Bob Doyle, Columnist
Last year at this time, there was a lot of talk about the world ending on Dec. 21, 2012. Of course, nothing of the sort happened — otherwise you wouldn’t be reading this column!
But in the distant past, there have been five global extinction events where over 75 per cent of all species died off over a two million years period (a relatively short time, geologically speaking).
How can we possibly know what happened many millions of years ago? Paleontologists use the principles of geology as well as radiometric dating. Briefly, sedimentary layers are laid down one layer on top of another layer.
Of course, if there are plate movements or Earthquakes, the layers may be tilted. In these layers are found fossils, whose age is the same as the layers of rocks next to the fossils.
Radiometric dating is determining the age of materials or rocks by the concentration of radioactive nuclei and their daughter isotopes.
Each type of radioactive nuclei has a certain half life, the time for half of a collection of a certain radioactive nuclei to decay by emitting particles.
The best known radioactive nuclei is Carbon 14 with a half life of 5,730 years. Every living animal or plant intakes Carbon 14 as it is formed in our atmosphere due to cosmic rays.
The normal concentration of Carbon 14 is one Carbon 14 in a trillion carbon atoms. By carbon 14 dating, we have been able to determine the age of earliest human settlements in North and South America. Radioactive potassium with a short half life is used in some stess tests for your heart.
A fascinating account of the five global extinction events is in the book “Scatter, Adapt, and Remember: How Humans Will Survive a Mass Extinction” by Annalee Newitz.
Unlike many books that either deny unsustainable trends in our civilization or insist that our civilization is about to drive off a cliff, Dr. Newitz’s approach is to point out how humanity can persist despite adverse conditions (of our own doing) or natural disasters (megavolcanoes or asteroid impact).
“Scatter, Adapt and Remember” is a Doubleday book, published in 2013 with ISBN 978-0-385-53591-5.
The earliest mass extinction was at the end of Ordovician Period (roughly 470 million years ago) . The Earth was then a greenhouse world with a level of Carbon Dioxide over a dozen times higher than at present. Plate tectonics had pushed all the continents into two huge land masses, each with a vast tropical shelf area. Most life forms were in the ocean, with few living on land. The two continents collided, creating a huge mountain range called the Appalachians. These high mountains of volcanic rocks began to slowly erode away, washing pieces of rock into the sea. These rocks and the sediments they formed absorbed much of the carbon dioxide in the atmosphere, changing our planet into an ice house.
So 450 million years ago, glaciers began to spread outward from the poles, destroying much of the marine life forms. Then in less than a million years, the glaciers receded, due to a resurgence of volcanoes, injecting carbon dioxide into the air. Another factor in this mass extinction might have been the position of our sun above the central plane of our galaxy, leading to a shower of cosmic rays.
The third extinction occurred at the end of the Permian Period, about 250 million years ago. This event, called “The Great Dying” resulted in 95 per cent of all species dying off.
In Siberia, there was a megavolcano, erupting as much as one million square miles of lava. From 13 to 43 billion tons of carbon were injected into the atmosphere as well as sulfur particles that blocked off the sunlight. At this time, all the land masses were joined into Pangaea, which spread from pole to pole.
Acid rain poured into the ocean and on land, killing both marine and land creatures. This die off was quicker than most, lasting only a hundred thousand years.
The last great extinction event occurred 65 million years ago when an asteroid six miles wide impacted central America, injecting a huge amount of dust and sulfur in the stratosphere, which took several years to settle out.
This extinction event was preceded several million years earlier by a megavolcano eruption in India (Decca Plateau). While the dinosaurs died off, their avian ancestors (birds) survived and today flourish.
Small furry animals called mammals emerged from the ground and began a rapid transformation that led to most of wild life we have today. Now one species of mammals (Homo Sapiens) rules the Earth.
SKY SIGHTS AHEAD: The moon will appear half full in the tomorrow’s southwestern evening sky. Tomorrow evening, the planet Venus will be brightest.
Bob Doyle invites any readers comments and questions. E-mail him at firstname.lastname@example.org . He is available as a speaker on his column topics.