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 chlorine 36 -- 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.

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