The field of chemical oceanography has evolved over the past several decades from one of discovery to an interdisciplinary science that uses chemical distributions to understand physical, biological, geological and chemical processes in the sea. The study of chemical oceanography includes much of the background required to understand the global carbon cycle on all time scales because of the primary role of the marine carbonate system. Thus, we present this book about Chemical Oceanography and the Marine Carbon Cycle as a natural outgrowth of the evolution of our scientific field and a necessary background for building intuition to manage the anthropogenic intrusion into the global carbon cycle.

After a long deliberation about whether we had the time, stamina and personalities to write a book about our subject, John Hedges and I decided to do it, using as a guide, the notes we had compiled from teaching Chemical Oceanography together in the School of Oceanography at the University of Washington. During the first three years of the new century we used sabbatical leaves and time borrowed from teaching and research to compile about half of the book. Then, in 2003 John died suddenly and unexpectedly. Everyone John touched was thrown into a state of shock at the loss of a good friend, reliable colleague and brilliant organic geochemist. At this point we had put so much of ourselves into this undertaking that I felt there was no turning back, and I continued to complete what you see here.

Analyses of stable and radioactive isotope compositions have become a mainstay of the chemical perspective of oceanography, owing in large part to their value as tracers of important oceanographic processes. The utility of isotopes as tracers of biological, physical and geological ocean processes is perhaps the main reason that chemical oceanography has become a strongly interdisciplinary science. Small contrasts in stable isotope compositions can carry geographic information for discriminating sources such as different ocean water masses, and marine versus terrestrially derived organic matter. Within fossils, isotope distributions afford information about the temperatures, geographic settings, transport mechanisms, and ecology (e.g. who ate whom) of ancient environments. Stable isotope compositions also integrate the cumulative results of ongoing processes such as the passage of organic elements up trophic levels, climate change, marine productivity, and the formation and melting of continental glaciers. Stable isotopic signatures can persist over geologic time, even through severe changes in chemical composition. Radioactive isotopes have the additional property of being useful as nuclear clocks that, regardless of environmental conditions, dependably tick away to indicate the age of an object or the dynamics (e.g. turnover time) of a pool of materials. In addition, nuclear decay events often involve conversions of parent isotopes to daughter elements with very different physical and chemical properties, which then can be sensitively traced as they seek new chemical forms and locations in the ocean.

One of the great advantages of the chemical perspective on ocean processes is the ability to predict whether specific reactions between molecules and ions might occur in aqueous media. Given the extreme complexity of natural systems, this type of fundamental constraint is invaluable. Such predictions are based on concepts and energetic information that have been painstakingly generated over the past several centuries and assembled into the discipline known as thermodynamics. The purpose of this chapter is to present an introduction to the properties of water and ions, and the basic concepts of the thermodynamics of chemical reactions. Rather than cover the breadth of thermodynamics, we seek to demonstrate the applications of free energy to the prediction of equilibrium distributions of chemical species among gaseous, liquid, and solid phases. The goal is to establish the conceptual foundation and tools that can be applied to the following chapters on ocean processes.

The properties of water and ions

The structure of water

Water accounts for approximately 96.5 mass percent of seawater. The innate characteristics of water affect almost all the properties of the ocean (e.g. density, salinity, and gas solubility) and the processes (e.g. circulation, heat exchange, chemical reactions, and biochemical transformations) that occur within it. Water is so much a part of our world and daily lives that it is easy to overlook how unusual this substance is in its physical and chemical properties.

We end Part 1 of this book with a study of past changes in the Earth's atmosphere, oceans and ice volume. Interpretation of past climatic conditions from chemical tracers and isotopes preserved in the geological record requires knowledge and intuition developed from the study of present-day oceanography. For this reason descriptions of how paleoceanographic tracers are used to unravel insights about past ocean circulation and biogeochemistry serve as a review of the geochemical perspectives presented in the first six chapters of this book.

Present human activities create chemical sources to the environment that are, in some cases, comparable to those of the natural (pre-industrial) Earth. Since some of these anthropogenic additions may affect the natural order of the Earth's climate system, it is urgent to understand how the natural system functions mechanistically. For example, the effect of rapidly rising anthropogenic atmospheric CO2 on the climate system of the Earth is a first-order question (see Chapter 11). Even though global models that incorporate physical and biological interactions among the atmosphere, ocean, terrestrial and ice “spheres” now allow scientists to recreate the Earth's system, reasons for even the most first-order observations of climate change during the past million years are still poorly understood. The waxing and waning of glacial ice with roughly a 100 ky cycle is likely triggered by variations in the amount of solar energy reaching the Earth's surface because of changes in the Earth's orbital motions around the sun.

A chemical perspective

This book describes a chemical perspective on the science of oceanography. The goal is to understand the mechanisms that control the distributions of chemical compounds in the sea. The “chemical perspective” uses measured chemical distributions to infer the biological, physical, chemical and geological processes in the sea. This method has enormous information potential because of the variety of chemical compounds and the diversity of their chemical behaviors and distributions. It is complicated by the requirement that one must understand something about the reactions and time scales that control the chemical distributions. Chemical concentrations in the sea “remember” the mechanisms that shape them over their oceanic lifetime. The time scales of important mechanisms range from seconds or less for very rapid photochemical reactions to more than 100 million years for the mineral-forming reactions that control relatively unreactive elements in seawater. The great range in time scales is associated with an equally large range in space scales, from chemical fluxes associated with individual organisms to global processes like river inflow and hydrothermal circulation.

Studies of chemical oceanography have evolved from those focused on discovering what is in seawater and the physical–chemical interactions among constituents to those that seek to identify the rates and mechanisms responsible for distributions. Although there is still important research that might be labeled “pure marine chemistry,” much of the field has turned to the chemical perspective described here, resulting in a fascinating array of new research frontiers.

Cycling of carbon among the ocean, atmosphere and land is a fundamental component of the chemical perspective of oceanography because the fugacity () or partial pressure () of carbon dioxide is the most important greenhouse gas in the atmosphere (except for H2O, which behaves in a feedback rather than forcing capacity). Since there is about 50 times as much inorganic carbon dissolved in the sea as there is CO2 in the atmosphere, ocean carbonate chemistry has a great impact on in the atmosphere. On time scales of hundreds to a thousand years the main marine processes that influence in the atmosphere are the thermodynamic temperature dependence of CO2 solubility in seawater (the solubility pump), and the interplay between the rate of ocean circulation and the rate of biological carbon removal from the euphotic zone to the deeper reservoirs of the ocean (the biological pump). On longer time scales, of the order of one to tens of thousands of years, the preservation and dissolution of calcium carbonate along with the rate of weathering and the transport of bicarbonate to the sea come more into play.

The Earth is presently in the early stages of a grand acid–base titration of seawater by CO2. Anthropogenic CO2 is being added to the atmosphere at a rate fast enough to have resulted in an approximately 30% increase in the of the atmosphere since pre-industrial time.