Microscopic air bubbles trapped in fossilized tree resin reveal that the Earth's atmosphere 80 million years ago was about 50% richer in oxygen than it is today, a finding that could have profound implications for the evolution of life, scientists reported Thursday.
The discovery, if confirmed, reveals that the Earth's atmosphere is dramatically different today than it was during the Cretaceous period before the last of the dinosaurs died out more than 65 million years ago. Robert A. Berner of Yale University and Gary P. Landis of the U.S. Geological Survey in Denver presented their findings at the annual convention of the Geological Society of America in Phoenix.
The scientists analyzed air trapped in tiny bubbles in four pieces of amber that formed when resin from coniferous trees hardened at different periods in the Earth's history.
The discovery caught many experts by surprise, and even came as somewhat of a jolt to the scientists themselves.
They had been studying pieces of amber that are 25 million and 40 million years old, and they had found that air in those samples was comparable to the air today. But two weeks ago, when they were analyzing samples from a piece of 80-million-year-old amber from Canada, "we hit the jackpot," Berner said.
The air in the amber was as high as 32% oxygen compared to around 21% oxygen in today's air.
Although the scientists had only one piece of Cretaceous amber, it had many air bubbles, and all the results supported the conclusion that the atmosphere then was far richer in oxygen, Berner said.
There are few opportunities to study ancient air, because air is affected by so many processes that change its character. Prior to Thursday, the oldest samples studied were extracted from the ice below Antarctica and dated at about 160,000 years old.
Other experts around the country reacted with surprise to the finding. Although the mix of gases in the Earth's atmosphere has changed over many millions of years, there has been no indication in the past that the air was ever far richer in oxygen.
"If the production of oxygen was that far ahead of what it is now, it has very interesting implications," said Hale Johnston, an atmospheric expert at the University of California, Berkeley.
Link to Dinosaurs
Some scientists could not resist the urge to link the discovery with the mass extinction of the dinosaurs. A radical change in the percentage of oxygen could have had a severe impact on animals whose biological systems were dependent on a different level, experts said.
A sudden rise in oxygen could kill animals accustomed to a lower level, because oxygen is toxic and a dramatic increase could overwhelm an animal's defensive mechanisms. And a sudden drop could prove fatal to animals equipped with smaller respiratory systems that had been adequate in an atmosphere rich with oxygen.
"A large number of plants and animals would be put under a big evolutionary stress," Johnston said. "It could contribute to mass extinctions."
Some said they were not surprised that the percentage of oxygen has changed, because oxygen depends on the balance between plants, which produce it, and animals, which consume it. The ratio between plants and animals does not remain constant over millions of years, and thus the atmosphere does change, but few scientists expected to see the dramatic change that Berner and Landis believe they have found.
"Oxygen is in dynamic equilibrium, because it is produced at a certain rate, and used up at a certain rate, and it strikes a balance," said Yuk Yung, professor of planetary science at Caltech and a specialist in atmospheric evolution. "It's not like a rock. It's more like the water in a lake, and the level depends on how much water you are getting in and how much you are using."
Some scientists noted that the age of the amber coincides with major changes that were taking place in the Earth's environment, some of which could explain the higher percentage of oxygen.
George Poinar, an entomologist at UC Berkeley who is a leading expert on amber--long cherished by insect specialists because almost any sample contains at least a few fossilized specimens--noted that 80 million years ago the planet was quite literally beginning to blossom.
"Angiosperms, the flowering plants, were just spreading at that time," Poinar said in a telephone interview. Plants produce oxygen through photosynthesis, and "it could make sense that you have this increase in oxygen at that time."
"The flowering plants evolved during the Cretaceous and they were spreading rapidly over the entire surface of the Earth, so you would have a sudden increase in plants, and it's conceivable you would get a sudden increase in oxygen."
Impact Could Be Serious
If it jumped by several percent, the impact on some animals could be serious, he added.
"If you raised the oxygen (level), it could be toxic to them," he said.