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By Peter Rejcek
Scientists Douglas Wilson and Bruce Luyendyk haven’t
found the lost continent of Atlantis, but their discovery
that far more of West Antarctica may have existed above
sea level millions of years ago could help solve one of
the great mysteries in the climate history of the continent.
In a paper published in Geophysical Research Letters,
a journal of the American Geophysical Union, the University
of California Santa Barbara researchers suggest that at
least twice as much land existed in West Antarctica than
today, increasing the total landmass of Antarctica by
10 to 20 percent.
The idea not only inserts an intriguing chapter into
the geologic evolution story of Antarctica but also provides
new insight into how much ice the continent may have held
as the so-called greenhouse world suddenly flipped —
in geologic time — to an icehouse Earth about 34
million years ago.
In about 100,000 years, Antarctica went from relatively
ice-free to hosting the sort of huge continental ice sheets
that exist today. However, until now, many scientists
believed most of the ice accumulated on East Antarctica,
which sits mostly above sea level even today. In contrast,
modern West Antarctica is an archipelago, and the base
of its marine-based ice sheet sits on bedrock that is
below sea level in many places, which makes it more susceptible
to climate change.
Such topography 34 million years ago would have taken
far longer for ice to build up into a heavyweight ice
sheet in the west — about 20 million years more based
on current models, according to the Wilson and Luyendyk.
The problem is that the estimated sea level drop near
the Eocene-Oligocene transition suggests more ice existed
than what scientific models predict based on using the
present topography for the early icehouse Earth.
The mystery of the missing ice becomes less of a mystery
with a larger landmass in West Antarctica, according to
the UCSB scientists.
“If you jack up that whole area by several hundred
meters, you have a huge increase in land area where ice
can accumulate,” Luyendyk explained. “A lot
of the things discussed in the paper are not terribly
surprising — and I think mildly controversial —
but what I think we’ve done is just put it all together.
Everything was higher in the past, and it’s subsided
since then.”
The reconstructed topography includes more land not
only in the deep interior of West Antarctica but also
where today the hundreds-of-meters-thick Ross Ice Shelf
floats.
“The ice sitting there will move much more slowly,”
Wilson said of the ancient landmass. “Not only will
you have ice there, but that will cause ice on the rest
of the continent to back up a little bit. Between the
two effects, that creates potential for quite a bit more
ice that can affect global sea level on the order of 10
to 20 meters.”
Wilson said it appears the western subcontinent has been
particularly active over the last 100 million years, undergoing
a great amount of horizontal stretching — what geologists
refer to as extension — that created large basins.
Eventually, the extension ended, the crust cooled, and
subsidence began, as the land began to shift down and
submerge under water.
“It’s a really big change based on a hundred
million years of geology,” Wilson said.
In addition to subsidence, large glaciers did their slow
if efficient job of erosion, though Luyendyk said they
need more data to constrain, or determine, how much land
those rivers of ice grinded away.
One key piece of evidence for the theory comes from a
sediment core drilled into the seafloor in 1973. Deep
Sea Drilling Project (DSDP) site 270 contained sediments
about 25 to 30 million years old, the oldest being beach
sediments above hard continental rock, according to Wilson.
“What that sediment record tells us is that we had
something that was above water, and some time after the
continental glaciers started in Antarctica, it went from
above sea level and it sank to being flooded by the ocean,”
he said. “That’s an important check on what’s
going up and what’s going down.”
David Pollard , a senior research scientist at Pennsylvania
State University , and Robert DeConto at the University
of Massachusetts , have previously modeled ice sheet growth
at the Eocene-Oligocene (E-O) transition 34 million years
ago. The work by the UCSB scientists will have a bearing
on their modeling efforts.
“The main goals were to understand the timing and
amplitude of this transition in the Antarctic ice sheet,
and to establish the ranges of climate forcing for major
growth and decay,” Pollard explained via e-mail.
The key icehouse driver in their simulation, he said,
was a decline in atmospheric carbon dioxide, the leading
greenhouse gas of today.
Some scientists also believe a major ocean current started
around the same time — perhaps initiated by the opening
of ocean gateways such as the ones between South America
and Antarctica or Australia and Antarctica — helping
to isolate the continent from warmer waters to the north.
Pollard, who is collaborating with Wilson and Luyendyk,
but was not an author in the AGU paper, said the idea
of larger landmass in West Antarctica bolsters the E-O
ice sheet model.
“An outstanding mismatch in our E-O simulations
was the total volume of Antarctic ice after the transition,
with data suggesting a much greater volume than modeled,”
he said. “The extra land surface above sea level
in the new reconstruction supports greater area and volume
of ice, and so helps to reduce this discrepancy.
“The new topography is important not only for the
initial growth at the Eocene-Oligocene boundary, but is
the first step in incorporating evolving topography and
its interactions with ice sheets over millions of years
through the later Cenozoic,” he added.
Wilson and Luyendyk said they’re interested in finding
additional evidence like that from DSDP site 270 to fill
in more details about the geologic evolution of West Antarctica.
Antarctic sediment drilling programs like ANDRILL and
SHALDRIL may offer an opportunity for additional data
points — sites to confirm and refine their theory
— if proposals receive funding in the future.
“A study like this, which is going to have a major
impact on the evolution of Antarctic ice sheets, just
serves to remind us that there are lots of things to be
found out,” Luyendyk.
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