UNH Scientist Helps Show
Collapse of Antarctic Ice Shelf is “Unprecedented”
Contact: David Sims
Institute for the Study of Earth, Oceans, and Space
Aug. 3, 2005
DURHAM, N.H. -- A paper to be published in the August 4 issue of
the journal Nature asserts that the recent collapse of the
Larsen B Ice Shelf in the Weddell Sea of Antarctica is “unprecedented”
in recent times. The ice shelf – the third largest in the
Antarctic – has undergone catastrophic decay in recent years.
A total of about 3, 250 square kilometers of the shelf area disintegrated
in a five-day period in the winter of 2002. Over the last five years
the ice mass has lost some 5,700 square kilometers and is currently
40 percent the size it was previously when stable.
Using marine sediment cores from the ocean floor formerly covered
by the ice shelf, scientists found no evidence for “episodes
of open marine conditions” indicating that the ice shelf has
been in existence for last 8,000 years – a period of time
referred to as the Holocene Epoch.
“Our unique observation, that for the first time the Larsen
B is involved in collapse, indicates that the current warming trend
in the NW Weddell Sea has exceeded past warm episodes in its magnitude,”
states the paper, whose lead author is Eugene Domack of Hamilton
University of New Hampshire scientist Michael Prentice, one of the
article’s authors, is an expert in the paleoceanographic technique
used to extract past ocean properties from the seafloor sediment.
The technique involves analyzing the chemical composition of the
remains of tiny, one-celled animals called Foraminifera or “forams.”
The technique, according to Prentice, is technically challenging
in polar regions but highly accurate at giving a clear picture of
past water temperatures and salinity levels.
Says Prentice, “Some of the forams in the cores lived and
died in the surface water adjacent to the ice shelf before settling
to the seafloor to be incorporated into the sediment that we recovered.
A pristine record of ocean surface water can be had from analyzing
them.” Prentice calls forams the “workhorse” in
the field of paleoclimatology.
The method involves dissolving the carbonate shells of the tiny
forams and, using a mass spectrometer, measuring the oxygen isotopes
contained within the carbon dioxide gas that comes from the shells.
Because the ratio of oxygen isotopes in the shells is controlled
by the water conditions at the time the forams were living, forams
from layers of accumulating sediment give a clear record of water
conditions from the present back deep into the past.
“We got a pretty good fix on what the longer history of ice
shelf extent and melting has been,” Prentice says. This fix,
in turn, gives scientists a context for judging the significance
of the current collapse. Prentice adds, “These data are the
first good isotopic record adjacent to a crumbling ice shelf.”
And that isotope record, he says, suggests that there has been a
progressive slow melting/thinning of the ice shelf over the last
8,000 years. “This is due in part to climate warming. But
melting over the last 8,000 years was never close to what it is
today, and so the current collapse and glacier surge that it has
unleashed are unprecedented.” He adds, “The Larsen B
is considered a harbinger for the massive ice shelves to its south,
which, for now, dam the large majority of the world’s ice.