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The Origin of Coal
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18193 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
11 / 1990 |
2,846 Words |
| Author
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Jennifer M. Robinson
Jennifer M. Robinson is a research associate at the Earth
System Science Center at Pennsylvania State University. |
The greatest coal age in Earth history came in the Late Paleozoic era, between 320 and 250 million years ago. The first part of this period, the Carboniferous, produced much of the coal in Europe and North America. It is portrayed as a time of vast warm tropical swamps, with dragonflies with two-foot wingspans, and giant club moss trees. The second part, the Permian, yielded more coal, but is less familiar to us because the coal was deposited in Siberia, Australia, China, and India. Unlike Carboniferous coals, Permian coals were formed in cool climates. The forests that produced them were much more like modern forests than the giant club moss forests of the Carboniferous. Their dominant trees were ancient relatives of modern Southern Hemisphere conifers, but unlike those of today, most of them had broad leaves. The second great coal age, that of the Cretaceous and Early Tertiary, had rates of coal formation that were only about half those of the Late Paleozoic.
Why was this time of geologic history, just before the emergence of the dinosaurs, so peculiarly prolific in depositing coal within the earth? Various ideas have been proposed, but each has suffered serious deficiencies. Recently, new evidence suggests that those ancient trees could resist decay allowing their carbon to be deposited and compressed into black lumps of energy - and simultaneously changing the earth's chemistry and climate.
Coal and geochemistry
The late Robert Garrels, one of the foremost geochemists of this century, pioneered the now-flourishing study of global biogeochemical cycles; he developed a model to study global balances of carbon, sulfur, iron, and other compounds over geologic history. Later geochemists, including Robert Berner at Yale and Lee Kump at Penn State, have adapted this model in a way that allows them to study the effects of coal ages and other geochemical events. They conclude that the Late Paleozoic coal age could have greatly altered global chemical balances. Specifically, whenever an atom of carbon is buried in sediment, one molecule of carbon dioxide is removed from the atmosphere and one molecule of oxygen is released into the atmosphere.
The amount of carbon buried during the Late Paleozoic could have driven up atmospheric oxygen to levels of 30 percent or higher (the modern value is 21 percent) and driven down atmospheric carbon dioxide, which probably had been many times more abundant than today, to near modern levels. High oxygen is a possible cause for the large amounts of what appears to be fossil charcoal in
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