Due to its thick and vast ice sheet, Antarctica appears to be a continuous land mass centered over the South Pole and spanning both hemispheres of the planet. The western sector of the ice sheet is shaped like a “hitchhiker’s thumb,” an apt simile, because the West Antarctic Ice Sheet is on the move. Affected by warming oceans and atmosphere, this ice sheet is melting, flowing outward, and shrinking in size, all at an astonishing rate.
Much of the discussion about the melting of huge ice sheets in times of climate change focuses on its effects on people. This makes sense: millions will see their homes damaged or destroyed by rising sea levels and storm surges.
But what will happen to Antarctica itself as the ice melts?
In layers of sediment built up on the seafloor over millions of years, researchers like us are finding evidence that when West Antarctica melted, there was a rapid increase in geological activity on land in the area. The evidence anticipates what could happen in the future.
A voyage of discovery
About 30 million years ago, an ice sheet covered much of what we now call Antarctica. But during the Pliocene, between 5.3 and 2.6 million years ago, the West Antarctic ice sheet retreated dramatically. Instead of a continuous ice sheet, only high ice caps and glaciers on or near mountain peaks remained.
About 5 million years ago, conditions around Antarctica began to warm and the western ice diminished. About 3 million years ago, the entire Earth entered a warm climate phase, similar to what occurs today.
Glaciers are not static. These large masses of ice form on land and flow toward the sea, moving over bedrock, scraping material from the landscape they cover and dragging that debris, almost like a conveyor belt. This process is accelerated by climate warming, as is the fragmentation in the sea that forms icebergs. Debris-laden icebergs can transport continental rock material out to sea and deposit it on the seafloor as they melt.
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In early 2019, we joined a major scientific voyage – Expedition 379 of the International Ocean Discovery Program – to the Amundsen Sea, south of the Pacific Ocean. Our expedition sought to recover material from the seabed to learn what happened in West Antarctica during its melting period millions of years ago.
Aboard the drill ship JOIDES Resolution, workers lowered a drill nearly 3,962 meters to the sea floor and drilled 794 meters into the ocean floor, just off the most vulnerable part of the West Antarctic ice sheet.
The drill extracted long tubes called “cores,” which contained layers of sediment deposited between 6 million years ago and today. Our research focused on sections of sediment from the Pliocene, when Antarctica was not completely covered in ice.
An unexpected find
During the trip, one of us, Christine Siddoway, was surprised to discover a rare sandstone pebble in a disturbed section of the core. Sandstone fragments were rare, so the origin of the pebble was of great interest. Tests showed it came from mountains deep inside Antarctica, about 1,300 km from the drilling site.
For this to occur, icebergs had to break off from glaciers flowing from the inland mountains and then float toward the Pacific Ocean. The pebble provided evidence that a deep ocean passage existed – rather than today’s thick ice sheet – through the interior of what is now Antarctica.
After the expedition, when the researchers returned to their laboratories, this finding was confirmed through the analysis of silt, mud, rock fragments and microfossils extracted from the cores. The chemical and magnetic properties of the material revealed a detailed timeline of the ice sheet’s retreats and advances over many years.
A key indicator came from analyzes led by Keiji Horikawa. He attempted to correlate thin layers of mud in the core with continental rocks, to prove that icebergs had transported materials over long distances.
Each layer of mud was deposited just after a deglaciation episode, when the ice sheet receded, creating a clay bed with pebbles carried by icebergs. By measuring the amount of elements such as strontium, neodymium and lead, he was able to link specific layers of the core with chemical signatures in outcrops in the Ellsworth Mountains, 1,400 km away.
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Horikawa discovered not just one case of this material, but up to five layers of mud deposited between 4.7 and 3.3 million years ago. This suggests that the ice sheet melted and formed open ocean, then the ice sheet grew back, filling the interior, repeatedly, over short periods of thousands to tens of thousands of years.
Creating a more complete picture
Fellow Ruthie Halberstadt combined this chemical and chronological evidence into computer models that show how an archipelago of rugged ice-capped islands emerged as the ocean replaced the thick sheets of ice that now fill Antarctica’s interior basins.
The biggest changes occurred along the coast. The simulations show a rapid increase in iceberg production and a dramatic retreat of the ice sheet edge toward the Ellsworth Mountains. The Amundsen Sea was filled with icebergs coming from all directions. Rocks and pebbles embedded in the glaciers floated out to sea inside the icebergs and were deposited on the seafloor as they melted.
Long-standing geological evidence from Antarctica and other parts of the world shows that as the ice melts and flows, the land rises because the ice no longer presses on it. This change can cause earthquakes, especially in West Antarctica, which lies above particularly hot areas of the Earth’s mantle, which can react quickly as the ice covering them melts.
The release of pressure on the land also increases volcanic activity, as is currently happening in Iceland. The evidence in Antarctica comes from a layer of volcanic ash identified in the cores by Siddoway and Horikawa, formed 3 million years ago.
Ice loss and upward movements in West Antarctica also caused massive avalanches and rockslides in fractured terrain, forming glacial valley walls and coastal cliffs. Undersea collapses displaced large amounts of sediment from the marine slope. No longer held back by the weight of the ice and water, huge masses of rock broke loose and fell into the water, producing tsunamis that caused further coastal destruction.
The rapid onset of all these changes turned deglaciated West Antarctica into an example of what has been called “catastrophic geology.”
The sudden increase in activity resembles what has happened in other parts of the planet. For example, at the end of the last ice age in the Northern Hemisphere, 15,000 to 18,000 years ago, the region between Utah and British Columbia suffered flooding from glacial lake calving, land rebound, rock avalanches, and increased volcanic activity. On the coast of Canada and Alaska, these events continue to occur today.
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Dynamic ice sheet retreat
Our team’s chemical analysis of the rocks makes it clear that West Antarctica is not necessarily undergoing a gradual, massive shift from ice to ice-free, but instead oscillates between very different states. Every time the ice sheet disappeared in the past, geological chaos ensued.
The future implication for West Antarctica is that when its ice sheet collapses again, catastrophic events will return. This will happen repeatedly, as the ice sheet retreats and advances, opening and closing connections between different areas of the world’s oceans.
This dynamic future could generate equally rapid responses in the biosphere, such as algal blooms around icebergs, triggering the arrival of marine species into newly opened ocean waterways. Large tracts of land on the islands of West Antarctica could allow the growth of moss and coastal vegetation, making Antarctica greener than its current icy whiteness.
Our data on the past of the Amundsen Sea and the resulting forecast indicate that changes on land in West Antarctica will not be slow, gradual or imperceptible from the human perspective. Rather, what happened in the past will likely repeat itself: rapid geological changes that will be felt locally as apocalyptic events such as earthquakes, eruptions, landslides and tsunamis, with effects around the world.
This article was originally published on The Conversation
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