Using a new time measuring device geochronologists study the orbital pacing of the ice ages
OCEANOGRAPHERS had found evidence
that the march of ice ages over the last
million years was paced by the cyclical
stretching and squeezing of earth's orbit
around the sun, which would have
altered the way sunlight fell on the
planet's surface. But in 1988, researchers
scuba diving in Nevada's Devils Hole
came up with a climate record - captured in carbonate deposits in the crack
- that seemed to contradict this
chronology. Tie Devils Hole record
traced climate swings of about the same
length as the marine record, but they
were out of step with the variations of
earth's orbit. Most glaringly, these carbonates indicated a profound warming trend, which appeared to signal the end
of the penultimate ice age, thousands of
years before orbital variations could
have be, melt the ice (Science, Vol 276, No 5313).
Almost after a decade of wrangling
over whether inaccuracies in the dating
of one record or the other might
account for the conflict, an arbiter has
come forward. Geochronologists
Lawrence Edwards and Hai Cheng of
the University of Minnesota and
Michael Murrell and Steven Goldstein
of Los Alamos National Laboratory in
New Mexico used a new clock, based on
the radioactive decay of uranium-235 to
protactinium-231, to check the dates of
both the Devil Hole record and records
of sea-level change in Barbados coral.
The result: the marine record is right
and the astronomical theory is on solid ground.
To most oceanographers, the Devil's
Hole and the marine records, "are two
fundamentally different beasts," says
Steven Clemens of Brown University.
He and other oceanographers suggest
that while the marine records trace the
ebb and flow of the ice ages, Devils Hole
may chronicle only the climate of a
region as small as southwestern North America.
The original confirmation of the
astronomical theory came in the late 70s
from sea-floor sediment, where the ratio
of two oxygen isotopes traces how much
water was locked up in the ice sheets
when the sediment was deposited. This
ice-volume signal showed that water
flooded into the ocean from melting ice
sheets about 128,000 years ago, marking
the start of the last warm inter
glacial period. That was just
when orbital variations would
have maximised the amount of
sunlight falling on Northern
Hemisphere ice sheets. The coincidence had helped convince
oceanographers that orbital variations paced the ice ages.
Marine sediments are so difficult to date, a team of
oceanographers studying the
isotope record had only two or
three direct dates for the sediments of the past million years.
To fill in the chronology, they
simply counted the 'ticks' left in
the isotope record by two known
periods of orbital clock, 21,000
and 41,000 years long. But a
better, more directly dated
record tended to confirm the
sea-floor chronology. Corals that
formed at the ocean's surface
when melting ice pushed up the
sea level, then died when the ice
returned and sea level fell, have left
terraces that can be dated by measuring
the accumulation of thorium-230 from the decay of uranium.
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