April 2020, ScientificAmerican.com 57hundreds of meters of water, fed by streams tumbling
down the mountains. This idea is incomplete, however,
because the valleys are open on their seaward ends, with
nothing to hold in such deep water.
Scientists surmise that sometime during a previous
ice age, the West Antarctic Ice Sheet had advanced hun-
dreds of kilometers farther north than it currently sits,
approaching the mountains and damming the mouths
of the valleys at the sea, allowing big lakes to form. One
of them, Glacial Lake Washburn, was at least 300
meters deep.
During the 1990s Brenda Hall, a geologist at the Uni-
versity of Maine, dug into the ancient sediments high
up on the Lake Washburn valley wall and collected
freeze-dried tatters of algal mats that had grown there.
Using radiocarbon dating, she estimated that the algae—
and hence the lake—had existed 23,000 to 13,000 years
ago, at roughly the pinnacle of the last ice age. This find-
ing led to a curious contention, Hall says: during the
ice age, the thinking went, “the glaciers were probably
melting more than they do now.”
Scientists have strained to explain how that could
happen because the climate was colder. One theory is
that the surrounding oceans were more widely covered
in ice than they are today—leading to less evaporation
and therefore fewer clouds, less snowfall and more sun-
light warming the dark rocks of the mountains. This, in
turn, would cause more melt high up. This increased
melt could have happened along the entire length of the
mountains, including where Tullbergia was found.
Closely related is a strange phenomenon that scien-
tists now call the solid-state greenhouse effect. Most sun-
light that strikes a glacier is reflected by its snowy exte-
rior. But in the Transantarctic Mountains, where hard,
dry winds slowly evaporate snow and ice, glaciers often
have deep, relatively transparent ice exposed on the sur-
face. Sunlight can penetrate a meter into this ice, warm-
ing and melting it from within. Andrew Fountain, a gla-
ciologist at Portland State University, has found that this
can occur at air temperatures down to –10 degrees C.
Hall has witnessed this phenomenon high in the
southern mountains, as far as 200 kilometers south of
Shackleton Glacier. “I’ve seen on sunny, clear days,” she
says, “these films of water creeping down the front of
the ice cliff.”
To Hogg and Adams, these mechanisms offer impor-
tant clues into how Tullbergia and Antarctophorus, as
well as small worms, mites and other animals, might
have survived dozens of ice ages along the edges of gla-
ciers such as Shackleton. Adams calls them “Goldilocks
habitats”—north-facing (sun-facing) hollows with just
the right configuration of dark rocks and transparent
ice. Along the edge of that ice would be a narrow hab-
itable band, maybe just a few meters wide, where slight,
occasional meltwater could flush the soil of salts and
also help critters rehydrate, “at least every so many
years,” Adams says. As an ice age moved in, gradually
pushing ice farther up the slopes, Tullbergia could have
slowly moved upslope as well, maybe just a meter a year,
if it was lucky enough to encounter Goldilocks habitats
along the way.
These explanations sound plausible but are unfin-
ished. Hogg and Adams, neither of whom has been back
to Shackleton Glacier, need to connect the genetics to
a clearer time line of how Antarctica’s ice has waxed
and waned. They also need to see if the pattern holds
for other species. They and their students are now try-
ing to sequence DNA from the same cytochrome gene
in a species of mite and a species of nematode worm
they found at Shackleton Glacier and at other locations
around the southern Transantarctic Mountains. They
hope that the genetic sequences will help explain how
long these other animals have lived here, how they
moved around in the past and how they stayed alive.
What is already apparent is that some species sur-
vived by the thinnest of margins. During glacial retreats,
they could have established new outposts on nearby
mountains. But with each new ice age, most of the pop-
ulations died off. Tullbergia bears the scars of that bru-
tal history in its DNA. The fact that every individual
from around Shackleton Glacier carries virtually iden-
tical gene sequences suggests that at some point in the
past, as few as two of the animals managed to survive.
Every representative alive today is descended from
those progenitors, which may have been lucky enough
to be blown by a windstorm onto a patch of Goldilocks
ground the size of a basketball court. Tullbergia “came
extremely close to extinction,” Adams says.
Of course, entire communities of plants and animals
have disappeared from Antarctica, part of the waves of
extinctions that have occurred across Earth’s history.
Would a warmer, wetter Antarctica help Tullbergia
rebound? Adams was back in the McMurdo Dry Valleys
in January. Lake levels are rising, dry soils are getting
moister and numbers of small animals such as certain
nematode worms that live in the ground are increasing.
At the same time, animals that have survived the real-
ly cold, dry, harsh soils “are decreasing in abundance,
and their range across the landscape is contracting,”
Adams says. Perhaps newcomers are crowding out the
old hangers-on.
The question is whether Tullbergia will suffer a sim-
ilar fate. “Based on what they’ve done in the past, my
guess would be that they’d do quite well,” Adams says.
“Just so long as they don’t have to compete with inva-
sive species.”MORE TO EXPLORE
Nematodes in a Polar Desert Reveal the Relative Role of Biotic Interactions in the Coexistence of
Soil Animals. Tancredi Caruso et al. in Communications Biology, Vol. 2, Article 63; February 2019.
Spatial and Temporal Scales Matter When Assessing the Species and Genetic Diversity of Springtails
(Collembola) in Antarctica. Gemma E. Collins et al. in Frontiers in Ecology and Evolution, Vol. 7, Article
76; March 2019.
FROM OUR ARCHIVES
Life at Hell’s Gate. Douglas Fox; July 2015.
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