The external aspects of geochemical cycles, the phenomena that occur at relatively lowtemperatures (typically from 0 to + 30 °C) in the ocean, the atmosphere, and in rivers, are largely governed by chemical equilibria in solution or at the water–mineral interface. The cycles themselves imply transfers controlled primarily by water–rock interaction (erosion, sedimentation, hydrothermal reactions) and by biological activity. A central role is played by the carbonate system. We will apply these concepts to the geochemistry of erosion and of the ocean, with a discussion of the impact of these cycles on climates in particular.

Basic concepts

A few important concepts that are part of college chemistry are required.

1. Acidity is the concentration (mol kg-1) of [H+] protons in a solution. The exact form, H+ or H3O+, in which these protons occur is of little significance. A scale of acidity is defined by the potential pH of the protons in the solution, such that pH = –log [H+]. At 25 °C, pure water has a pH of 7. A lower pH indicates an acidic solution and a higher pH a basic solution.

2. Ion behavior is dictated by the dissociation of acids and bases. An acid is a proton–donor molecule. A strong acid such as HCl or a strong base such as NaOH become completely dissociated to produce Cl- and Na+ ions, which behave essentially like inert species and are of relevance only in terms of charge. […]

The analytical methods of geochemistry are many and varied, but they can be grouped by family depending on what is to be analyzed. Setting aside the high-temperature and high-pressure experiments that involve methods often borrowed from mineralogy and petrology, these methods fall roughly into three groups depending on what is to be measured:

• concentrations;

• isotopic ratios;

• speciation of elements in solutions, mineral phases, or organic matter.

We will omit the last item here, as the variety of methods would involve substantial developments with a large physics content about spectroscopic methods.

Measurement of concentrations

Two general principles are commonly used. The first one uses comparison with a reference material by means of calibration curves and only requires from-the-shelf reagents, while the second one, isotope dilution, requires artificially altered nuclide mixtures. In the first case, the operator compares the response to physical stimulation (radiation, ionization) by means of a suitable detector upon the passage of a solution containing the dissolved sample and a set of reference solutions. The first step of most procedures is the dissolution of the powdered sample in hydrofluoric acid (HF), the only acid to dissolve silicates. Often this attack phase is replaced by melting of the sample powder in a lithium meta-borate “flux,” the addition of which lowers the melting point of the mixture for all the minerals, even the most refractory ones (zircon, oxides).

I am specially happy to preface this book. First, because it is always a pleasure to be able to speak well of a friend's work; and Francis Albarède is a friend of long standing!We both embarked on our academic careers at about the same time. After some solid grounding in geology at the University of Montpellier, we were fortunate enough to begin our doctoral research in geochemistry in the 1970s in Professor Claude Allègre's laboratory at the Paris Institut de Physique du Globe, at a time when the discipline was really taking off in France. We also helped set up degree courses in geochemistry at the recently founded University of Paris 7, where we were appointed Assistant Lecturers. Our work together resulted in the publication of a short book in 1976, primarily for students, which quickly sold out and curiously enough was never reprinted! Few universities in those days offered specialist courses in geochemistry.

Times have clearly changed since then! Geochemistry is now taught in most universities and it is needless to recall here the fundamental contribution that this discipline has made to all areas of Earth sciences and cosmochemistry. It is always helpful, though, for students and for non-specialist faculty to have a textbook that provides a review of the basic concepts and the most recent contributions to the discipline. And this is the second reason why I am happy to present this book; because Francis Albarède's work fulfills both these requirements.

The speed at which the universe is expanding as measured by the “red shift” of light from the stars indicates that the Big Bang occurred about 15 billion years ago. Because our star, the Sun, is a mere 4.5 billion-year old youngster, it is clear that the Solar System recycles chemical elements with a long history. These elements form in stars by a combination of processes of thermonuclear fusion, nucleon absorption, and radioactive decay. Who would think that our warm sunshine is actually the output of the largest nuclear reactor for light years around? The processes leading to the formation of elements are known collectively as nucleosynthesis.

The formation of elements

The most abundant element in the universe is hydrogen, an element formed by an electron orbiting around a single proton. Let us begin with the formation of a Sun-like star by gravitational collapse of an interstellar nebula, a cloud of gas and dust. The Sun itself, which makes up 98% of the total mass of the Solar System, consists of 71% H, 27% He, and 2% of heavier elements. The composition of the Solar System is given by spectroscopic analysis of sunlight, and it is this composition that will concern us here (Fig. 9.1). The accumulation of potential energy released by the collapse of the star's enormous mass raises the elements at its center to extremely high temperatures, typically several million K, until thermal agitation prevents further contraction.

The 92 naturally occurring chemical elements (90, in fact, because promethium and technetium are no longer found in their natural state on Earth) are composed of a nucleus of subatomic nucleons orbited by negatively charged electrons. Nucleons are positively charged protons and neutral neutrons. As an atom contains equal numbers of protons and electrons with equal but opposite charges, it carries no net electrical charge. The mass of a proton is 1836 times that of an electron. The chemical properties of elements are largely, although not entirely, determined by the way their outermost shells of electrons interact with other elements. Ions are formed when atoms capture surplus electrons to give negatively charged anions or when they shed electrons to give positively charged cations. An atom may form several types of ions. Iron, for example, forms both ferric (Fe3+) ions and ferrous (Fe2+) ions, while it also occurs in the Fe0 elemental form.

A nuclide is an atomic nucleus characterized by the number Z of its protons and the number N of its neutrons regardless of its cloud of electrons. The mass number A is the sum of the nucleons N + Z. Different interactions act in the nucleus and explain its binding: the short-range (nuclear) strong force, the electromagnetic force, and the mysterious weak force. Two nuclides with the same number Z of protons but different numbers N of neutrons will be accompanied by the same suite of electrons and so have very similar chemical properties; they will be isotopes of the same element.

Before discussing the composition of the different systems of geological interest and the exchanges of matter that make those systems evolve relative to each other, it is worth recalling the principles governing the geochemical differentiation of our planet. These seemingly simple principles conceal what are often daunting complexities. They are: the principle of conservation of mass, elementary and isotopic fractionation induced by phase changes, kinetic fractionation, and radioactivity.

In the Introduction, we alluded to the contrast between mixing processes and differentiation processes. We will now look at a number of examples.

1. Partial fusion of the mantle beneath mid-ocean ridges produces basaltic liquids whose chemical composition is different from the ultramafic chemical composition of the source peridotite. This chemical fractionation of elements between the molten fluid and its parent medium can be described by thermodynamic rules. The former makes up the oceanic crust while the latter forms the refractory base of the lithosphere (located in the oceanic plates beneath the crust). When the oceanic crust and oceanic lithospheric mantle plunge at subduction zones, they begin a long journey within the mantle, where convection folds and stretches them, in much the same way as a baker kneads dough, progressively eliminating the differences that initially existed between the two constituent parts. Convective mixing therefore undoes the effect of magmatic differentiation.

2. The erosion of a granite yields clay minerals and quartz grains, and adds to the dissolved load of run-off water; the clay accumulates on the ocean floor, the quartz is deposited as sandstone on the continental shelf or slope, and the river water mixes with the ocean waters. […]

This chapter looks at the changes that over time affect the geochemical properties of a system or a set of systems, such as the mantle, the crust, or the ocean, when subjected to disturbances whether caused naturally or by human activity. The essential concepts utilized – residence time and forcing – are taken from chemical engineering. Viewing system Earth as a chemical factory composed of reactors, valves, sources, and sinks, has proved to be a simple and robust model. The theory goes by various names, with the “box model” probably the most widely used. We will first set out the principles by describing the behavior of a system with a single reservoir and then go on to generalize the approach.

Single reservoir dynamics

Let us begin by considering a lake (Fig. 5.1) containing a mass of water M that we will take to be constant. A river flows through the lake with a rate of flow Q, which we will express in kilograms per year. Q is therefore the same upstream and downstream. We are interested in the balance of a chemical species in the lake. A chemical element introduced upstream may be carried away downstream or be taken up by sedimentation, at a rate P also expressed in kilograms per year. The lake itself is considered homogeneous, being well mixed by turbulent flow and by convection.

The concentration Ci of an element i is therefore the same in the lake and in the downstream outflux.

Geochemistry utilizes the principles of chemistry to explain the mechanisms regulating the workings – past and present – of the major geological systems such as the Earth's mantle, its crust, its oceans, and its atmosphere. Geochemistry only really came of age as a science in the 1950s, when it was able to provide geologists with the means to analyze chemical elements or to determine the abundances of isotopes, and more significantly still when geologists, chemists, and physicists managed to bridge the chasms of mutual ignorance that had separated their various fields of inquiry. Geochemistry has been at the forefront of advances in a number of widely differing domains. It has made important contributions to our understanding of many terrestrial and planetary processes, such as mantle convection, the formation of planets, the origin of granite and basalt, sedimentation, changes in the Earth's oceans and climates, the origin of mineral deposits, to mention only a few important issues. And the way geochemists are perceived has also changed substantially over recent decades, from laboratory workers in their white coats providing age measurements for geologists or assays for mining engineers to today's perception of them as scientists in their own right developing their own areas of investigation, testing their own models, and making daily use of the most demanding concepts of chemistry and physics. Moreover, because geochemists generate much of their raw data in the form of chemical or isotopic analyses of rocks and fluids, the development of analytical techniques has become particularly significant within this discipline.

This chapter is intended to provide a geochemical overview of a number of important elements. The elements will be grouped according to mixed criteria, in particular their position in the periodic table and their geochemical properties. We will describe the major mineral phases that host these elements in the mantle and the crust, their properties in solution, and the processes by which they are transferred from any major reservoir (mantle, crust, ocean) to its neighbors. We will not reproduce here the terrestrial abundances, which can be found in Appendix A. We will nevertheless provide the reader with some important data. Condensation temperatures in the solar nebula (Wasson, 1985) define the volatile versus refractory character of the element. The solubility and complexation data in surface waters (Morel and Hering, 1993) and the residence times in seawater (Broecker and Peng, 1982) constrain the concentration level and speciation in natural waters at low temperature. Different parts of geochemical cycles may receive uneven attention. This inhomogeneous treatment reflects the power of geochemistry: different elements are used to trace different processes. The elements that essentially fractionate in atmospheric and oceanic processes (N, O, H) have not been dealt with, while only the long-term aspects of the carbon cycle have been considered.

The study of air pollution begins with the study of chemicals that make up the air. These chemicals include molecules in the gas, liquid, or solid phases. Because the air contains so many different types of molecules, it is helpful to become familiar with important ones through the history of their discovery. Such a history also gives insight into characteristics of atmospheric chemicals and an understanding of how much our knowledge of air pollution today relies on the scientific achievements of alchemists, chemists, natural scientists, and physicists of the past. This chapter starts with some basic chemistry definitions, then proceeds to examine historical discoveries of chemicals of atmospheric importance. Finally, types of chemical reactions that occur in the atmosphere are identified, and chemical lifetimes are defined.

BASIC DEFINITIONS

Air is a mixture of gases and particles, both of which are made of atoms. In this section, atoms, elements, molecules, compounds, gases, and particles are defined.

Atoms, Elements, Molecules, and Compounds

In 1913, Niels Bohr (1885–1962), a Danish physicist, proposed that an atom consists of one or more negatively charged electrons in discrete circular orbits around a positively charged nucleus. Each electron carries a charge of –1 and a tiny mass. The nucleus consists of 1–92 protons and 0–146 neutrons. Protons have a net charge of +1 and a mass 1,836 times that of an electron. Neutrons have zero net charge and a mass 1,839 times that of an electron.