1.3: Systems, Cycles, Reservoirs, and Fluxes

Thinking about the Earth as a system has become, in recent years, much in vogue. The basic idea of a system is that it is something that consists of a number of distinctive and diverse parts that function together by a variety of interactions and exchanges of energy and matter. Your motor vehicle is a good example of a fairly small and simple system: it is a machine with a number of moving or nonmoving parts, which consumes motor fuel and emits various exhaust substances as it travels from place to place. The Earth is, of course, a far larger and also far more complicated system, whose component parts operate on time scales from seconds to many millions of years. This is the basis for what is nowadays called Earth system science. Here I’m only touching upon some important aspects of Earth systems science that are relevant to this course: the hydrologic cycle and the carbon cycle. A whole course could be devoted to those and certain other cycles that are important in the context of the Earth’s surface environment.

The concept of cycles plays a fundamental role in systems thinking— although it’s not easy to frame a suitable definition of such cycles. A cycle might best be described as a characteristic succession of events and processes , involving certain kinds of Earth materials, by which the materials reside in certain kinds of places and move among such places in certain ways. Cycles operate through an indefinitely long span of time.

Much of the material that has been covered so far in this course involve, in one way or another, water and sediment. Earlier chapters have dealt in much detail with the Earth’s water and sediment: where they are, and how the move. Now it’s time for us to be more integrative in our thinking about the Earth’s water and sediment. To that end, the next two sections deal with what are called the hydrologic cycle and the sediment cycle.

Geochemists, in particular, are fond of thinking about certain substances from the standpoint of cycles. Such cycles are called geochemical cycles. The most important, in the context of the Earth’s surface environment, is the carbon cycle. The carbon cycle is especially important because carbon is the fundamental basis for life. It is also of great importance for the Earth’s climate, because carbon dioxide (CO2) is the second important of the so-called greenhouse gases, which play a key role in climate. (Water vapor is the most important, but we humans have virtually no control over the content of water vapor in the atmosphere, whereas we have great influence on the content of carbon dioxide in the atmosphere, owing to the burning of fossil fuels.) The concluding section of this chapter is devoted to some aspects of the Earth’s carbon cycle that are most relevant to the Earth’s surface environment. Geochemical cycling of certain other elements, most notably nitrogen and phosphorus, have received study because of their role as nutrients.

What pops into your mind when you hear the word reservoir? Probably a body of water, small or large, that is impounded behind a dam. In Earth systems science, the term reservoir is used for a distinctive kind of place where a certain kind of material is stored, or resides, for some period of time. That definition probably doesn’t enlighten you much, so here are some examples of near-surface reservoirs you have encountered earlier in the course: the atmosphere; glaciers; the soil layer; the aggregate of bodies of fresh water on the continents (rivers and lakes).

Material moves into and out of reservoirs. The rate at which a given material moves between reservoirs is called a flux. If the flux of material into and out of a given reservoir is the same for some period of time, that reservoir is said to be in a steady state. Commonly, however, the flux in and the flux out are not equal.

Another concept that is useful to know about when thinking about systems is residence time. In an earlier chapter you learned about the residence time of water in lakes. The same concept can be applied to any substance that resides in some reservoir for some period of time.