The Biosphere Puts It Together Study Guide (page 4)

Updated on Sep 26, 2011


How much carbon goes to the ocean from rivers, carried from the land's soils? The amount is about 0.5 billion tons per year, mostly in the form of dissolved bicarbonate ions. But some organic matter, too, is carried along to the ocean. This half-a-billion tons per year seems smaller than all the fluxes we have considered so far, but its existence will lead us to a very deep understanding, in a moment, of the relationship between the biosphere and Earth's rocks below.

Note that in the numbers given so far for the natural cycle (ignore the human input of carbon dioxide), the ocean as a reservoir is not in balance. Across the ocean's surface, gas exchange with the atmosphere is 100 billion tons per year in both directions. Then we have 0.5 billion tons per year coming in from the soils via the rivers, creating an excess supply to the ocean. If that were to continue, year after year, the ocean's carbon content would double (from 35,000 to 70,000 billion tons) in 70,000 years (35,000 tons added/0.5 tons per year = 70,000 years). There must be another flux that leaves the ocean that we have not considered.

The additional exiting flux comes about from the organic detritus and carbonate shells of organisms that are buried in the ocean's sediments. Not all the productions of living things are recycled. Some of the productions "leak" out from the ocean and are trapped in the mud of the ocean bottom, to be buried deeper and deeper, someday becoming new rock. This burial flux is 0.5 billion tons per year. With these numbers, the natural ocean is in balance.

Here we come to the profound understanding of the relationship between the biosphere (air, water, soil, and life) and the deep earth below. With the burial of 0.5 billion tons of carbon from the ocean into the sediments each year, the biosphere as a whole is not in balance. The biosphere is losing 0.5 billion tons of carbon each year. At this rate, the biosphere will lose its carbon in how many years? Add up all the carbon in the four reservoirs and divide by the loss rate. You will be asked to provide the answer in a practice question.

For the biosphere not to be depleted of its carbon, new carbon must be entering the biosphere from below, from the geological processes that interact with the biosphere. What processes? Basically, plate tectonics.

Consider volcanoes. They release gases that happen to be high in carbon dioxide. Thus, volcanoes are a source of carbon in the form of carbon dioxide to the atmosphere. Consider, too, the rocks that are brought into the biosphere when the forces of plate tectonics expose fresh rock to the weathering processes of the biosphere during mountain building. Physical and chemical weathering first reduce the rock to particles and then dissolve the elements, thereby bringing those elements into the cycles of the biosphere.

All told, it is estimated that about 0.5 billion tons of carbon per year enter the biosphere as new carbon from below. This amount balances the loss of carbon from the biosphere that takes place in the sediments of the ocean. Thus, the connection between the surface biosphere—where organisms live—and the deep geological processes of the planet is essential for the maintenance of the biosphere and thus life itself.

Study Figure 17.1, which summarizes the reservoirs and fluxes of the carbon cycle, and then move on to the practice questions.

Figure 17.1

Practice problems of this concept can be found at: The Biosphere Puts it Together Practice Questions

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