It’s one of the great — and unresolved — debates of Antarctic science.
In 1984, a team of researchers from Ohio State University reported on a surprising fossil find: More than a mile above sea level, in Antarctica’s freezing and far inland Transantarctic mountain range, fossilized deposits of tiny marine organisms called diatoms were found in rock layers dated to the Pliocene era, some 2 to 5 million years ago. But how did they get all the way up there? Diatoms, ubiquitous marine microorganisms whose tiny shells coat the ocean floor when they die, don’t show up in high mountain rocks unless something rather dramatic happened long ago to get them there.
So began the debate over this rock formation, dubbed the “Sirius Group” after Mount Sirius, one of the range’s many peaks. It was between the “dynamicists”— who argued that the enormous ice sheet of East Antarctica had dramatically collapsed in the Pliocene, bringing the ocean far closer in to the Transantarctic range, and that subsequent upthrusts of the Earth and re-advances of glaciers had then delivered the diatoms from the seafloor to great heights — and the so-called “stabilists.” To the contrary, these scientists argued, the ice sheet had stayed intact, but powerful winds had swept the diatoms all the way from the distant sea surface into the mountains.
“It became very much split into two camps,” remembers Reed Scherer, an Antarctic researcher at Northern Illinois University. “It got really nasty.” Some researchers even tried to resolve matters by suggesting that a meteorite, and subsequent cataclysms, could account for the odd fossil locations.
But the decades have given way to new research tools and new perspectives. And Scherer has now paired up with two researchers behind what is arguably the hottest (and most troubling) new computer simulation of how Antarctica’s ice behaves in order to revisit the tale of those pesky diatoms. Their solution, published Tuesday in Nature Communications, isn’t good news — for it suggests that large parts of East Antarctica can indeed collapse in conditions not too dissimilar from those we’re creating today with all of our greenhouse gas emissions.
If we steer the Earth back to those Pliocene-type conditions — when sea levels are believed to have been radically higher around the globe — oddly located diatoms will be the least of our problems.
The new study is co-authored by Rob DeConto of the University of Massachusetts, Amherst, and David Pollard of Penn State University, who recently published a new ice sheet model of Antarctica that predicts the ice continent can raise sea levels by nearly a meter on its own during this century. They reached this result by adding several new dynamic ice collapse processes to glacial models that, in the past, had been slow to melt East Antarctica even in quite warm conditions — simultaneously lending weight to the views of the stabilists in the debate over the Sirius fossils, while also seeming to suggest that we needn’t worry about truly radical sea-level rise from Antarctica.
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The result is that in the Pliocene — and especially the mid-Pliocene warm period, when atmospheric carbon dioxide was at about the level where it is now, 400 parts per million, but global temperatures were 1 or 2 degrees warmer than at present — the model not only collapses the entirety of West Antarctica (driving some 10 feet of global sea-level rise) but also shows the oceans eating substantially into key parts of East Antarctica. In particular, the multi-kilometer thick ice that currently fills the extremely deep Aurora and Wilkes basins of the eastern ice sheet retreats inland for hundreds of miles — which would have driven global seas to a much higher level than caused by a West Antarctic collapse alone.
Here’s a figure from the study, showing as much:
From Scherer et al, Nature Communications. The images depicts a computer simulation of what Antarctica may have looked like in the Pliocene era, with West Antarctica gone and major retreat of the ice in the Wilkes Basin (WB) and Aurora Basin (AB) of East Antarctica. The red spots indicate locations where diatoms have been found in Sirius Group rocks in the Transantarctic range. Brown areas depict previously submerged regions that rebounded after ice retreat.
Not only is this the world we could be headed to if global warming continues, but it’s a world that can throw diatoms up into the Transantarctic Mountains, the new study argues. Here’s how that would work.
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At first, in the wake of ice retreats in the Aurora and Wilkes basins, what would be left behind are ocean bays filled with life — and many, many diatoms. But Scherer and his colleagues do not believe that winds simply scooped them out of the water and hurled them to the mountains — living, wet diatoms suspended in water would have been too heavy to travel so far, Scherer says.
So instead, the study postulates another development. After a few thousand years of seas filled with happy diatoms, dying and lining the ocean floor in front of the remnant glaciers of the Wilkes and Aurora basins, the once submerged Earth would slowly rebound in some spots (a process sometimes called “isostatic uplift” or “postglacial rebound”). This would create an archipelago of islands, new landmasses free to rise to the surface now that so much ice has sloughed off their backs.
These islands, then, were the source of the diatoms, the study postulates.
The computer model “did show the ice retreated along the margins of East Antarctica, and isostatic uplift would then expose these areas that become new seaways, and with it would have been highly productive for plankton,” says Scherer. “So you would have been accumulating massive numbers of diatoms across this new basin, and with the loss of the ice, the land flexed upward, became exposed to winds, and the wind carried them to the mountains.”
Scherer notes that his new scenario doesn’t really proclaim either the dynamicists or the stabilists the victors. His view is clearly reliant on a substantial amount of dynamics, but it also doesn’t show the East Antarctica ice retreated nearly as far back as earlier proposals. Nor does it use glacial processes to move the deposited diatoms. Rather, it borrows the stabilist idea of wind-blown transport, albeit only after ice has retreated and land has risen in its wake.
Commenting on this new compromise proposal Monday, one Antarctic researcher praised the work as representing an advance on old ways of thinking. “The paper is a great example of how much [paleo]climate modelling has improved in the last decade[s], particularly in the last few years,” said Simone Galeotti, an Antarctic researcher at the Università degli Studi di Urbino in Italy, by email.
The research also earned praise from David Harwood, one of the original ‘dynamicists’ and now a professor at the University of Nebraska-Lincoln.
“This paper’s integration of climate, ice sheet, and atmospheric models provides interesting new perspective on potential source regions for the Antarctic, marine Pliocene diatoms present in glacial sediments of the Transantarctic Mountains, from interior basins of East Antarctica,” said Harwood in an emailed statement. “Their origin from deglaciated, exposed, rebounded marine basin floors in the Aurora and Wilkes basins is plausible, and the new model-derived wind patterns support their trajectory toward the [Transantarctic Mountains].”
But beyond solving the riddle of the Sirius deposits in the Transantarctic Mountains, the new study speaks to the present moment. After all, the warm Pliocene, with its much higher seas, is one of the key past eras that scientists regularly look to for an analogue for where we are currently driving the planet with our greenhouse gases.
[Why the Earth’s past has scientists so worried about sea level rise]
And thus, the new work suggests that if we keep pushing the system, we’ll not only have to worry about the loss of Greenland’s and West Antarctica’s ice, but also major losses from the biggest ice sheet of them all, East Antarctica.
Scherer, DeConto, and Pollard also have a fourth author on the study, the noted Penn State glaciologist Richard Alley, who has become more and more outspoken of late about his concerns that the world’s great ice sheets could be unstable. In a media statement accompanying the study’s release, Alley had this to say:
This is another piece of a jigsaw puzzle that the community is rapidly putting together, and which appears to show that the ice sheets are more sensitive to warming than we had hoped. If humans continue to warm the climate, we are likely to commit to large and perhaps rapid sea-level rise that could be very costly. No one piece of the puzzle shows this, but as they fit together, the picture is becoming clearer.
In other words, solving this key scientific problem from Antarctica’s past turns out to immediately raise major concerns about its future.
“We have now reached a point where atmospheric CO2 levels are as high as that during the Pliocene, 400 ppm, when geological evidence and new model results suggest substantial retreat of the EAIS [East Antarctic Ice Sheet] margin into interior basins. These perspectives bear fundamentally on predictions of future EAIS behavior,” said Harwood by email.
Granted, on a scientific and individual level, there’s also the satisfaction of finally being able to unify quite a lot of information into an explanation that fits the data and also matches our growing present day understanding of Antarctic vulnerability.
“Personally, I find the story rather cathartic, because it does explain the observations, I think, in a much better way than had been done before,” says Scherer.