By analyzing unusual rock samples collected years ago in Antarctica, scientists at the University of California, Santa Cruz, have discovered an impressive record of how the East Antarctic ice sheet responded to changes in climate over 100,000 years during the late Ice Age.
The East Antarctic Ice Sheet is the largest ice mass in the world. Understanding their sensitivity to climate change is critical to efforts to project the amount of sea level rise as global temperatures rise. Recent studies suggest that it may be more susceptible to ice loss than previously thought.
The new study Posted on September 15 in Nature Connections, Evidence for changes in the ice sheet base over a wide area in response to periodic changes in climate during the Ice Age. Changes in the types of minerals deposited at the base of the ice sheet are reflected.
“One of the key findings is that the ice sheet was responding to temperature changes in the Southern Ocean,” said co-author Terence Blackburn, associate professor of Earth and planetary sciences at the University of California, Santa Cruz. “Warm water eats away at the edges of the ice sheet and causes the ice to flow faster, and this response reaches deep into the core of the ice sheet.”
The rock samples analyzed in the study consist of alternating layers of chalcedony and calcite that formed as mineral deposits at the base of the ice sheet, recording periodic changes in the composition of subglacial fluids.
“Each layer in these samples is a manifestation of change at the base of the ice sheet driven by changes in the movement of glacial currents,” said first author Gavin Piccione, PhD. Candidate working with Blackburn at the University of California.
By dating the layers, the researchers found a surprising correlation between the layers of mineral deposits and the record of polar sea surface temperatures derived from ice cores. Agate is deposited during cold periods, and calcite during warm periods.
said co-author Slawek Tulaczyk, a professor of Earth and planetary sciences at UCSC who has been studying the behavior of ice sheets and glaciers for decades.
Climate cycles that correspond to the mineral strata are relatively small fluctuations that occur every few thousand years during the more pronounced interglacial cycles that occur every 100,000 years or so throughout the Ice Age. The interglacial cycles are primarily driven by changes in the Earth’s orbit around the Sun. Smaller millennium-scale climate cycles involve fluctuations in polar temperature driven by the weakening and strengthening of a major ocean current (the Atlantic Meridian Inversion Circulation, or AMOC) that transports large amounts of heat northward across the Atlantic.
Tulaczyk said the new findings reveal the sensitivity of the Antarctic ice sheet to small, short-term climate fluctuations.
“Despite the importance of the Antarctic ice sheet – it is responsible for nearly 17 meters of sea level rise since the last ice cap – we really know very little about how it responds to climate variability,” he said. “We know the last 20,000 years pretty well, but then we were almost blind. That’s why these results were so amazing. People have been banging their heads against the wall over this for decades.”
The rock samples analyzed for this study were collected from Moraine glaciers separated by more than 900 kilometers (560 mi), and formed over different periods covering a total of more than 100,000 years. In other words, they record similar cycles of mineral deposition under ice that occur over a wide area and over long periods of time.
“The chemistry of the two samples matched, although they came from a very far distance, which gave us confidence that a large-scale systematic process was going on,” Piccione said.
Layers of opal and calcite
The mechanism behind the formation of layers of opal and calcite is somewhat complex and requires an understanding of not only the mineral chemistry but also the unusual hydrology under the Antarctic ice sheet. Heat from the Earth’s interior (“geothermal heating”) causes melting at the base of the ice sheet, insulated from freezing polar temperatures by the thickness of the ice. As the ice becomes thinner toward the edges of the ice sheet, the subglacial meltwater begins to refreeze, concentrating the dissolved minerals and eventually forming highly saline brine.
Mineral deposits form when water is concentrated by refreezing, and the first thing that precipitates is calcite, the most common form of calcium carbonate. The opal (amorphous silica) will eventually precipitate from old supersaturated brines that have no carbon remaining in them.
“Antarctica has these interesting carbonless brines, because they were all deposited earlier, so when these brines are isolated from other water sources, they form opals,” Piccione explained.
To obtain a layer of calcite over garnet requires an influx of carbon-containing glacial meltwater, which occurs during warm periods in climate cycles, when the AMOC slows. This warms the Southern Hemisphere and brings warm water to the floating ice shelves at the edges of the ice sheet. As warm water erodes the bottom of the ice shelves, the “land line” where the ice comes into contact with the ground begins to retreat and the ice flows more rapidly from the interior to the edges.
Tolaczyk explained that the movement of ice over the bedrock generates heat, which increases the amount of meltwater at the base of the ice sheet. “If you imagine a map of where the meltwater is under the ice sheet, that area expands in warm periods and contracts in cold periods, like the heartbeat,” he said.
The resulting “freeze flow cycles” at the base of the ice explain the alternating layers of garnet and calcite in the rocks.
Southern Ocean temperatures
The results suggest that water temperatures in the Southern Ocean are the main mechanism driving the response of the Antarctic ice sheet to changes in global climate. The temperatures in Antarctica are so cold that even a few degrees of warming wouldn’t cause the surface to melt, Blackburn said, yet scientists know that the ice sheet has melted in the past and parts of it have collapsed. “It was hard to understand, but it clearly shows that ocean warming is the driving mechanism,” he said.
“If you look at the places that are losing ice today, they are concentrated along the edges of the ice sheet where they are in contact with the warm ocean,” Tolachik added. “The primary driver of ocean warming now is atmospheric carbon dioxide, not AMOC, but I don’t think the ice sheet cares about what’s causing the warming.”
Tulaczyk said the results show that the ice sheet can retreat during warm periods and then recover during subsequent cooling. He said, “In the context of the threshold question – is the ice sheet sitting on a sill then there can be runaway melt and it will disappear – that’s not what I’m seeing here.” “Ice is sensitive to these short-term fluctuations, but the magnitude of ice loss is small enough that it can recover with cooling.”
In addition to Piccione, Blackburn, and Tulaczyk, co-authors of the paper include Mathis Heine, Chloe Tingloff, and B. Cheney at the University of California, Santa Cruz; Troy Rasberry and Paul Northrup of Stony Brook University; de Ibarra and Katharina Mithner at UC Berkeley; and Kathy Licht of Indiana University-Purdue University Indianapolis. This research was funded by the National Science Foundation.