The scientific determination of the age of fossils

StoryImage( ‘/Images/Story//Auto-img-112188372217973.jpg’, ‘Photo provided by Burpee Museum’, ‘JANE’);
StoryImage( ‘/Images/Story//Auto-img-112188380317280.jpg’, ‘Photo provided by Burpee Museum’, ‘JANE, the lean, mean, killing machine roamed the Montana Badlands about 65 million years ago.’);

The newly opened exhibit of Jane the juvenile Tyrannosaurus rex (Latin for tyrant king) dinosaur at the Burpee Museum of Natural History is truly outstanding. All aspects of this creature’s life some 65 million years ago are covered in detail, and those with an active imagination can easily transport themselves back in time to the badlands of Montana when this creature and its kin dominated the earth. All of those responsible for this exceptional exhibit are to be congratulated.

However, I recently noted a letter to the editor of the Rockford daily paper questioning the stated approximate age of the fossilized Jane. The writer said there was no scientific method to determine time in terms of millions of years, and the stated age of Jane of 65 million years was merely a philosophical guess. He suggested the true age of Jane was closer to 7,000 years ago when Noah’s flood occurred. According to his Bible, “All flesh died that moved on the earth.” He further declared it would have been impossible for the dinosaur’s bones to have survived that long without being obliterated by the forces of nature.

Obviously, this individual is not up to date on the science of paleontology and geochemistry and does not realize the bones of Jane long ago were impregnated with minerals and were transformed into rock. Over the years, science has developed several methods of accurately dating fossils. For ages up to perhaps 50,000 years, the radiocarbon technique is very accurate. Living organisms utilize a portion of their organic carbon in the radioactive form. The half life of radioactive carbon is approximately 5,760 years, so fossils containing carbon such as bone wood, or other carbon containing compounds, may be assayed for their radiocarbon content. The difference between the amount in fresh tissue and the fossil is attributed to disintegration of radiaocarbon, and the age of the specimen can be calculated from the half life, the time it takes for half of the radiocarbon to break down.

This method is highly reliable for up to 50,000 years as it has been checked numerous times against objects of known age such as wool and other artifacts from Egyptian tombs.

For periods in excess of 50,000 years, the so-called lead method is sometimes employed. Over a period of time, the element uranium gradually disintegrates into lead and helium. With any amount of uranium, half of the molecules will break down forming lead and helium during the course of 4,510,000,000 years. This figure represents the half life of uranium and, by utilizing this figure, the age of the rock can be calculated.

This method has serious limitations because uranium is not a common element and may not be present in the fossil being dated or in the geological formations surrounding the specimen in question.

However, other geochemical methods have been developed that are used when uranium is not involved. For example, an isotope of the common element potassium (an isotope is an alternate form of an element involving a different number of neutron particles in the atomic nucleus) breaks down to yield calcium and argon. An isotope of rubidium decays into strontium, and thorium yields lead. The potassium-argon pair is more abundant in nature than uranium and the others, and it is more widely used. Rocks dated at 4.5 billion years old have been identified by this method, and that figure is widely accepted as the approximate age of the Earth.

The scientist in the field will take a specimen of the stratum immediately above and below the fossil he or she has found, and then send them to one of the few laboratories in the world capable of performing the complex dating procedure.

Though it is impossible to put an exact age on a fossil, the radioactive uranium or potassium-argon methodology is accurate within a few million years. So, Jane may not be 65 million years old, but 62 or 68 million. In the grand scale of geological history, a few million in this case is not important. The geological formation in which Jane was discovered is widely accepted to be about 65 million years old.

With Jane, we are not talking billions in comparison to what the late Illinois Senator Everett McKinley Dirksen once said: “A billion here, a billion there, and we are beginning to talk about real money.”

Dr. Robert Hedeen is a former resident of Maryland’s eastern shore and resided in the Chicago area from 1960 to 1971. He is a retired professor emeritus of biological sciences in the University of Maryland system. He has published more than 30 scientific papers, has written numerous magazine articles, and is the author of two books on the natural history of the Chesapeake Bay.

From the July 20-26, 2005, issue

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