Record of Antarctic ice sheet’s response to climate cycles found in rock samples – Raised by that?
The influence of the global climate cycle on Southern Ocean temperatures has prompted a cycle of melting and freezing in East Antarctica every few thousand years, according to a new study.
UNIVERSITY OF CALIFORNIA – SANTA CRUZ
CREDIT: GAVIN PICCIONE
By analyzing unusual rock samples collected years ago in Antarctica, scientists at the University of California, Santa Cruz, have discovered a remarkable record of how the East Antarctic Ice Sheet response to climate changes over a period of 100,000 years during the Late Pleistocene.
The East Antarctic Ice Sheet is the largest ice sheet in the world. Understanding how sensitive it is to climate change is crucial to efforts to forecast how much sea levels will rise as global temperatures rise. Recent studies suggest it may be more susceptible to ice loss than previously thought.
new research, published September 15 in Nature Communications, which provides evidence of ice-bottom changes over a large area in response to cyclical changes in climate during the Pleistocene. The changes are reflected in the types of minerals deposited at the bottom of the ice sheet.
“One of the key findings is that the ice sheet responded to temperature changes in the Southern Ocean,” said co-author Terrence Blackburn, associate professor of Earth and Planetary Sciences at UC Santa Cruz. “Warm water eats into the edges of the iceberg and makes the ice flow faster, and that reaction goes deeper into the center of the iceberg.”
The rock samples analyzed in the study included alternating layers of opal and calcite that formed as mineral deposits at the bottom of the ice sheet, recording cyclical changes in the composition of the subglacial fluids.
“Each layer in these samples is indicative of changes in the bottom of the ice sheet caused by changes in the motion of the ice streams,” said first author Gavin Piccione, PhD. candidate working with Blackburn at UCSC.
By dating the layers, the researchers found a striking correlation between the layers of mineral deposits and the record of polar sea surface temperatures derived from ice cores. Opal is deposited during cold periods, and calcite during warm periods.
Co-author Slawek Tulaczyk, professor of Earth and Planetary Sciences at UCSC, who has studied the behavior of ice sheets and glaciers for decades.
Climate cycles consistent with mineral layers are relatively small fluctuations occurring every few thousand years during the more pronounced glacial-interglacial cycles occurring every 100,000 years or so throughout the Pleistocene . Glacial-interglacial cycles are driven primarily by changes in the Earth’s orbit around the sun. Smaller millennium-scale climate cycles involve fluctuations in polar temperatures due to the weakening and strengthening of a major ocean current (Atlantic Overturning Circulation, or AMOC) that circulates. transfers large amounts of heat northward across the Atlantic Ocean.
Tulaczyk said the new findings demonstrate the sensitivity of the Antarctic Ice Sheet to small, short-term climate fluctuations.
“The Antarctic ice sheet is just as important – it’s been responsible for nearly 17 meters of sea level rise since the last glaciation – we really know very little about how it reacts,” he said. with climate change,” he said. “We know pretty well about the last 20,000 years, but beyond that, we’re pretty much clueless. That’s why these results are so impressive. People have been banging their heads against the wall for this for decades. “
The two rock samples analyzed for this study were collected from glaciers more than 900 kilometers (560 miles) apart, and they formed over different periods totaling more than 100,000 years. In other words, they record similar cycles of mineral deposition beneath the ice over a large area and over long periods of time.
“The chemistry of the two samples is consistent, even though they come from very far apart, which leads us to believe that some large-scale, systematic process is at play,” says Piccione.
The mechanism behind the formation of the opal and calcite layers is somewhat complex and requires an understanding not only of the mineral chemistry but also of the unusual hydrology beneath the Antarctic Ice Sheet. Heat from the Earth’s interior (“geothermal heating”) causes melting at the base of the ice sheet, which is insulated from icy polar temperatures by the thickness of the ice. As the ice thins towards the edge of the ice sheet, the meltwater beneath the ice begins to freeze, gathering dissolved minerals and eventually forming super alkaline brine.
Mineral deposits form when water becomes concentrated by refiltration, and the first thing to precipitate is calcite, the most common form of calcium carbonate. Opals (amorphous silica) will eventually precipitate from older, oversaturated brines with no carbon left in them.
“Antarctica has these interesting brines without the carbon, because it all precipitates earlier, so when those brines are isolated from other water sources, they form,” explains Piccione. opal.
To get a calcite layer on top of the opal requires a stream of carbonaceous water, which occurs during warm periods in the climate cycle, when the AMOC slows down. That leads to warming in the Southern Hemisphere and brings warm water into contact with icebergs floating at the edge of the ice sheet. As warm water eats away at the bottom of the ice shelves, the “landing line” where the ice comes in contact with the soil begins to recede and the ice flows faster from the inside to the edge.
Tulaczyk explains that the movement of the ice across the platform generates heat, which increases the amount of meltwater at the bottom of the ice sheet. “If you imagine a map of where meltwater is under the ice, that area expands during warm periods and contracts during cold periods, like a heartbeat,” he said.
The result are “freezing cycles” at the bottom of the ice that create alternating layers of opal and calcite in the rock.
The finding points to water temperature in the Southern Ocean as a key mechanism driving the response of the Antarctic Ice Sheet to changes in global climate. Antarctica’s temperatures are so cold that warming a few degrees won’t melt the ice on the surface, but scientists know that the ice sheet has melted in the past and parts of it have collapsed, Blackburn said. “It’s confusing, but this clearly shows that ocean warming is the driving mechanism,” he said.
Tulaczyk added: “If you look at places that are losing ice today, they’re concentrated along the edges of the ice sheet, where it’s in contact with the warming ocean. “The main driver of ocean warming right now is atmospheric carbon dioxide, not AMOC, but I don’t think the ice sheet is of interest to what causes warming.”
Tulaczyk said the findings suggest the ice sheet may recede during a warm period and then recover during subsequent cooling. “In the context of the threshold question – whether the iceberg is at a threshold beyond which it will be able to melt and all go away – that is not what I see here,” he said. “Ice is sensitive to these short-term fluctuations, but the extent of the ice loss is small enough that it can recover from cooling.”
In addition to Piccione, Blackburn, and Tulaczyk, the paper’s co-authors include Mathis Hain, Chloe Tinglof, and B. Cheney at UC Santa Cruz; Troy Rasbury and Paul Northrup at Stony Brook University; DE Ibarra at UC Berkeley and Brown University; Katharina Methner at Stanford University; and Kathy Licht at Indiana University-Purdue University Indianapolis. This research was funded by the National Science Foundation.
JOURNEYS
Nature Communications
DOI
ARTICLE TITLE
Sub-glacial precipitation records Antarctic ice sheet response to late Pleistocene millennium climate cycle
ARTICLE PUBLICATION DATE
September 15, 2022
REPORT REPORT
The authors declare no competing interests.
Related
