This Evolving Atmosphere

This Evolving Atmosphere

The astronauts who first stood on the moon were amazed at the incredible number of stars they could see in the black sky above. One can scarcely conceive the clarity of a night sky unimpeded by the layers of gases that make up the atmosphere encircling our earth. But there was a time when the sky as seen from Earth was as clear as that now observed from the moon. In the very primal stages of our planet's evolution, about 4.5 billion years ago, the earth did not have an atmosphere, and the stars shone very brightly against an extremely black night sky.

Scientists believe the first atmosphere was produced about four billion years ago. This primal atmosphere was released through active volcanism in the landscape of this primeval world. The original gases included water vapor, carbon dioxide, chlorine and sulfur compounds, and methane, nitrogen, and ammonia. Additional nitrogen may have been formed by the breakdown of ammonia (NH3) under the influence of light.

Subsequently the water vapor condensed to form the oceans; the carbon dioxide reacted chemically with materials in the earth's crust and still remains locked up in many of the oldest sedimentary rocks. Of the original atmosphere, only nitrogen remains in high concentration (78 percent). The atmosphere has consistently changed with time, especially since humanity arrived on this planet and assumed control.

The dissociation of water molecules through discharges of lightning produced free oxygen in the primeval atmosphere some 3.5 billion years ago. By three billion years ago oxygen was becoming relatively abundant. Rocks and fossils of life forms of that early era indicate this. But the atmosphere was still not breathable by human standards. By the age of the coal—making forests, 360 million years ago, it reached a level of concentration somewhat similar to the present. During this, the Carboniferous Period, much of the land surface of the earth was an endless array of immense jungles and marshes. The intense activity of photosynthesis constantly released free oxygen into the atmosphere. As a result, the oxygen level by the time of the dinosaur, 100 to 200 million years later, was much higher than at present. Scientists determined this recently through the study of amber that had been in existence during the Cretaceous Period, 180—65 million years before the present (mybp), the last stage of the dinosaur era.

Amber is the fossilized resin or sap secreted through the bark of prehistoric pine trees, similar to the fluid that exudes from conifers of today. The soft, sticky nature of the original secretions is indicated by the abundance of perfectly preserved insects, spiders, and even small lizards trapped in the semifluid sap. In time the resin hardened to become amber, which has been used as a gemstone since prehistoric man. Only recently has amber been prized by scientists for the record of ancient life that it can provide. Along with creatures, the resin preserved, most significantly, bubbles of air that would represent the atmosphere of 80 mybp. This was the air that the dinosaurs breathed.

In 1987 two scientists obtained samples of amber that were 80 million years old. The air bubbles in the amber were a part of the atmosphere from the earth's Cretaceous Period. The scientists crushed pieces of amber in a vacuum and analyzed the air bubbles with a highly sensitive machine that can detect the concentration of various gases in extremely tiny samples. Their results were quite illuminating, for the amber proved to be inert and didn't contaminate the samples of air. As expected, the atmosphere for the dinosaurs was far different from that of today. In the preliminary analysis the oxygen level appeared to have been about 30 percent of the atmosphere, compared with 21 percent today.

A number of scientists now believe that the much higher concentration of oxygen is related to the giantism that characterizes many species of Cretaceous dinosaurs. Some stood as high as five stories, and the estimated weight of some herbivores was well over 100 tons. The abundance and variety of dinosaurs were also impressive. Their renaissance during the late Cretaceous seems to have followed the new, varied, and luxuriant plant life that could maintain a sizable population of large, well—fed herbivorous dinosaurs. This abundance could in turn support a plentiful array of giant carnivores, and all could enjoy the golden age of plentiful oxygen.

Amber of a more recent age, about 40 million years old, was obtained from the Baltic Sea area. Analysis of its air bubbles shows the oxygen level to be about that of today. Analyses of air bubbles 25 million years old show a similar oxygen content. The distinctly lower level following the time of the dinosaur requires an explanation, or at least a plausible conjecture.

Scientists generally agree that the late—Cretaceous dinosaurs were terminated abruptly by the collision of a gigantic celestial body from outer space with the earth, about 65 mybp. Wildfires raged everywhere; the earth was truly scorched. When the fires died down, the forests of the earth were gone. The release of oxygen through photosynthesis being at a much lower scale, the oxygen content of the atmosphere dwindled to the present 21 percent.

For a study of the earth's atmosphere during more recent times, scientists have examined drill cores obtained from ice sheets of Greenland and Antarctica. The cores are significant scientific treasures, for they preserve continuous layered records of annual snowfalls going back about 125,000 years. The air bubbles trapped in the ice provide the same information as those encased in the older amber. The earliest of the ice cores were trapped during an ice age, and chemical analysis of the trapped air samples showed a somewhat lower content of carbon dioxide. This is significant because carbon dioxide is essential in retaining the warmth of the sun. The analysis also showed that the atmosphere contained a heavy burden of sunlight—screening dust, much of it volcanic. These two factors doubtless contributed to the ice age but were not its primary causes.

Our atmosphere today is becoming contaminated at an alarming rate. Recently scientists studied ice cores taken from a Yukon glacier about a thousand years old. They contained many trapped air bubbles. The research team found that the carbon dioxide level in today's atmosphere is 27 percent higher than it was before 1850, the beginning of the Industrial Revolution. And over one—third of this alarming increase has been within the last 25 years. Global warming, here we come!

From the book: 
Petrified Lightning