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:

• 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 beyond the scope of this book.

Two general principles are commonly used. The first one uses comparison with a reference material by means of calibration curves and only requires off-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 sample–flux mixture for all the minerals, even the most refractory ones (zircon, oxides). The resulting glass can be dissolved in hydrochloric acid, which is far less dangerous than HF.

The theory of element transport is a way of representing the spatial changes in geochemical properties in various contexts, such as movement in the ocean or mantle, the migration of geological fluids or magmatic liquids within a rock matrix, or the attainment of chemical and isotopic equilibrium among minerals within the same rock, etc. It is, in fact, a set of rather complex theories involving some heavy-going mathematics, which we can only touch lightly upon in this book.

The essential concepts forming the core of this theory are those of conservation, flux, sources, and sinks. A conservative property is additive and can only be altered by addition or subtraction at the system boundaries or by the presence of sources and sinks. Mass or number of moles are conservative properties; concentration is not: if a mole of salt is added to a solution already containing one mole, the resulting solution will contain two moles, regardless of how the salt is added. In contrast, two solutions of one mole per liter combine to form two liters of a solution at one mole per liter. A flux is a quantity of something (mass, moles, energy, etc.) crossing a unit surface per unit time. The most familiar of these fluxes is volume flow, which is quite simply the velocity υ(in cubic meters per square meter per second).

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 long-range electromagnetic force, and the mysterious intermediate weak force.

A major achievement of low-temperature geochemistry is its ability to provide estimates of variables such as ocean temperature, atmospheric composition and pressure, erosion intensity, and biological productivity. These estimates come through geochemical observables, known as proxies, which can be related with some confidence to a variety of parameters of our environment. The understanding of ancient climates, oceans, atmospheres, and biological activity would be very poor in the absence of these proxies and would remain qualitative and highly speculative. The derived environmental information, however uncertain it may be, can always be tested against predictions and with the help of improved observations can be continuously improved.

Let us first briefly review some of the most important environmental proxies for modern environments (<65 Ma).

1. As shown by Dansgaard in 1964, mean annual air temperature can be determined (or estimated) from the mean δD or δ18O value of the local precipitation (rain or snow).

2. The amount of ice locked up in polar regions is derived from the average δ18O value of seawater.

3. The temperature of deep oceanic water can be obtained from the δ18O values of benthic foraminifera.

4. The surface ocean δ18O is perturbed by evaporation, precipitation, and continental run-off. The sea-surface temperature (SST) can instead be obtained from the Mg/Ca and Sr/Ca ratios in the carbonates produced by organisms living in the photic zone, typically corals, pelagic foraminifera, and coccolithophores. The δ18O values of fish tooth enamel (phosphate), which is more resistant to diagenetic modification than carbonates, are a useful temperature proxy. Sea-surface temperature is also obtained from the relative abundances of alkenones extracted from sediments.

5. […]

The Earth is, together with Mercury, Venus, and Mars, one of the rocky planets of the inner Solar System. Our sampling of the Solar System is rather limited. Terrestrial samples are plentiful and the Apollo astronauts brought back hundreds of kilograms of lunar rocks. More than 30 meteorites from the Moon and 40 meteorites from Mars (the SNC meteorites) complete the well-identified planetary material. Achondrites are basaltic rocks which have been suggested to come from the asteroid Vesta in the asteroid belt, between Mars and Jupiter. Chondrites derive their name from the presence of abundant mm-sized molten blebs known as chondrules. They are fragments of much smaller, undifferentiated asteroids, and are divided into carbonaceous, ordinary, and enstatite meteorites. Carbonaceous meteorites are remarkable because they contain hydrous minerals, organic matter, refractory inclusions, and pre-solar grains (SiC, diamonds). Iron meteorites are actually Fe–Ni alloys: some of them represent the core of some small planetary bodies disrupted by impacts.

The composition and the conditions prevailing at the surface of each rocky planet are very different and distinct from those of the giant gas-rich planets, such as Jupiter and Saturn, of the outer Solar System. Astronomical observations indicate that the Sun and its companion planets formed by condensation of a cloud of gas and dust, called the solar nebula. The topic of this chapter is to describe the major processes that led from the early stages of the Universe to the formation of the elements, to the birth of the Solar System and finally to the formation of the Earth and its sister planets.

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. 6.1) 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 i introduced upstream with a concentration Ci in is either lost through the lake outlet or entrained into sediments. The sedimentation rate P is also expressed in kilograms per year. The lake itself is considered homogeneous, being well mixed by turbulent flow and by convection.

Tracing natural processes using isotopic abundances is probably the most successful aspect of modern geochemistry. The methods relying on isotopic data are analytically intensive since they depend on complex separation procedures and expensive equipment, but overall they are conceptually simple and robust. The natural variability of the abundances of isotopes in nature results from different processes:

1. Under a variety of thermodynamic and kinetic conditions, isotopes distribute themselves unevenly among co-existing phases (minerals, liquids, gases). These effects are in general very subtle as are the differences between the isotopes that create them, which explains why they escaped detection until the 1950s.

2. Radioactivity removes the parent isotope from an element and adds the decay product to another: 87Rb becomes 87Sr. We will see in the next chapter that the rate of removal of the parent isotope is identical anywhere and at any time in the universe, so this process only affects the relative abundances of the radiogenic isotopes.

3. Cosmic rays are particles produced outside the Solar System and their origin is still not quite understood. The energy of some of these particles, mostly protons and α particles, occasionally exceeds the nuclear binding energy. In the upper atmosphere, some nuclei, such as nitrogen and oxygen, get chipped by the collision, a process known as “spallation.” Most particles reaching the ground are actually secondary and can be accounted for by spallation.

4. […]

Modern geochemistry is a discipline that pervades nearly all of Earth science, from measuring geological time through tracing the origin of magmas to unravelling the composition and evolution of continents, oceans and the mantle, all the way to the understanding of environmental changes. It is a comparatively young discipline that was initiated largely by Goldschmidt in the 1930s, but its modern development and phenomenal growth started only in the 1950s. Although there are many journals dedicated to geochemical research, there have been remarkably few general geochemical textbooks that cover more than a limited segment of the full scope of modern geochemistry. This is one reason why Francis Albarède's new book is most welcome. It is written by the author of the authoritative and widely acclaimed Introduction to Geochemical Modeling (Cambridge University Press, 1995), and it is intended as an undergraduate introductory course in geochemistry. Its scope is large, though not all-inclusive, concentrating on the inorganic chemistry of the condensed part of our planet. Although it started out as a translation from the original French book, the new English-language edition is much more than just a translation. The entire text has been substantially revised and in some parts expanded, and it is really a new book. Yet it retains a distinctly French flavor, particularly in the way many subjects are addressed via mathematical description.

The purpose of this Chapter is a short review of how biological processes affect the geochemical pathways that would prevail in the absence of life and how they contribute to the production of specific components that can occasionally form the bulk of some biological material such as oil. Expertise in biogeochemistry requires a strong background in biology and biochemistry and also some understanding of how biomass interacts as a whole with the mineral world. Background is well beyond the scope of the present book and we will hence try to restrict ourselves to the simplest of concepts.

The geological record

Oxidized rocks, limestones, cherts, and phosphates contain the biological materials with the most spectacular contribution to the geological record. Modern limestones are largely formed by the accumulation of carbonate tests of foraminifera and unicellular algae such as coccolithophores. Diatom frustules contribute massive amounts of silica to sediments at the bottom of the Southern oceans. The gigantic phosphorites of Africa represent fossil hard parts (teeth and bones) or their remobilization by diagenetic fluids: they are mined to produce fertilizers for agriculture. On the sea floor, these three types of rocks are often associated with each other in areas rich in nutrients, continental platforms, wind-driven upwellings of deep seawater such as next to the coasts of Morocco and Peru and the older seawater from the Southern oceans.

The material of several chapters has been deeply revised and rewritten for better intelligibility and to account for some major recent scientific developments. Many figures have been redrawn. I stuck to the black-and-white option to make it easier for teachers to distribute photocopied material for educational purpose: with due credit, I will gladly provide the figures as eps files that can be dressed with colors for classes. New chapters have been added, one on stable isotope fractionation, one on biogeochemistry, and one on paleoenvironments; and existing chapters have been complemented with new material. Overall, the new edition is 50 percent longer than the first one.

Although I used graphic analogies whenever I thought it could spare the reader a difficult mathematical derivation without losing the substance of basic concepts, some of this new material will certainly be felt as a turn-off and I apologize for that. Boxes have been added for very specific material, such as the derivation of some equations, common misconceptions, or more anecdotal material, all of which can be left out without interrupting the main flow of the presentation. Quoting too much old work is pedantic but we can't really ignore the papers that created the basic concepts we use every day. I have marked as “must read” with a ♠ sign some references which laid the groundwork of entire fields.

Finally, at the end of most chapters I have incorporated an additional section of exercises, which have been found previously only on my website.

The external aspects of geochemical cycles, the phenomena that occur at relatively low temperatures (typically from 0 to +3°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 transfer 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 [H+] (mol kg–1) of 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. In an acid–base reaction, the acid is the proton donor and the base is the acceptor. 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.

3. […]

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; Lodders, 2003) 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 atmophile elements, which essentially fractionate in to the atmosphere and the ocean are not considered in this chapter (N, O, H), while only the long-term aspects of the carbon cycle have been addressed.