The environmental spheres analogue: Atmosphere, hydrosphere, lithosphere, and biosphere

Chemicals on earth are distributed among four major environmental compartments or conceptual spheres: atmosphere, hydrosphere, lithosphere, and biosphere. While such a compartmentalization of nature is rather arbitrary, it helps in organizing our existing knowledge on the distribution and flow of chemicals. A schematic representation of the four environmental compartments and their interrelationships is shown in figure 3.

The circles represent the spheres and the curved arrows the flow pathways of matter. These are used instead of boxes and straight-line connections to emphasize the close, dynamic, inseparable, organic coupling among the environmental compartments; if one compartment or linkage changes, all other compartments respond.

In this conceptual frame, every sphere has a two-way linkage to every other sphere, including itself. The two-way linkage signifies that matter may flow from one compartment to another in both directions; the two-way transfer within a given compartment indicates movement of the substance from one physical location to another without changing the sphere. Since matter cannot be created or destroyed, the question one seeks to answer is the location and chemical form of the substance at a given time.

The four spheres

The atmosphere is best envisioned as a transport-conveyer compartment that moves substances from the atmospheric sources to the receptors. Its storage capacity for matter is small compared to the other spheres, but it has an immense capability for spatially redistributing matter.

The hydrosphere may be envisioned as two compartments: a conveyor (river system) collects the substances within the watershed and delivers them to the second hydrologic compartment, oceans.

Fig. 3 The four environmental spheres

The lithosphere is the solid shell of inorganic material at the surface of the earth. It is composed of soil particles and of the underlying rocks down to a depth of 50 kilometres. The soil layer is also referred to as the pedosphere, and is a mixture of inorganic and organic solid matter, air, water, and microorganisms. Within the soil, biochemical reactions by microorganisms are responsible for most of the chemical changes of matter. However, soil and rock are mainly storage compartments for deposited matter.

The biosphere is the thin shell of organic matter on the earth's surface. It occupies the least volume of all of the spheres but it is the heart, or the chemical pump, of much of the flow of matter through nature. Weathering through the hydrological cycle, wind, and volcanic releases are the other mobilizing agents. The biosphere is responsible for the grand-scale recycling of energy and matter on Earth. The mobilization of matter by biota is by no means restricted to small geographic regions. The periodic burning of forests and savannas, for example, not only changes the chemical form of matter, but also results in long-range atmospheric transport and deposition. Some of the biologically released chemicals, including carbon, nitrogen, and sulphur, have long atmospheric residence times, resulting in redistribution on a continental and a global scale.

Man and the biosphere

Human activities most closely resemble the function of the biosphere. In more than one way, humans are part of the biosphere. Humans and biota are responsible for grand-scale redistribution of chemicals on earth - once again with major similarities and differences. Fires and other forms of combustion result in an oxidation of both biogenic and anthropogenic elements. In nature, living plants tend to reduce their metabolized chemicals, thus ensuring a cycling of the chemicals that make up living matter.

Once again, the anthroposystem has no built-in mechanisms for reducing oxidized compounds. Man-induced oxidation products have instead to rely on biota for reduction, i.e recycling. Given the limited reduction capacity of the biosphere, many of the combustion products remain in stable oxidized form and are ultimately deposited in another long-term geochemical reservoir.

The atmosphere and the hydrosphere (rivers) are effective conveyors of matter. Consequently, many of the anthropogenic chemicals are transferred to the land oceans where they are subsequently incorporated in these long-term geochemical reservoirs. Much of the environmental damage is done in the atmosphere, hydrosphere, lithosphere, and the biosphere during the transit from one long-term geochemical reservoir to another.