The Biosphere Puts It Together Study Guide
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.
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