November 2002 AGGMAN Geology

Carbonate Rocks — The Secret to Life on Earth

A view of the eastern hemisphere from space. Image courtesy of NASA.

Perhaps no other mineral commodity has as many uses as limestone, dolomite, and marble. These carbonate rocks and products derived from them are used as aggregates, cement, fluxes, glass raw material, refractories, fillers, abrasives, soil conditioners, environmental applications, ingredients in a host of consumer products, and much, much more.
Carbonate rocks form about 15 percent of the earth’s sedimentary crust and occur in rock formations that range in age from the youngest (Holocene) to the most ancient (Precambrian). You can thank your lucky stars for that, because billions of years ago carbonate rocks played an absolutely essential role in the creation of Earth’s unique combination of moderate temperatures, atmospheric gases (specifically nitrogen and oxygen), and the presence of liquid water that make Earth an inhabitable planet for organic life (including mankind) that is powered by aerobic combustion. Over the eons, carbonate rocks have continued to carry out their job of maintaining those life-sustaining conditions.
So, what complex processes are at work on Earth to enable the creation and maintenance of our unique combination of temperature, gases, and water? And how do carbonate rocks fit into those processes?
Certainly our position in the solar system helps. Our “sister planet” Venus is the second planet from the Sun; Earth is the third. The temperature on the surface of Venus reaches greater than 800° Fahrenheit, hot enough to melt zinc. However, Mercury is closer to the Sun than Venus, yet Venus is hotter than Mercury. So distance from the Sun is not the only factor. Something else is happening.
That “something else” is the presence of carbon dioxide in the atmosphere. The Venusian atmosphere is composed mostly of carbon dioxide and is about 90 times heavier than the atmosphere on Earth. (The pressure of the atmosphere if standing on the surface of Venus would feel the same as being under 3,000 ft. of water on Earth!) The carbon dioxide in the atmosphere on Venus allows incoming solar radiation to pass through, but blocks outgoing thermal radiation. This process is referred to as the greenhouse effect. On Venus, the greenhouse effect raises the surface temperature hundreds of degrees above what it would be otherwise. With such high temperatures, much of the water on Venus boiled away. The fate of the rest of the water is very interesting, but is well beyond the scope of this article.
Although the Earth’s atmosphere may have started out like that of Venus, Earth’s atmosphere today is quite different. Instead of being 95 percent carbon dioxide, the Earth’s atmosphere is 77 percent nitrogen and 21 percent oxygen, with only traces of carbon dioxide and other gasses. If we had 95 percent carbon dioxide like Venus, the results of the greenhouse effect would be unbearable; our oceans would boil away, and we would burn up.
How the Earth ended up with this unique life-supporting atmosphere of nitrogen and oxygen with only a trace of carbon dioxide is the result of a fascinating interaction between life itself (the biosphere) and rocks (the lithosphere).
Almost 3 billion years ago, bacteria and photosynthetic algae existed in the ancient oceans of Earth. These small, simple life forms extracted carbon dioxide from the atmosphere and released oxygen back into the atmosphere. Because the oceans have remained for millions of years, the life forms in the oceans were able to remove the original huge mass of carbon dioxide gas from the atmosphere by incorporating it into limestone and other carbonate rocks. Gradually — over immense geological time — the atmospheric gas content was altered from the dominance of carbon dioxide to the dominance of a mixture of nitrogen and oxygen — an atmosphere that favorably supports organic life. So the Earth’s limestone (and other carbonate rocks) serves as a life saving “sink” — a place to store excess carbon dioxide.
Even today the biosphere is removing carbon dioxide from the atmosphere, storing it as carbonate rock such as coral. However, it is extremely important to have a tiny amount of carbon dioxide present in order to maintain the Earth’s surface temperature. By having a small amount of carbon dioxide, the greenhouse effect on Earth raises the average surface temperature about 63° Fahrenheit, from a frigid -6° Fahrenheit to a comfortable +57° Fahrenheit. Without a trace of carbon dioxide in our atmosphere, the oceans would freeze and life as we know it would be impossible.
But if the biosphere is constantly removing carbon dioxide from the atmosphere, why is there any carbon dioxide left in the atmosphere? The answer to this question lies in the lithosphere (rocks).
Unlike the surface of Venus, the Earth’s crust is divided into several separate solid plates that float around independently on top of the hot mantle below. This ages-long process is referred to as plate tectonics, which was described in an earlier article in this column (See “Plate Tectonics — Building the Earth’s Surface,” AggMan, August 2002). When gigantic plates on the earth’s surface crash into one another, they push up huge mountain ranges. Precipitation and other natural forces erode the rocks in those mountains, including carbonate rocks, thus recycling carbon dioxide back into the atmosphere. Elsewhere, movement of the plates drag carbonate rocks down into the Earth, while volcanic eruptions release carbon dioxide into the atmosphere.
Working together, geologic processes and biological processes maintain a continual flow of carbon dioxide from the atmosphere to rocks, and back again, all the while fueling the greenhouse effect that ultimately sustains life on Earth.

William H. Langer is a geologist with the Mineral Resources Team of the U.S. Geological Survey.

November 2002 Carved in Stone Carbonate Rocks — The Secret to Life on Earth				Carbonate Rocks — The Secret to Life on Earth By Bill Langer A view of the eastern hemisphere from space. Image courtesy of NASA. Perhaps no other mineral commodity has as many uses as limestone, dolomite, and marble. These carbonate rocks and products derived from them are used as aggregates, cement, fluxes, glass raw material, refractories, fillers, abrasives, soil conditioners, environmental applications, ingredients in a host of consumer products, and much, much more. Carbonate rocks form about 15 percent of the earth’s sedimentary crust and occur in rock formations that range in age from the youngest (Holocene) to the most ancient (Precambrian). You can thank your lucky stars for that, because billions of years ago carbonate rocks played an absolutely essential role in the creation of Earth’s unique combination of moderate temperatures, atmospheric gases (specifically nitrogen and oxygen), and the presence of liquid water that make Earth an inhabitable planet for organic life (including mankind) that is powered by aerobic combustion. Over the eons, carbonate rocks have continued to carry out their job of maintaining those life-sustaining conditions. So, what complex processes are at work on Earth to enable the creation and maintenance of our unique combination of temperature, gases, and water? And how do carbonate rocks fit into those processes? Certainly our position in the solar system helps. Our “sister planet” Venus is the second planet from the Sun; Earth is the third. The temperature on the surface of Venus reaches greater than 800° Fahrenheit, hot enough to melt zinc. However, Mercury is closer to the Sun than Venus, yet Venus is hotter than Mercury. So distance from the Sun is not the only factor. Something else is happening. That “something else” is the presence of carbon dioxide in the atmosphere. The Venusian atmosphere is composed mostly of carbon dioxide and is about 90 times heavier than the atmosphere on Earth. (The pressure of the atmosphere if standing on the surface of Venus would feel the same as being under 3,000 ft. of water on Earth!) The carbon dioxide in the atmosphere on Venus allows incoming solar radiation to pass through, but blocks outgoing thermal radiation. This process is referred to as the greenhouse effect. On Venus, the greenhouse effect raises the surface temperature hundreds of degrees above what it would be otherwise. With such high temperatures, much of the water on Venus boiled away. The fate of the rest of the water is very interesting, but is well beyond the scope of this article. Although the Earth’s atmosphere may have started out like that of Venus, Earth’s atmosphere today is quite different. Instead of being 95 percent carbon dioxide, the Earth’s atmosphere is 77 percent nitrogen and 21 percent oxygen, with only traces of carbon dioxide and other gasses. If we had 95 percent carbon dioxide like Venus, the results of the greenhouse effect would be unbearable; our oceans would boil away, and we would burn up. How the Earth ended up with this unique life-supporting atmosphere of nitrogen and oxygen with only a trace of carbon dioxide is the result of a fascinating interaction between life itself (the biosphere) and rocks (the lithosphere). Almost 3 billion years ago, bacteria and photosynthetic algae existed in the ancient oceans of Earth. These small, simple life forms extracted carbon dioxide from the atmosphere and released oxygen back into the atmosphere. Because the oceans have remained for millions of years, the life forms in the oceans were able to remove the original huge mass of carbon dioxide gas from the atmosphere by incorporating it into limestone and other carbonate rocks. Gradually — over immense geological time — the atmospheric gas content was altered from the dominance of carbon dioxide to the dominance of a mixture of nitrogen and oxygen — an atmosphere that favorably supports organic life. So the Earth’s limestone (and other carbonate rocks) serves as a life saving “sink” — a place to store excess carbon dioxide. Even today the biosphere is removing carbon dioxide from the atmosphere, storing it as carbonate rock such as coral. However, it is extremely important to have a tiny amount of carbon dioxide present in order to maintain the Earth’s surface temperature. By having a small amount of carbon dioxide, the greenhouse effect on Earth raises the average surface temperature about 63° Fahrenheit, from a frigid -6° Fahrenheit to a comfortable +57° Fahrenheit. Without a trace of carbon dioxide in our atmosphere, the oceans would freeze and life as we know it would be impossible. But if the biosphere is constantly removing carbon dioxide from the atmosphere, why is there any carbon dioxide left in the atmosphere? The answer to this question lies in the lithosphere (rocks). Unlike the surface of Venus, the Earth’s crust is divided into several separate solid plates that float around independently on top of the hot mantle below. This ages-long process is referred to as plate tectonics, which was described in an earlier article in this column (See “Plate Tectonics — Building the Earth’s Surface,” AggMan, August 2002). When gigantic plates on the earth’s surface crash into one another, they push up huge mountain ranges. Precipitation and other natural forces erode the rocks in those mountains, including carbonate rocks, thus recycling carbon dioxide back into the atmosphere. Elsewhere, movement of the plates drag carbonate rocks down into the Earth, while volcanic eruptions release carbon dioxide into the atmosphere. Working together, geologic processes and biological processes maintain a continual flow of carbon dioxide from the atmosphere to rocks, and back again, all the while fueling the greenhouse effect that ultimately sustains life on Earth. William H. Langer is a geologist with the Mineral Resources Team of the U.S. Geological Survey.