By Lori Stiles - University of Arizona News
 A
team of American and British scientists report that radiocarbon
levels in Earth's atmosphere during the last Ice Age were more
than twice as high as today, higher even than the nuclear weapons
tests of nearly half a century ago. They also reported in the
May 11 issue of the journal Science of having extended the record
for atmospheric radiocarbon more than 45,000 years.
The researchers,
who come from the University of Arizona, University of Bristol
(U.K.) and the University of Minnesota, were able to extract a
precise and near-continuous record of atmospheric carbon dioxide
levels in a half-meter-long stalagmite that formed during the
last glacial period in a cave that now lies underwater in the
Bahamas.
Marking time
with carbon 14 requires an accurate record of atmospheric radiocarbon
through time. Archaeologists, for example, use the radiocarbon
time scale to date artifacts, but dates were only accurate as
far back as 16,000 years. The information contained in the stalagmite
effectively triples the calibration period.
University
of Arizona physicist J. Warren Beck and his colleagues also discovered
that atmospheric carbon 14 levels soared dramatically between
45,000 and 33,000 years ago. Beck says even more interesting was
a dramatic spike in radiocarbon levels during a millennium that
began 44,300 years ago, nearly twice as high as the "bomb
pulse" produced during nuclear weapons testing in the 1950s
and 60s.
The radiocarbon
peak Beck and his colleagues found correlates to other peaks for
other radioactive isotopes - beryllium 10 and chlorine36 - found
in polar ice cores and lake sediments. All three isotopes are
produced when cosmic rays bombard Earth's upper atmosphere. Beck
says this suggests much higher levels of cosmic rays were striking
the atmosphere during the Ice Age.
While scientists
have known for some time that atmospheric carbon 14 levels were
higher and more variable in the Ice Age atmosphere than today,
"the magnitude of variation revealed by our stalagmite is
surprising," Beck and the others write in Science.
The researchers
used mathematical simulations to examine which of four possible
factors might have produced high concentrations of radiocarbon
in the atmosphere.
Three phenomena
that affect the rate at which cosmogenic isotopes are produced
in Earth's stratosphere. One is cosmic ray flux, the intensity
of very high-energy "galactic" radiation coming from
beyond the solar system. Two others are the strength of the sun's
electromagnetic field and Earth's own magnetic field, both of
which deflect cosmic radiation.
From rigorous
theoretical modeling, Beck and the others conclude that variations
in the strength of the solar electromagnetic field and in the
intensity of Earth's magnetic field alone aren't enough to explain
the fluctuations in radioisotope levels found in their stalagmite.
And, Beck
adds, while it is possible that a burst of galactic cosmic rays
from a nearby supernova explosion dramatically increased production
of cosmogenic isotopes -- as previously hypothesized in other
research by UA geoscientists Alex McCord and Paul Damon -- whether
a supernova explosion would be powerful enough to push aside the
heliosphere that shields Earth from galactic cosmic rays "is
an open question."
Any one or
a combination of the three cosmogenic scenarios may have contributed
to elevated CO2 levels during the Ice Age.
"But
the bottom line is that Earth's carbon cycle was significantly
different than it is today," Beck said.
The fourth
factor is the structure of Earth's carbon cycle. Most carbon on
Earth is locked up in limestone and fossil fuel. A relatively
small amount of "active" carbon circulates through the
atmosphere, oceans, soils and biota.
The authors
concluded - based on their mathematical models - that changes
in the carbon cycle must also be partly to blame for the large
fluctuations in the amount of atmospheric radiocarbon they observed.
In particular, they say that the carbon cycle must have operated
more slowly during the last Ice Age than today.
One way the
carbon cycle may have differed during the Ice Age is a slower
rate of ocean organism deposition on the deep ocean floor. More
organic carbon would in that case be exchanged with the surface
ocean and atmosphere. Modeling that scenario comes closer to the
actual evidence, "but even this doesn't match the high concentrations
in the observed record," Beck said.
What had to
have been going on, according to their models, is a slower rate
of carbon exchange between the surface ocean and the deep ocean.
"If we slow that rate by about a third of the modern exchange
rate, we get a simulation that looks like the observed evidence.
Ocean mixing during the glacial period must have taken longer
than it is today, " Beck said.
Ocean mixing
in the geological present occurs primarily at two high latitude
regions, in the North Atlantic and off the shores of Antarctica.
A breakup of western Antarctic ice sheets, or an increase in freshwater
icebergs floating into these regions, or changes in wind that
contributes to ocean mixing influence ocean mixing rates and could
trigger abrupt change in the carbon cycle to produce greater concentrations
of atmospheric carbon dioxide, Beck said.
While the
cause of implied changes in ocean mixing rates or carbonate sedimentation
rates is unknown, the authors conclude, the observation that the
carbon cycle was significantly more sluggish in the recent past
"may have profound implications regarding the oceans capacity
to take up anthropogenic CO2 emissions from fossil fuel burning.
"We should
take this as a warning that climate changes may affect the carbon
cycle in previously unexpected ways," Beck said.
|
