Record of Antarctic ice sheet response to climate cycles found in rock samples
UCSC researchers analyzed two rock samples collected in Eastern Antarctica in which alternating layers of calcite and opal reflect cyclic changes in the subglacial environment. Sample MA113 is 9 centimeters thick and comes from Mount Achernar Moraine on the side of Law Glacier. Credit: Gavin Piccione

Scientists at the University of California, Santa Cruz have found a remarkable record of how the East Antarctic Ice Sheet has responded to changes in climate over a 100,000 year period.

The world's largest ice mass is located in the East Antarctic. Understanding its sensitivity to climate change is important for estimating how much sea level will rise. It may be more vulnerable to ice loss than was thought.

Evidence of changes at the base of the ice sheet is provided in a new study. There are different types of minerals at the base of the ice sheet.

The ice sheet was responding to temperature changes in the Southern Ocean, according to one of the authors. Warm water eats at the edges of the ice sheet and causes the ice to flow faster and that response reaches deep into the heart of the ice sheet.

The rock samples analyzed in the study were formed as mineral deposits at the base of the ice sheet and recorded changes in the composition of subglacial fluids.

The samples show a change at the base of the ice sheet due to the movement of the ice streams.

The correlation between the layers of mineral deposits and the records of polar sea surface temperatures was found by dating the layers. During warm periods the calcite was deposited.

Slawek Tulaczyk, a professor of Earth and planetary sciences at UCSC, has studied the behavior of ice sheets and glaciers for decades.

The climate cycles that match the mineral layers are relatively small fluctuations that happen every few thousand years. Changes in Earth's position around the sun are what drives the glaciers. The AMOC is a major ocean current that transports heat through the Atlantic Ocean.

The new findings show that the ice sheet is sensitive to short term climate fluctuations.

We don't know a lot about how the ice sheet has responded to climate variability. We know the last 20,000 years, but we haven't been able to see much after that. These results are mind-blowing because of that. The people have been banging their heads against the wall for a long time.

Record of Antarctic ice sheet response to climate cycles found in rock samples
Sample PRR50489 is 3 centimeters thick and was found at Elephant Moraine in the Transantarctic Mountains. Credit: Gavin Piccione

The two rock samples analyzed for this study formed over a period of more than 100,000 years from different glaciers. They record cycles of mineral deposition beneath the ice over a long period of time.

The chemistry of the two samples matched and gave us confidence that a large-scale, systematic process was going on.

The mechanism behind the formation of layers of opal and calcite is a bit complicated and requires an understanding of not only mineral chemistry but also the unusual water underneath the ice sheet. The base of the ice sheet is insulated from the cold by the thickness of the ice. Hypersaline brines are formed where the ice gets thinner toward the margins of the ice sheet.

The first thing to happen when the water is refrozen is the formation of mineral deposits. Older, super saturated brines that don't have any carbon left in them will eventually lead to the formation of oy.

"Antarctica has interesting brines with no carbon in them, because it all precipitated out earlier, so when those brines are isolated from other sources of water they form opal," he said.

When the AMOC slows down, an influx of carbon-laden meltwater is needed to get a layer of calcite on the opal. Warming in the southern hemisphere brings warm water into contact with the floating ice shelves at the edges of the ice sheet. The "grounding line" where the ice contacts land begins to retreat as the warm water eats away at the bottom of the ice shelves.

The amount of meltwater at the base of the ice sheet increases when the ice over the bedrock moves. He said that a map of where there is meltwater under the ice sheet would expand in warm periods and contract in cold periods.

At the base of the ice, there are alternating layers of opal and calcite.

Water temperatures in the Southern Ocean are believed to be the main driver of the response to the ice sheet. A few degrees of warming won't cause the surface of the ice to melt, but scientists know that the ice sheet has collapsed in the past. It's been difficult to understand, but this shows that ocean warming is the main driver.

Tulaczyk said that the places that are losing ice today are concentrated along the edges of the ice sheet where it is in contact with the ocean. I don't think the ice sheet cares what causes the warming because the primary driver is atmospheric carbon dioxide.

The findings show that the ice sheet can retreat during warm periods and then recover. "In the context of the threshold question, is the ice sheet sitting on a threshold beyond which there will be runaway melting and it will all go," he said. The ice can recover with cooling even though it is sensitive to short-term fluctuations.

More information: Gavin Piccione et al, Subglacial precipitates record Antarctic ice sheet response to late Pleistocene millennial climate cycles, Nature Communications (2022). DOI: 10.1038/s41467-022-33009-1 Journal information: Nature Communications