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Monday, February 15, 2010

Sea levels erratic during latest ice age
Cave research finds new evidence of surprising rise 81,000 years ago


By Sid Perkins
Web edition : Thursday, February 11th, 2010


Pic at the citation
HIGH-RISE EVIDENCEThe mineral crusts on high-and-dry formations in
coastal caves of Majorca indicate that during the latest ice age, sea
level briefly and inexplicably rose more than one meter higher than
today’s level.© B.P.


Cave formations along the coast of an island in the Mediterranean Sea
hold evidence that sea level can rise and fall abruptly during an ice
age, a finding that casts some doubt on current notions about how
those lengthy cold spells develop and progress.


At the height of an ice age, immense volumes of water are locked up in
land-based ice sheets, and ocean levels can be as much as 130 meters
below where they are today. By contrast, when that ice melts during
warm periods, sea level can be a few meters higher than the modern-day
standard, says Jeffrey Dorale, a paleoclimatologist at the University
of Iowa in Iowa City. Now, Dorale and his colleagues report in the
Feb. 12 Science that during a brief interval well within the most
recent ice age, sea level suddenly and inexplicably rose to a height
more than one meter above today’s.


Evidence supporting that conclusion comes from cave formations on the
Spanish island of Majorca, the researchers say. As sea levels rose and
fell, waters sloshing into coastal caves left crusts of minerals on
their walls and floors as well as on existing cave formations, Dorale
says.


Radioisotope dating of mineral crusts in one cave along Majorca’s
southern coast indicates that sea level sat about 2.6 meters higher
than today between 121,000 and 116,000 years ago, during the last warm
spell between ice ages. That level is consistent with sea level data
gathered at other sites worldwide, Dorale notes.


But three samples from other crusts in the same cave — samples
deposited about 1.5 meters above modern-day sea level — yielded
surprising results. Those crusts formed around 81,000 years ago, well
after the most recent ice age — which lasted from roughly 110,000
until 10,000 years ago — had begun, Dorale says. Similar analyses of
samples from nearby caves show that between 80,000 and 82,000 years
ago, sea level ranged between 1.25 and 1.6 meters above today’s
standard.


“The [team’s] results are strong but not absolutely watertight,”
comments R. Lawrence Edwards, a paleoclimatologist at the University
of Minnesota in Minneapolis. One possible confounding factor, for
instance, could be the rebound of Earth’s crust in the region since
the end of the most recent ice age. After the ice mass smothering
Northern Europe melted and ran to the sea, pressure from viscous
material at the top of Earth’s mantle would have lifted the area,
thereby influencing apparent sea level.


But Dorale and his colleagues contend that tectonic uplift hasn’t
affected their data. He cites similar analyses of now-submerged
mineral crusts in Majorcan caves indicating sea level was about 20
meters below today’s level about 85,000 years ago and about 15 meters
below the modern standard about 79,000 years ago — readings that match
most data gleaned elsewhere at those times.


Studies at a handful of sites worldwide have noted that sea level
reached an exceedingly brief and similarly enigmatic high point around
81,000 years ago, says Dorale. Those results have been controversial
and, for the most part, have been “politely ignored because they don’t
fit the presumed pattern” of how ice ages develop and progress, he
says.


Scientists have long noted erratic dips and jumps in sea level during
Earth’s ice ages, but debate has typically focused on the magnitude of
those swings, says Dorale. The new findings are somewhat disturbing
because they suggest that at some points during an ice age, sea level
can rise as much as 2 meters over the course of a century. “It’s tough
to explain how to melt that much ice that fast,” he admits.

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