynamic than we previously appreciated, and we need to probe this idea of reversibility - what were the forcing mechanisms that caused the ice sheet to re-advance to where it is today? - so we can better predict future scenarios," said Venturelli.