I think that a good case can be made that the three outstanding characteristics of the chemical environment of the near-surface zone of the Earth are that
(1) it is pervaded with water;
(2) it is largely oxygenated, and
(3) it involves the chemical element carbon in various important ways.
1.5.2 The Role of Water
Almost all of the chemical reactions that are important in the surface zone of the Earth take place in an aqueous medium. Water is the great reaction medium for near-surface chemical processes on the Earth. (The important exceptions are certain gaseous reactions in the Earth’s lower atmosphere, which will not concern us in this course. That brings in a whole other field of science: atmospheric chemistry.)
Water is often described as the universal solvent. More specifically, water dissolves far more substances than any other liquid, largely because the hydrogen ions and hydroxyl ions, as well as the polar water molecules themselves, can insinuate themselves into the ionic crystals of which most of the solid Earth is composed.
Chemical reactions of the kind that are important in the Earth’s near- surface solid materials just don’t happen in a waterless medium. All you need to do to convince yourself of that is to compare the rates of weathering processes on Earth (there will be a long section on that in the following chapter) with weathering processes on a dry planetary body like the Moon. All of the important reactions by which the primary minerals of igneous and metamorphic rocks are broken down into other minerals, which are sable under Earth-surface conditions, along with dissolved materials, are hosted by water.
1.5.3 The Role of Oxygen
The geologic evidence is good that the Earth’s atmosphere has contained abundant oxygen for about the latter half of geologic time. At present, the Earth’s atmosphere consists of about twenty percent oxygen, by volume. You know, of course, that oxygen is essential to animal life on Earth. What is probably less obvious to you is that the presence of that oxygen means that chemical compounds in the reduced state which become exposed to the Earth-surface conditions are very susceptible to oxidation. This is especially true of rock- forming minerals that contain ions ferrous iron (Fe2+). When such rocks undergo weathering at the Earth’s sauce, about which you will learn in the next chapter, the ferrous iron is oxidized to ferric iron (Fe3+).
Also, organic material (the tissues of plants and animals), which consists of a great variety of chemical compounds made up largely of carbon and hydrogen, become oxidized back to carbon dioxide and water when they are exposed to the oxygen-rich waters of the Earth’s surface. Only in places where ferrous-iron-bearing material and organic material are sealed off from the oxidizing environment can these materials survive in the reduced state. This happens, for example, in places like lake bottoms or the sea floor where the materials are rapidly buried under fine-grained sediments (muds), which are largely impermeable to the passage of surface waters. Under such conditions, all of the oxygen is consumed during oxidation, leaving variable amounts of the reduced materials intact. With regard to organic materials, the consequences for human society are enormous: that’s the origin of all of the fossil fuels we burn in today’s world!
1.5.4 The Role of Carbon
This is a good place for some comments on the important role of carbon dioxide in the Earth’s surface environment. Carbon dioxide, a gas, CO2, is a minor—but extremely significant—constituent of the Earth’s atmosphere. The average concentration of CO2 in the modern but pre-industrial atmosphere is (was!) about 280 parts per million (ppm), or about three percent, by volume. As a consequence of our burning of fossil fuels, it’s now up around 370 ppm, and climbing rapidly.
Carbon dioxide has an essential role in the Earth’s atmosphere: after water vapor, it is the most important greenhouse gas, as described in an earlier section of this chapter. (And, in contrast to water vapor, which is a dependent variable in the sense that its concentration in the atmosphere is itself a function of variety of atmospheric processes, the concentration of carbon dioxide is, nowadays at least, an independent variable, in the sense that humankind is acting to raise its concentration independently of atmospheric processes.)
Carbon dioxide, like the other atmospheric gases, dissolves in water. For a given concentration of carbon dioxide in the atmosphere, there is a corresponding equilibrium concentration of dissolved carbon dioxide in waters exposed to the atmosphere. Incidentally, that concentration depends on the temperature of the carbon dioxide–water system: the higher the temperature, the lower the equilibrium concentration. It also depends strongly on pressure, as anyone who has opened a bottle of champagne knows well, but that’s not a major factor in the behavior of carbon dioxide in the Earth’s surface environment because the atmospheric pressure varies over only a small range.
The carbon dioxide dissolved in water reacts with the water to form a weak acid, carbonic acid, H2CO3, according to the reaction
CO2 + H2O ⇔ H2CO3
In turn, the carbonic acid dissociates to form hydrogen ions and bicarbonate ions, according to the reaction
HCO3 ⇔ H+ + HCO3-
The consequence is that the Earth’s natural waters in contact with the atmosphere are mildly acidic, with a pH between 5 and 6. That natural acidity plays a major role in the chemical weathering of rock, as you will see in the following chapter.