7 - Global cycles of the elements

Summary

Biogeochemical cycles

The Wilson cycles circulate elements are circulating through the crust over long time periods, as discussed in Section 1.4. Cycling of material within and between the atmosphere, hydrosphere, and biosphere is typically much faster.

Biogeochemical cycles are flows of matter within and between the spheres as defined in the preceding Chapters 3 to 6. These flows are driven by solar energy, and they begin and end in the crust, which acts as the main reservoir of nearly all elements. Motions within the crust are driven by energy from radioactive decay, and the circulations of the atmosphere and ocean are driven by the sun. In fact, biogeochemical cycles are processes that take place within an open physicochemical system as depicted in Figure 7.1. The spheres are marked by ellipses, and their mutual interactions are indicated by arrows. We refer to the discussion of open systems in Section 2.2b, in particular with respect to the significance of the surroundings. The system can be treated in its entirety, but usually the focus is on a few chemical elements and their flow within and through the subsystems. Since 1971, the International Council for Science (ICSU) committee SCOPE has published a series of reports on the global turnover of elements central to the environment. Because of their importance for life, the nonmetals C, N, P, and S have been taken up several times, but Cl, Pb, Hg, Cd, and As have also been investigated. In Sections 7.2 through 7.6 of this chapter, we will follow the approach of SCOPE, that is, little emphasis is given to the details of the biochemical processes involved.