Amber yields samples of ancient air.

Amber yields samples of ancient air

Air bubbles trapped in 80-million-year-old amber are giving scientists an unprecedented opportunity to sample and analyze the atmosphere from the earth's Cretaceous period, when dinosaurs roamed the planet. And the preliminary results are suggesting that these creatures breathed an air far different from the atmosphere of today.

"We were able to recalculate the original concentration of oxygen,' says Robert A. Berner of Yale University, who has analyzed the samples along with Gary P. Landis of the U.S. Geological Survey in Denver. "The oxygen level appeared to be around 30 percent [of the atmosphere] as opposed to 21 percent today,' Berner said last week at the Geological Society of America's annual meeting in Phoenix.

Previously, the oldest samples of the atmosphere came from ice buried deep in the glacial cap of the Antarctic, dating back only 160,000 years--a tiny portion of the earth's 4.5-billion-year history. Scientists have therefore had to rely on indirect methods for studying atmospheric evolution.

The rock and fossil records show that oxygen first appeared in the atmosphere around 2 billion years ago and continued to increase in concentration over time. But this record reveals little about the last half billion years, when plants and animals left the seas, evolving into the complex forms of today.

Air samples from amber might help fill in this important gap, says Harvard University's Heinrich Holland, who has studied the evolution of the atmosphere and oceans. "It's very exciting because it potentially gives us the first real handle on variations of oxygen over the time scale of millions of years.'

Amber is fossilized tree resin or sap. As sap oozes out of a tree it often encases air, insects and even frogs (SN: 9/26/87, p. 205). Landis and Berner crushed their amber samples in a vacuum and analyzed the escaping gas with a quadrupole mass spectrometer.

In addition to the Cretaceous amber, which came from north-central Canada, the researchers also analyzed 40-million-year-old amber from the Baltic Sea and 25-million-year-old amber from the Dominican Rupublic. They found that oxygen in the Baltic amber equaled today's level, but the air from the younger Dominican amber contained less oxygen. However, Berner stresses that he and Landis completed these measurements only a week before the conference: "These are all very preliminary results. They need much further testing.'

Holland and others had previously thought of analyzing the air in amber, but they abandoned the idea, believing that the amber might have contaminated the trapped air over millions of years. Berner says the results show that the amber was relatively inert. Instead, the trapped air seems to contain some contamination-- in the form of suspiciously high values of carbon dioxide--from the respiration of trapped microbes.

The carbon dioxide concentration in the atmosphere has never risen above a few tenths of a percent, says Berner. But the high levels in the amber lead him to believe that microbe respiration converted oxygen to carbon dioxide. Because this process is a simple one-to-one conversion, he was able to recalculate the original oxygen in the trapped air.

Berner's results appear to mesh with the notions of modelers of the oceanatmosphere system, who have predicted that oxygen levels might have been higher during the Cretaceous period. These modelers believe that the concentration of atmospheric oxygen represents a balance between two pervasive processes on the earth: weathering of rocks and burial of organic matter. While weathering consumes oxygen, the burial of plants and animal material in the oceans causes oxygen levels to rise.

"It's a very complicated interplay, which we are just starting to understand,' says Holland. "But I think [the amber finding] fits beautifully into what we know about the burial of organic matter during the Cretaceous.'

Some researchers had suggested that oxygen concentrations could never rise much above the 20 percent mark because high oxygen levels would support world-wide forest fires. Since there is no record of such burning, says Berner, these fire estimates may need revision.

William Berry, a paleontologist at the University of California at Berkeley, believes the new oxygen measurements might help explain how the largest dinosaurs --some of which stood five stories tall--could have developed and flourished. "You begin to think about big animals having a little bit easier time,' he told SCIENCE NEWS.