Alternate View Column AV-27
Keywords: cretaceous, air, oxygen, amber, quetzalcotalus, pterosaur
Published in the July-1988 issue of Analog Science Fiction & Fact Magazine;
This column was written and submitted 12/5/87 and is copyrighted © 1987, John G. Cramer. All rights reserved.
No part may be reproduced in any form without the explicit permission of the author.
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The largest flying creature alive today is the Andean condor Vultur gryphus. At maximum size it weighs about 22 pounds and has a wingspread of about 10 feet. But 65 million years ago in the late cretaceous period, the last age of dinosaurs, there was another larger flying animal, the giant pterosaur Quetzalcotalus. It had a wingspread of over 40 feet, the size of a small airplane. Other pterosaurs were also quite large. The pteranodons of the late jurassic period, the classic flying dinosaurs of magazine illustrations, had a maximum wingspan of about 33 feet.
This presents a puzzle: how is it that the largest flying animals of the cretaceous were able to attain so much greater size than modern birds? There are severe physical limits associated with flight. It is difficult for large birds to generate enough lift to take off. Consider the well-known square-cube law: if you double the size of a bird by simple scaling, its wing area and associated lift go up by 22, or a factor of 4, while the body weight that must be lifted goes up by a factor of 23, or a factor of 8. When an evolving flying animal species increases in size the basic design must be altered to accommodate the reduced lift-to-payload ratio. But if anything, the pterosaurs were less well designed than modern birds. They lacked the birds' efficient keelbone muscle structure and the aerodynamic advantages of feathers. How, then, could pterosaurs have grown so large?
A missing piece of this puzzle may have been discovered. There are indications that the cretaceous atmosphere may have been much richer in oxygen. Today, Earth's atmosphere contains about 77% nitrogen, 21% oxygen, 1% water vapor, 0.9% argon, and 0.03% carbon dioxide, with traces of about a dozen other gases. It's been commonly assumed that earth-air stabilized at about this composition a few million years after life evolved on our planet when the early plants photosynthesized most of the primordial carbon dioxide into free oxygen, and the early bacteria converted most of the primordial ammonia to free nitrogen and water.
But now there is evidence that the cretaceous atmosphere may have been very different. Samples of 80 million year old air have been analyzed. You well might ask how there could be samples of air trapped and preserved for 80 million years. One might say that nature has provided the sample bottle.
Great forests of the extinct pine species Pinus succinifer once covered large areas of the world. Like modern pine trees the Pinus succinifer when injured by storms or boring insects had a tendency to drip pitch. This sticky resin would fall to the ground, accumulate, and eventually be buried. Over a multi-million year time span solidified pitch is fossilized into the hard resin amber. Amber in jewelry-grade specimens is almost clear, with a characteristic pale yellow color. Sometimes one finds ancient insects encased in amber. These were originally trapped in the sticky pitch and fossilized along with it, and they now provide an important base of knowledge about the insect life of past geologic eras.
As lumps of pine pitch fall to the ground and aggregate, pockets of air are sometimes enclosed, becoming tiny bubbles of air trapped in amber. The amber thus forms a natural "sample bottle", trapping air samples from millions of years in the past and preserving it for present analysis. Moreover, the sample bottles are labelled with a date of collection. Geological analysis of the rock strata in which the amber samples are found and evidence provided by organisms trapped in the amber along with the bubbles can be used to establish the age of the sample. In many cases the pressure inside such bubbles has become as high as 10 atmospheres from compression by the geological forces that converted the pitch to amber.
The bubbles are typically small, some only 0.01 millimeters in diameter. The quantity of air in such amber bubbles is minute. Even when a sizable sample of amber is crushed to release the trapped gases, the volume of air obtained is very small. Normal chemical analysis techniques would be utterly useless for such almost infinitesimal air volumes. But there is a better way.
A modern analytical instrument, the quadrupole mass spectrometer (QMS), is capable of analyzing very small gas samples into their constituent chemical elements. The gas sample is ionized, and an electrical discharge removes an electron from the gas atoms. These charged atoms are accelerated by a high voltage and passed between four charged bars. These bars run parallel through the instrument to form a "quadrupole" electric field. Adjacent bars have opposite voltages and are driven with a rapidly oscillating electric field. For ions of just the right mass, that varying electric field focuses and collects the atoms and delivers them very efficiently to the collection electrode at the end of the apparatus. For atoms of the wrong mass the field values are inappropriate, and they collide with the bar electrodes and are lost. The sensitivity and mass discrimination of this instrument are very good. It is relatively easy for the QMS to determine chemical abundances and even isotope ratios in a very small gas sample. That is what was done.
At a meeting of the Geological Society of America held last Fall in Phoenix, Robert Brenner of Yale University and Gary Landis of the U. S. Geological Survey reported the results of a QMS analysis of ancient air bubbles trapped in amber. They obtained a remarkable result. The atmosphere of the Earth 80 million years ago was discovered to have 50% more oxygen than modern air. Brenner and Landis found that for all gas samples taken from amber 80 million years old the oxygen content ranged between 25% to 35% and averaged about 30% oxygen. Cretaceous air was supercharged with oxygen.
On the other hand, 40 million year old samples similarly analyzed showed about the same oxygen content as modern air, and 25 million year old samples showed slightly less oxygen than modern air. The composition of air has been shifting with time over a far broader range than geologists had thought possible. The cause of these excursions is not understood. Perhaps they are caused by a shift in the delicate balance between oxygen production by photosynthesis and oxygen trapping by exposed iron, sulfur, and organic reducing materials.
There is, of course, concern about whether these bubble samples accurately reflect the true atmospheric content. Is it possible that they have changed with time, leading to a false result? Atmospheric gases might have diffused in or out through the amber at different diffusion rates, changing the net composition of the trapped gas. Internal evidence of the samples themselves, however, argues against this. Most of the processes that are of concern, for example oxidation of the amber, would tend to reduce the oxygen content of the bubbles. Moreover, traces of hydrogen, a gas that diffuses far more readily than oxygen, are found in the bubbles with approximately modern concentrations. Analysis of bubble samples taken from modern tree resins also agrees with present atmospheric composition. The case for high oxygen in the cretaceous atmosphere seems, in a manner of speaking, air tight.
This result has very interesting implications about the era of the dinosaurs. The dinosaurs apparently breathed air that was much richer in oxygen than our air and lived in forests and grasslands that were far more combustible than ours. The metabolisms evolved to live is such an atmosphere might be radically different from ours. This new information may be relevant to many puzzles of the cretaceous and jurassic periods.
The problem of how the giant pterosaurs were able to generate enough energy to become airborne has troubled many paleontologists. For example, the Encyclopedia Britannica makes the unlikely suggestion that the pteranodon may have launched itself by "running downhill" on its stubby legs. The discovery of the oxygen enriched atmosphere of the cretaceous period sheds new light on this problem. In such an atmosphere many of the constraints of metabolism are relaxed. The creatures of the cretaceous may have been literally turbo-charged like race cars by the oxygen enriched atmosphere. It becomes plausible that a flying creature that evolved during that period could reach size limits that are impossible in today's anemic atmosphere.
Another puzzle from the era of the dinosaurs is the carbon layer at the cretaceous-tertiary boundary. The Alvarez hypothesis attributes the end of the cretaceous period to the collision of the Earth with a large chondritic meteor. The disintegrating meteor dumped vast quantities of fine iridium-rich dust into the atmosphere, bringing on a sort of "nuclear winter" that was connected with the extinction of the dinosaurs. A curious feature of this event, the "cretaceous catastrophe", is that a world-wide layer of finely divided carbon has been found at the cretaceous-tertiary boundary stratum beside the iridium rich dust from the Alvarez meteor. If this carbon is soot from a fire then the quantity of soot involved is truly enormous. Its quantity would require the simultaneous burning of a large fraction of the plant life on the Earth's surface, a sort of world-wide fire storm.
What produced this carbon layer and how? Was a fire ignited by the cretaceous meteor strike, or did the fire come later? How could such a world wide conflagration have occurred? Was it the meteor dust and its effects on climate and vegetation or the fire that killed the dinosaurs? The new information of oxygen content may provide important clues to these questions. The atmospheric oxygen data described above imply that the drop in atmospheric oxygen corresponded at least roughly to the cretaceous catastrophe.
One can imagine a scenario in which the Alvarez meteor dust blocks sunlight for several years, causing a large fraction of the surface plant life to wither and die. The brown dead vegetable matter would then provide excellent fuel in the oxygen rich atmosphere. Spontaneous combustion or lightning might trigger a fire that would spread over the brown landscape, producing the worldwide fire storm. A fire of this magnitude might well consume enough oxygen to account for the observed composition drop. In any case, the combination of dust, decimated vegetation, colder climate, a world-wide fire, and a 1/3 drop in atmospheric oxygen could certainly have combined to bring about the extinction of the dinosaurs.
From the viewpoint of the follower of science fiction there are important lessons here. Time travellers must be aware that the cretaceous period is not the same as the 20th century. Oxygen is present in incendiary quantities. Use cigarettes and matches only with great caution. Do not wear flammable clothing during your cretaceous travels. Do not leave camp fires unattended. Smokey the Tyrannosaur says, "Only you can prevent forest fires!"
Bubbles in Amber:
Richard A. Kerr, Science 238 #4829, 890 (13 November, 1987).
This page was created by John G. Cramer on 7/12/96.