THE world鈥檚 oceans may have become stagnant for several thousand years during
the last ice age, after their circulation system abruptly stopped. And thanks to
global warming it could happen again. So say researchers who studied a
half-metre chunk of stalactite hacked from a flooded cave on the island of Grand
Bahamas.
They looked at layers of calcium carbonate in the 45,000-year-old stalactite
to estimate prehistoric levels of carbon-14. This radioactive isotope is created
by cosmic rays bombarding the atmosphere. Levels usually remain fairly constant,
but knowing of any fluctuations allows archaeological remains to be accurately
carbon dated.
However, the work took an exciting turn when David Richards of the University
of Bristol and his team found that carbon-14 levels jumped to twice
pre-industrial levels between 45,000 and 33,000 years ago, peaking in a
鈥渄ramatic spike鈥� 44,000 years ago. The results will be published in a future
issue of Science.
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One explanation is that the Earth was being hit by much more cosmic radiation
than today. A nearby supernova may have exploded, or the magnetic fields of
either the Sun or the Earth鈥攚hich deflect cosmic rays鈥攎ay have
decreased. But while either of these events may have happened, says Richards,
鈥渢hey cannot explain the intensity of the fluctuations in carbon-14 that we have
蹿辞耻苍诲.鈥�
Richards and Warren Beck of the University of Arizona in Tucson believe the
only explanation for the radiocarbon roller coaster is that the Earth was not
able to remove carbon from the air as efficiently as today. They blame the ocean
circulation system, which pulls surface water into the deep ocean and returns it
a thousand years later. The circulation also removes large amounts of carbon
dioxide from the air, and releases it back.
If the ocean circulation was as vigorous as it is today, peak carbon-14
levels in the past should have been lower than the researchers found in their
stalactite. And more of the carbon-14 would have returned to the air before it
decayed away in the deep ocean. Richards says the abrupt appearance and
disappearance of the spike only makes sense if the ocean circulation had
faltered.
Ocean circulation is driven by dense, saline water falling to the depths,
says Richard Alley of Pennsylvania State University in State College. When ice
sheets on land collapsed, icebergs reduced the salinity of the ocean, he
believes, halting circulation.
Global warming is today reducing sea ice and making polar waters less saline,
warns Richards. This could cause the oceans to stagnate once more鈥攁nd
accelerate the build-up of CO2 in the air. 鈥淲e are poking the climate
with sticks,鈥� says Beck.