Antarctica’s Pine Island and Thwaites glaciers are dropping ice extra rapidly than they’ve at any time in the previous few thousand years, historic penguin bones and limpet shells recommend.
Scientists are apprehensive that the glaciers, two of Antarctica’s fastest-shrinking ones, are within the means of unstable, runaway retreat. By reconstructing the historical past of the glaciers utilizing the previous bones and shells, researchers wished to search out out whether or not these glaciers have ever been smaller than they’re right now.
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“If the ice has been smaller in the past, and did readvance, that shows that we’re not necessarily in runaway retreat” proper now, says glacial geologist Brenda Hall of the University of Maine in Orono. The new consequence, described June 9 in Nature Geoscience, “doesn’t give us any comfort,” Hall says. “We can’t refute the hypothesis of a runaway retreat.”
Pine Island and Thwaites glaciers sit in a broad ocean basin formed like a bowl, deepening towards the center. This makes the ice susceptible to heat currents of dense, salty water that hug the ocean flooring (SN: 4/9/21). Scientists have speculated that because the glaciers retreat farther inland, they might tip into an irreversible collapse (SN: 12/13/21). That collapse may play out over centuries and lift the ocean stage by roughly a meter.
To reconstruct how the glaciers have modified over 1000’s of years, the researchers turned to previous penguin bones and shells, collected by Scott Braddock, a glacial geologist in Hall’s lab, throughout a analysis cruise in 2019 on the U.S. icebreaker Nathaniel B. Palmer.
One afternoon, Braddock clambered from a bobbing inflatable boat onto the barren shores of Lindsey 1 — certainly one of a dozen or extra rocky islands that sit roughly 100 kilometers from the place Pine Island Glacier terminates within the ocean. As he climbed the slope, his boots slipped over rocks lined in penguin guano and dotted with dingy white feathers. Then, he stumbled on a sequence of ridges — rocks and pebbles that have been piled up by waves throughout storms 1000’s of years earlier than — that marked historic shorelines.
Twelve thousand years in the past, simply because the final ice age was ending, this island would have been fully submerged within the ocean. But as close by glaciers shed billions of tons of ice, the removing of that weight allowed Earth’s crust to spring up like a mattress mattress — pushing Lindsey 1 and different close by islands out of the water, just a few millimeters per yr.
As Lindsey 1 rose, a sequence of shorelines fashioned on the sides of the island — after which have been lifted, one after one other, out of attain of the waves. By measuring the ages and heights of these stranded shorelines, the researchers may inform how rapidly the island had risen. Because the speed of uplift is set by the quantity of ice being misplaced from close by glaciers, this might reveal how rapidly Pine Island and Thwaites glaciers had retreated — and whether or not they had shrunk than they’re right now after which readvanced.
Braddock dug into the pebbly ridges, gathering historic cone-shaped limpet shells and marble-sized fragments of penguin bones deposited when the shorelines fashioned. Back in Maine, he and his colleagues radiocarbon dated these objects to estimate the ages of the shorelines. Ultimately, the researchers dated practically two dozen shorelines, unfold throughout a number of islands within the area.
These dates confirmed that the oldest and highest seaside fashioned 5,500 years in the past. Since that point, up till the previous few many years, the islands have risen at a gradual price of about 3.5 millimeters per yr. This is way slower than the 20 to 40 millimeters per yr that the land round Pine Island and Thwaites is at present rising, suggesting that the speed of ice loss from close by glaciers has skyrocketed because of the onset of fast human-caused warming, after 1000’s of years of relative stability.
“We’re going into unknown territory,” Braddock says. “We don’t have an analog to compare what’s going on today with what happened in the past.”
Slawek Tulaczyk, a glaciologist on the University of California, Santa Cruz, sees the newly dated shorelines as “an important piece of information.” But he cautions in opposition to overinterpreting the outcomes. While these islands are 100 kilometers from Pine Island and Thwaites, they’re lower than 50 kilometers from a number of smaller glaciers — and adjustments in these nearer glaciers might need obscured no matter was occurring at Pine Island and Thwaites way back. He suspects that Pine Island and Thwaites may nonetheless have retreated after which readvanced just a few dozen kilometers: “I don’t think this study settles it.”