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New Coral Dating Technique Helps Resolve Changes in Sea Level Rise in the Past

April 15, 2005

Corals from Papua New Guinea and Barbados indicate that changes in sea
level, one of the key indexes for global climate change, may have been
more frequent in the past than previously thought, according to a
report in today’s issue of Science.

Researchers from the Woods Hole Oceanographic Institution (WHOI) and
Columbia University developed a new set of dating equations to
determine the ages of corals from the Pacific and Atlantic Oceans to
help resolve a longstanding question about the influence of Earth’s
orbital variations on sea-level rise in the past. Their approach
improves sea-level reconstructions using coral ages and indicates that
past sea level changes have been too frequent to be explained solely by
orbital changes.  

Reef corals are commonly used to reconstruct changes in sea level over
time because they grow near the sea surface.  Fossil corals found
above or below current sea level show variations in sea level and can
be dated by radiocarbon for the past 40,000 years and by the
radioactive decay of uranium to thorium for the past 500,000
years.  Unfortunately, corals that appear to be otherwise pristine
often have more of the isotopes used for dating than can be explained
by radioactive decay, making their ages unreliable.

“Sea level is more variable than previously thought over a period
between 70,000 and 250,000 years ago,” said William Thompson, a postdoctoral
fellow in the WHOI Geology and Geophysics Department and lead author of
the study. “Substantial shifts occur over a few thousand years, during
both glacial and interglacial periods, with rates of change that exceed
estimates of modern sea level rise.  Although sea level over the
past few thousand years appears to have been relatively stable, this
seems to be the exception rather than the rule.”

The new method used by Thompson and co-author Steven Goldstein of
Columbia University has provided a detailed sea-level record for the
period of time between 250,000 to 70,000 years ago. Little accurate
data has been available for this period, when some of the changes could
be explained by orbital changes but others could not.

Most radiometric dating techniques rely on the assumption of a closed
system, meaning that once the ‘clock’ starts there is no gain or loss
of the isotopes used for dating.  The parent isotopes are put into
a box, and the box is closed.  At some later time, if you count
the number of parent and daughter isotopes, you can determine the
length of time the box has been closed very precisely.  

“In the case of corals, it’s been clear for a long time that most
samples have not behaved this way,” Thompson said. “We’ve discovered
that corals behave as a two-box system. There is a very small leakage,
usually an addition, of daughter isotopes to the coral from the
surrounding material.  The key is that this transfer also depends
on radioactive decay.  You can simply rewrite the decay equations
to account for the transfer of daughters between boxes, allowing you to
calculate ages for corals that have behaved as ‘leaky boxes’. We refer
to this new dating approach as ‘open-system’ dating.”

Thompson and Goldstein compared speleothem records of sea level and
climate from caves in the Austrian Alps, France, Tasmania and Brazil
with their sea-level reconstructions for corals. They found agreement
for high sea level at times of warm, wet climate conditions and lower
sea level at times of cold/drier climate conditions.  They also
compared their data with salinity records from the Red Sea and found
similar agreement, further verifying their model.

The team’s findings raise questions about the conditions required for
the growth of ice sheets and the causes of rapid changes in sea level.