April 2020, ScientificAmerican.com 55Mount
SpeedShackletonGlacierTr
an
san
tarctic (^) M
ou
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West
Antarctica
Ross
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McMurdo
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Area enlarged below
0 5 10 miles
0 5 10 kilometers
East
Antarctica
Map by Mapping Specialists
despite the fact that they lived just 10
kilometers apart, the width of the gap
that the glacier flows through. “It’s
quite surprising,” Hogg says. “Five mil-
lion years is a long time.” It appeared
that the species had not traveled at all.
Geologic evidence shows that dur-
ing an especially warm period three
million to five million years ago, the
West Antarctic Ice Sheet collapsed mul-
tiple times. Conceivably, this would
have allowed springtails to float along
the mountain chain as the ocean in-
truded. Springtails could have crossed
the 10-kilometer gap and bred with ge-
netically different springtails there. But
the Antarctophorus populations had
not. The genetic results in Hogg’s lab
also showed that groups of Antarcto
phorus from Shackleton Glacier had
not interbred with another population,
160 kilometers farther north along the
mountains, for at least eight million
years. These results suggested that even
when the West Antarctic ice sheet col-
lapsed, enough ice still remained in the
Transantarctic Mountains to prevent
the animals from moving around.
The analysis of Tullbergia collected
around Shackleton Glacier stunned the
researchers even more: the gene se -
quences from all four sites were virtu-
ally identical. “It’s like they’re all
clones,” Adams says. That could mean
that all the animals are descended from
a couple of individuals and that these
descendants have never bred with any
outside populations. “That is some-
thing that we’re all trying to wrestle
[with] to explain,” Adams says.
TOXIC QUANDARY
How could Tullbergia have persisted
for millions of years, pinned down by
ice during at least 30 ice ages, without
moving more than a few kilometers or
breeding with other populations? This question is all
the more puzzling because for much of that time, these
animals were trapped in a narrow zone between dead-
ly ice and deadly salt.
When Hogg and Adams were helicoptering up and
down Shackleton Glacier back in 2018, they often saw
a faint line running across the sides of the mountains:
A couple of hundred meters above the surface of the ice
the rock changed color, from lighter below the line to
darker above it. These “trimlines” show how high the
ice rose during the last ice age—the result of subtle dif-
ferences in the way that minerals oxidize when they are
exposed to air rather than covered.
It is easy to imagine that as the glaciers thickened,
the animals would have migrated farther up the moun-
tainside, to stay above the ice. But there is a major prob-
lem with that explanation: the upper reaches of the
mountains are loaded with toxic chemicals. Turn over
a rock above the trimline at Shackleton or any other
Transantarctic mountain, and the soil underneath is
often crusted in white salts. “It’s not a good salt. It’s not
Himalayan rock salt,” Adams quips. “Put your tongue
on this stuff, and it will light you up.”
The salt is high in nitrate, toxic to many living
things. Nitrate constantly rains down on Earth as ultra-
violet radiation reacts with atmospheric gases. In most
Home amid the Ice
The Transantarctic Mountains separate the vast East and West Antarctic Ice Sheets,
yet certain glaciers such as Shackleton ooze from east to west through gaps between
peaks. Researchers found the surprising Tullbergia critter on the glacier-facing slope
of Mount Speed ( bottom ). How it got there is a mystery. One
theory is that during warm times, the Ross Ice Shelf disap-
peared, the glaciers receded and sea water
flowed in, moving creatures along the
mountain bases.
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