Over the course of geological time, the amount of carbon trapped in land and the oceans has waxed and waned. So has the amount of carbon dioxide in the atmosphere.

These fluctuations correlate closely with changes in global temperatures. Therefore, studying the flow of carbon between land, water and atmosphere through geological ages can shed light on issues surrounding today's global warming.

New research described in the May 26 issue of Nature provides some missing pieces in the puzzle depicting the global carbon cycle over geological time.

During what geologists call "oceanic anoxic events," it has long been suggested that a large amount of the greenhouse gas carbon dioxide is removed from the atmosphere by millions of microscopic organisms that dwell in the oceans.

They do this by trapping carbon in their bodies. When they die, their bodies rain down to the ocean depths and are buried by sediment, locking away the trapped carbon from the atmosphere for million of years.

Scientists believe that during oceanic anoxic events the biological activity or "productivity" of these oceanic organisms is for some reason enhanced. On the other hand, perhaps the number of these oceanic organisms is somehow much greater during OAEs than during normal times.

Oceanic anoxic events are extremely unusual in other ways, too. They are often associated with mass extinction among many marine organisms and coincide with periods of intense global warming.

Scientists have long debated what causes OAEs. The prevailing theory is that a release of massive amounts of methane into the ocean is the root cause.

The methane, in turn, oxidizes creating colossal amounts of carbon dioxide in the ocean and atmosphere, which depletes the oceans of oxygen, warms the planet, and kills off plants and animal species.

Now, a new theory holds that OAEs � in particular the Toarcian OAE, which occurred about 183 million years ago during the age of dinosaurs � are triggered by the burning of vast underground coalfields. These coalfields were set ablaze by the intrusion of molten rock from the Earth's crust.

"The burnt coalfields are hundreds of feet thick and cover vast areas of the Transantarctic Mountains of Antarctica, as well as South Africa," said Jennifer McElwain, PhD, Associate Curator of Paleobotany at Chicago's Field Museum and lead author of the research.

"Huge quantities of methane and carbon dioxide would have been released from these coals as they were heated to high temperatures by the molten rock."

Although OAEs are not universally accepted as models upon which an understanding of modern climate change can be based, this new research sheds light on the possible consequences of the current level of consumption of carbon-based fuels.

"If the incredibly high global temperatures that occurred during the Toarcian oceanic anoxic event were caused by burning a significant amount of the Earth's coal deposits within one hundred thousand years, it doesn't take much imagination to realize what will happen if we burn most of the Earth's remaining fossil fuels over the coming century, which is what we are in the process of doing," McElwain said.

The scientists, who worked on this research for more than four years, also turned up a totally unexpected result: they identified a 200,000-year interval when atmospheric carbon dioxide dropped to surprisingly low levels at the start of the Toarcian Oceanic Anoxic Event.

This was probably due to the great number and activity of marine organisms at this time that effectively sucked carbon dioxide out of the atmosphere like a sponge. This drop cooled the Earth, maybe even enough to have enabled ice sheets to form and grow in the polar regions of the Arctic and Antarctic.

The idea of ice sheets during the age of dinosaurs has always been a controversial topic.

Nevertheless, McElwain and coauthors Steve Hesselbo, from the Department of Earth Sciences at the University of Oxford in England, and Jessica Wade Murphy, who was an undergraduate student in the Department of Ecology and Evolution at the University of Chicago at the time of this study, believe they have tantalizing evidence that the global temperatures were not as uniformly warm and ice free during the age of dinosaurs, as once assumed.

In this study, which was funded by the Comer Foundation of Science and Education, atmospheric carbon dioxide levels were determined by counting the stomata in small fossil leaves collected from Baga Formation, Denmark, of which 126 specimens were used.

Stomata are minute pores in the surface of leaves through which water vapor and gases, including carbon dioxide, pass. The fewer the stomata, the more carbon dioxide in the atmosphere, and vice versa.

"We were certainly surprised to find that our tiny fossil leaves from Denmark led us half way across the world to coalfields of the Transantarctic Mountains in Antarctica to form a new theory on how natural geological processes in the past have caused extreme global warming," McElwain said.

"It's very sobering to realize that humans are currently causing global warming by similar processes, that is, by burning fossil fuels like coal and oil. The difference today is that we are causing the atmosphere and climate to change at a greatly faster rate than has ever been observed in the Earth's history."

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