The Biosphere Puts It Together Study Guide (page 4)
The surface of the earth is a coordinated system that is called the biosphere, which includes the reservoirs of life, air, soil, and the ocean. Chemicals in these reservoirs cycle around and around, as rocks are turned into air and life, as ocean becomes soil, and soil becomes ocean. To understand the earth, we need to understand how the reservoirs of the biosphere interact, which is illustrated by the great cycle of carbon.
Structure of the Biosphere
The biosphere is the thin, dynamic outermost layer of our planet, which includes air, water, soil, and life. It stretches from the top of the atmosphere to the depths of the ocean. This zone is where most living things not only live, but are connected to each other by the circulations within the physical reservoirs of air, soil, and ocean. It is necessary to talk about life when we talk about the systems of the earth because life makes a huge impact on reservoirs, as we will see. But first, we will examine the time scales of circulation within the physical reservoirs.
The atmosphere circulates by great risings and fallings of air in the Hadley cells as well as by the tropical easterlies and midlatitude westerlies. Clockwise and counterclockwise winds blow around high- and low-pressure systems, respectively. By measuring certain gases in the Northern and Southern Hemispheres, scientists have determined that the entire atmosphere completely mixes in about one year.
Motions within the ocean are, for the most part, slower than the winds. But we have seen that the ocean, too, is a gigantic mixing machine, with tides and gyres, with the currents such as the Gulf Stream and Kuroshio Current, and with the deep overturning called the thermohaline circulation. Oceanographers are very interested in just how fast the ocean circulates. The number turns out to be about 1,000 years. That means any substance you put into the ocean will be mixed throughout the world's waters in about 1,000 years on average. That's quick for such a huge reservoir of water.
Although the soil doesn't have gyres and currents the way the atmosphere and soil do, the soil too does circulate. Winds and water can erode soil and deposit its grains elsewhere. Most important are the organisms, such as worms, that live in the soil and stir it. This process by which creatures mix the soil is called bioturbation. As we have seen, litter such as dead leaves from trees enters the soil at the top and degrades in the soil's deeper layers. The time scale for the mixing of soil varies with location, but in general, soil scientists have determined mixing times for soils on the order of 10 to 100 years.
All the reservoirs of the biosphere are connected to each other. This is similar to the way the reservoirs of the water cycle are all connected. The ocean water can become atmospheric water vapor, which becomes rain and then soil water, which can rise up into plant roots to be transpired back into the atmosphere. Indeed, the water cycle is an excellent example of how the four reservoirs of the biosphere—life, ocean, soil, and air—are interconnected. But this interconnection goes well beyond the water cycle. It includes cycles of other elements, such as carbon, nitrogen, and phosphorus.
Consider a system with just two reservoirs. How many back-and-forth connections can there be? Just one, a back and forth between the two reservoirs. Now consider a system of three reservoirs called A, B, and C. How many connections are there? Exactly three. Here they are: A-B, B-C, C-A. If the biosphere has four reservoirs (life, ocean, soil, and air), how many connections are there? You should come up with the correct answer of six. We must discuss six different types of connections between the four reservoirs of the biosphere: life-air, life-ocean, life-soil, air-soil, air-ocean, and soil-ocean.
The connection between life and air is primarily what is called gas exchange. Every time you breathe, you take in the atmosphere, extract some of its oxygen, and add some carbon dioxide waste from your body's cells; then you exhale the gases back into the atmosphere. This process is called respiration. The opposite process, extracting carbon dioxide and adding oxygen, takes places during photosynthesis, performed by green plants such as oak trees and grasses. Other examples of gas exchange occur too, but in general, through gas exchange, life affects the atmosphere and the atmosphere affects life.
Because the ocean is so huge, it is convenient to think of this connection as between life and the ocean, but the same reasoning applies to life and any bodies of water, such as rivers and lakes. Organisms in the ocean exchange chemicals with the water as the organisms take in nutrients and give back wastes. Some of the connection is by gas exchange, because water contains dissolved gases, such as oxygen and carbon dioxide, but other exchanges involve the transfer of ions and actual bits of matter.
For example, creatures such as coral that build shells of calcium carbonate obtain their calcium as ions from the ocean water itself. In this way, the contents of the water become the bodies of organisms. The nitrogen and phosphorus in the ocean, wastes from creatures sink downward into the ocean and are consumed by bacteria. Thus, a cycling of matter takes place between ocean water and marine organisms.
The soil's mineral grains of sand, silt, and clay contain many chemicals required by land creatures to live. Some silicates contain calcium and phosphorus, which are both required elements for life. (Chemical elements required by living things are called essential elements or, more simply, nutrients.) Many creatures actively obtain essential elements from soil minerals by secreting substances to dissolve those minerals. In this process, these creatures perform chemical weathering. In addition, all creatures in the soil, from worms to bacteria, put forth wastes into the soil matrix, and the soil creatures breathe the air in the pores of the soil. So gas exchange occurs between life and the soil, too. Apparently, the exchanges between life and soil are very complex.
Rain descends from the atmosphere, bringing with it certain chemicals in the air, and wind can carry off fine particles into the atmosphere, but the primary exchange between the soil and air is gas exchange. Gases move from the atmosphere into the air pockets (pores) of the soil, and they also move in the opposite direction, from the pores of the soil up into the atmosphere. Some of the gases will be used by organisms in the soil, and some gases are generated by organisms in the soil.
As in the case with soil, rain can bring not only water, but other chemicals from air to the ocean, and winds from the continents carry dust particles that are dropped onto the ocean's surface. Like the soil, the primary exchange with the atmosphere is through the movement of gases, back and forth across the surface between ocean and atmosphere. It doesn't appear to our eyes that the ocean is chock full of gases, but it is. The back-and-forth movement affects gases such as nitrogen, carbon dioxide, oxygen, and many more.
The connection between soil and ocean is a one-way flux, because the only way for substances to go back from ocean to soil is through the reservoir of the atmosphere. But substances do move from soil to the ocean all the time, transported by ground water and, most noticeably, by streams and rivers. As we have seen, rivers carry substances that are in the form of particles and substances that are dissolved ions, such as salts and phosphate ions. Rivers supply the oceans with new materials every day.
When we put together a picture of the four reservoirs and six connections, we have what is known as a biogeochemical cycle. Biogeochemical cycles are the cycles of elements essential to life. These cycles are thus biological (bio) and include geological processes (geo) and chemical reactions (chemical).
The Carbon Cycle
The most important biogeochemical cycle is that of carbon, the essential element in the organic molecules of life. In addition, carbon dioxide is one of the atmosphere's important greenhouse gases, crucial for maintaining Earth's surface above the freezing point of water, and also of concern in the future as a cause of global warming. To understand the global carbon cycle, we will first review the amounts and forms of carbon in the four main reservoirs and then look at the fluxes between the reservoirs.
Carbon in life is present in all organic molecules, the most common of which are proteins, carbohydrates, lipids, and nucleic acids (DNA). Carbohydrates, for example, come in different types, such as sugars, starches, and celluloses (in land plants). How much carbon is there in life? For this, we need to total both land life and marine life. Land life by far dominates the number, totaling at about 700 billion tons of carbon. In contrast, marine life has a miniscule amount of carbon, only about 2 billion tons. The difference comes about because most land life consists of trees, with huge trunks and root systems. Trees are great biological reservoirs of carbon, much of which is in the molecular form called cellulose.
The air contains two major forms of carbon: carbon dioxide (CO2) and methane (CH4). CO2 is 0.037% carbon and CH4 is 0.00014%; thus, CO2 is the dominant form. What is the total tonnage of carbon in the biosphere's atmosphere? The calculation requires knowing the total mass of the atmosphere and the molecular weights of the various gases, but let's go directly to the answer, rounded off: about 700 billion tons of carbon. It is intriguing that the amount of carbon as carbon dioxide gas in the atmosphere is about the same as the amount of carbon in the form of organic molecules in all of life globally.
Most of the carbon in the soil occurs in the top two layers, the O and A layers, which are the layers of organic litter and topsoil, respectively. Here we count only the carbon in the form of organic matter, because that is the form circulated by life's activities in the soil. Carbon also exists within certain minerals in the soil (such as carbonates), and that varies tremendously from place to place. The carbon in the organic matter of the world's soils is about three times the amount of carbon in the atmosphere, for a total of about 2,100 billion tons of carbon.
The oceans contain by far the largest reservoir of carbon. In addition to invisible amounts of organic carbon (dissolved proteins and carbohydrates from organisms, for example), seawater has carbon in other chemically active forms. One form is simply dissolved carbon dioxide gas, but this reacts with the water molecules to form the two other main types of carbon containing chemicals in seawater. The first is the bicarbonate ion (single negative charge), and the second is the carbonate ion (double negative charge). Of all the types of carbon in seawater, the one with the largest amount is the bicarbonate ion. All told, carbon in the ocean totals a whopping 35,000 billion tons.
Now we come to the fluxes of carbon between the reservoirs. Recall that water changes state as it moves between some of its reservoirs in the global water cycle, changing, for example, from liquid to gas as it moves from ocean to atmosphere during evaporation. Carbon is much more complicated because carbon changes the atoms it is bonded with into molecules during some (but not all) of its movements between reservoirs.
The major fluxes between life and air are driven by land organisms. Trees and grasses move about 60 billion tons of carbon per year from its gas state as carbon dioxide in the atmosphere into the various organic molecules inside plants. The process is called photosynthesis. In contrast, animals such as flying insects and deer, which consume and digest plants, thereby releasing the carbon from its organic form back into carbon dioxide, create a flux of only about 5 billion tons per year. This release of carbon dioxide by animals takes place during their metabolic process of respiration.
Photosynthesis and respiration also occur in the ocean between the various forms of carbon in seawater and the organic carbon in marine organisms. Estimates peg both processes within the ocean at about 40 billion tons of carbon per year.
The flux between life and the ocean seems in balance, with photosynthesis and respiration at about 40 billion tons a year. But the exchange between life and air on land seems out of balance. Land plants remove 60 billion tons of carbon per year, but land organisms (we mentioned deer and flying insects) return only 5 billion tons per year. What happens to the other 55 billion tons per year taken out of the air by photosynthesis on land? The answer is that much of the growth of plants falls to the soil. How much? About 55 billion tons per year. This yearly flux of detritus to the soil provides the food for the soil organisms (not included in the organisms that live above ground, such as flying insects and deer). Organisms such as worms and bacteria in the soil consume the organic matter in the soil and create carbon dioxide waste during their own processes of respiration, which occur underground. Roots of plants also contribute to this respiration flux of CO2 to the soil.
The 55 billion tons of carbon dioxide gas added into the air pockets of the soil each year have to go some-where. It travels up into the atmosphere in the process of gas exchange between the soil and atmosphere. The net result of this gas exchange is that carbon dioxide moves from the soil into the atmosphere in a flux of 55 billion tons of carbon per year.
As noted earlier, the main flux between the atmosphere and ocean occurs as gas exchange. In the global biogeochemical cycle of carbon, carbon dioxide moves back and forth between the ocean and atmosphere. The flux is about 100 billion tons per year. In the natural system of the biosphere, this back-and-forth flux is in balance. Today, because humans are adding car bon dioxide to the atmosphere during the combustion of coal, oil, and natural gas, a slight net flux (about 2% excess) is going into the ocean, compared to the carbon dioxide that is leaving the ocean into the atmosphere.
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.
Practice problems of this concept can be found at: The Biosphere Puts it Together Practice Questions
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