To illustrate the movement of material and energy among the spheres of the Earth system, let us take a brief look at the carbon cycle (Figure 1). Most carbon is bonded chemically to other elements to form compounds such as carbon dioxide, calcium carbonate, and the hydrocarbons found in coal and petroleum. Carbon is also the basic building block of life, as it readily combines with hydrogen and oxygen to form the fundamental organic compounds that compose living things.
Certainly one of the most active parts of the carbon cycle is the movement of carbon from the atmosphere to the biosphere and back again. In the atmosphere, carbon is found mainly as carbon dioxide (CO2). Atmospheric carbon dioxide is significant because it is a greenhouse gas, which means it is an efficient absorber of energy emitted by Earth and thus influences the heating of the atmosphere. Because many of the processes that operate on Earth involve carbon dioxide, this gas is constantly moving into and out of the atmosphere. Through the process of photosynthesis, plants absorb carbon dioxide from the atmosphere to produce the essential organic compounds needed for growth. Animals that consume these plants (or consume other animals that eat plants) use these organic compounds as a source of energy and, through the process of respiration, return carbon dioxide to the atmosphere. (Plants also return some CO2 to the atmosphere via respiration.) Further, when plants die and decay or are burned, this biomass is oxidized, and carbon dioxide is returned to the atmosphere.
Not all dead plant material decays immediately back to carbon dioxide. A small percentage is deposited as sediment. Over long spans of geologic time, considerable biomass is buried with sediment. Under the right conditions, some of these carbon-rich deposits are converted to fossil fuels—coal, petroleum, or natural gas. Some of the fuels are eventually recovered (mined or pumped from a well) and burned to generate electricity and fuel our transportation system. One result of fossil-fuel combustion is the release of huge quantities of carbon dioxide back into the atmosphere. Carbon also moves from the geosphere and hydrosphere to the atmosphere and back again. For example, volcanic activity early in Earth’s history is thought to be the source of much of our atmospheric carbon dioxide. One way that carbon dioxide makes its way back to the hydrosphere and then to the solid Earth is by first combining with water to form carbonic acid (H2CO3), which then attacks the rocks that compose Earth’s crust. One product of this chemical weathering of solid rock is the soluble bicarbonate ion (HCO3-) which is carried by groundwater and streams to the ocean. Water-dwelling organisms extract this dissolved material to produce hard parts of calcium carbonate (CaCO3). When the organisms die, these skeletal remains settle to the ocean floor as biochemical sediment and become sedimentary rock. In fact, the crust is by far Earth’s largest depository of carbon, where it is a constituent of a variety of rocks, the most abundant being limestone. Eventually, the limestone may be exposed at Earth’s surface, where chemical weathering will cause the carbon stored in the rock to be released to the atmosphere as carbon dioxide.
In summary, carbon moves among all four of Earth’s major spheres. It is essential to every living thing in the biosphere. In the atmosphere, carbon dioxide is an important greenhouse gas. In the hydrosphere, carbon dioxide is dissolved in lakes, rivers, and the ocean. In the geosphere, carbon is contained in carbonate sediments and sedimentary rocks and is stored as organic matter dispersed through sedimentary rocks and as deposits of coal and petroleum.
Author: E.J. Tarbuck, F.K. Lutgens illustrated by D.Tasa