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By
Peter Rejcek
As a geologist who studies paleoclimate, Brenda Hall generally
uses glaciers to help her reconstruct climate change through
history.
But the University of Maine researcher and three members
of her team spent the month of January picking through
the beaches between McMurdo Sound and Terra Nova Bay looking
for the remains of long-dead southern elephant seals.
Hall believes the presence of colonies along Victoria
Land as recently as a thousand years ago indicate the
region was warmer than it is today.
“We’re interested in them because they shouldn’t
be there,” Hall said during an interview at McMurdo
Station shortly before heading into the field for five
weeks. “Elephant seals don’t live in the Ross
Sea today.”
The reason they don’t call the Ross Sea region home
is the extensive sea ice cover now locked to the coast.
Simply put: elephant seals don’t care much for sea
ice. But evidence of colonies existing in the not-too-distant
past means there was little or no sea ice during the part
of the year when the elephant seals would normally come
ashore to breed and molt, according to Hall.
The absence of sea ice tells Hall that it was likely warmer
in the relatively recent geologic past. How much warmer,
she can’t say, but evidence suggests the Ross Ice
Shelf was still intact throughout the period that the
colonies existed.
“Basically we’re able to say: it got warmer,
and the Ross Ice Shelf survived,” Hall said. “We
came to realize these elephant seals can give us a climate
indicator.”
The connection is an important one for scientists studying
climate change and attempting to model how Antarctica’s
ice mass will respond to warming temperatures in the next
century. Some research suggests the Ross and the Filchner-Ronne
ice shelves are the keystones preventing the collapse
of the West Antarctic Ice Sheet, Hall said. Their disintegration
would open the floodgates for glaciers to empty out the
marine-based ice sheet.
“If a big ice shelf were to give way, the results
could be catastrophic,” said George Denton, a University
of Maine colleague, in a previous press release from the
university on the published findings, which appeared in
The Proceedings of the National Academy of Sciences in
June.
“Through her discovery of elephant seal remains over
a widespread area where they do not exist today, [Hall]
shows evidence not only that a warming occurred, but that
the Ross Ice Shelf survived that event. It’s important
because it speaks to the staying capacity of the ice shelf
in the face of global warming,” Denton said.
The most recent warming period occurred between 1,000
and 2,500 years ago, Hall noted. “We have pretty
good evidence of that time period,” she said.
A second significant warming might have occurred even
earlier, between 4,000 and 5,000 years ago with evidence
of elephant seal activity as much as 7,000 years in the
past. This season, the group is looking for more samples
for radiocarbon dating that will help it pinpoint this
second period.
When Hall and colleagues first discovered the skin and
hair from the locally extinct elephant seal colonies about
10 years ago, she had been on a different mission. She
was studying the retreat of ice sheets through the Ross
Sea and was literally combing the coast for organic material
to help date when the beaches had formed. It took a couple
of years before the scientists could genetically connect
the tiny fragments of skin and fur to elephant seals.
Southern elephant seals are sub-Antarctic mammals, with
huge breeding colonies on Macquarie Island and South Georgia.
Macquarie Island lies about halfway between Antarctica
and Australia and is the closest place today to the extinct
Ross Sea colonies, according to Hall.
Rus Hoelzel, at Durham University in the United Kingdom,
is one of two elephant seal specialists working on the
team. He is a molecular biologist performing the DNA work
on the samples that Hall and others in the group have
brought back from previous trips.
Hoelzel said his initial work in the project involved
identifying the remains of the elephant seals. Now his
focus is determining what the genetics of the population
can tell the team about the colonies’ history in
the context of environmental change over the last 5,000
years.
“We can use genetic diversity and simulation modeling
to estimate the pattern of demographic expansions and
contractions over time, and thereby compare population
dynamics with patterns of climate change,” Hoelzel
explained via e-mail. “We will also use the approximately
5,000-year time span to estimate a more accurate mutation
rate than would otherwise be possible, and this will facilitate
our estimations of population demographics.
“We will also compare the ancient samples with modern
populations to discover if seals from the Ross Sea may
have dispersed to one or more of the island populations,”
he added. A genetic link may exist between the Ross Sea
seals and the modern colonies on Macquarie Island, he
said, but that hypothesis is still being tested.
Hall and Paul Koch, co-principal investigator from the
University of California, Santa Cruz, will head as far
north as Terra Nova Bay to join Italian colleagues who
study Adélie penguins. The penguins serve as another
biological indicator for sea ice, Hall said.
Sea ice is a double-edged sword for Adélies. Big
sea ice years present foraging problems. However, scientists
working in the Antarctic Peninsula have identified a strong
link between the abundance of krill, a major food source
for Adélies, and the presence of sea ice.
Because they have different sea ice tolerances, the penguins
and elephant seals rarely share the same space, which
is proving true along coastal Victoria Land.
“It turns out when you combine different species,
because each species has a slightly different tolerance
to sea ice, you can get a more complete picture of sea
ice extent and duration,” Hall said.
This is the second and final field season for the three-year
grant from the National Science Foundation. Hall and colleagues
will use the third year to crunch the data and see how
it fits into the larger picture of global climate change.
Hall said it’s difficult to predict just how hardy
the ice shelves will prove to be, though she does not
believe the Earth has reached the tipping point yet for
the type of climate change that would cause major de-glaciation
of Antarctica. In the end, it will depend on just how
much the temperature really rises over the long term.
“It’s hard to tell; you don’t know what’s
going to happen to temperatures,” she said. “The
Ross Ice Shelf managed to survive warmer-than-present
temperatures 2,000 years ago, but its fate in the future
may depend on just how warm temperatures get.”
NSF-funded research in this story: Brenda Hall, University
of Maine; and Paul Koch, University of California, Santa
Cruz.
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