Introduction: Biogeochemical Cycles as Fundamental Constructs for Studying Earth System Science and Global Change

Michael C. Jacobson, Robert J. Charlson, and Henning Rodhe

1.1 Introduction

The latter part of the 20th century has seen remarkable advances in science and technology. Accomplishments in biochemistry and medicine, computer technology, and telecommunications have benefited nearly everyone on Earth to one degree or another. Along with these advances that have improved our quality of life, scientific research into the study of the Earth has revealed a planetary system that is more complex and dynamic than anyone would have imagined even 50 years ago. The Earth and the environment have become one of society's greatest concerns, perhaps as the result of these discoveries combined with the quick dissemination of information that is now possible with modern telecommunications.

The basis of most environmental issues is pollution. But what is pollution? Keep in mind that with very minor exceptions, virtually all of the atoms in the solid, liquid, and gaseous parts of the Earth have been a part of the planet for all of its approximately 4.5 billion years of existence. Very few of these atoms have changed (i.e., by radioactive decay) or departed to space.

This includes all of the atoms in your own body and in all other living things, which have also been permanent residents of the Earth through the eons. This means that the Earth is an essentially closed system with respect to atomic matter, and is therefore governed by the law of conservation of mass. This law dictates that all of the Earth's molecules must be made of the same aggregation of atoms even though molecular forms may vary, evolve, and be transported within and around the planetary system. Pollution, therefore, is a human-induced change in the distribution of atoms from one place on Earth to another.

In order to understand the impact of pollution on Earth, we must realize that the planet itself is not stagnant, but continually moving material around the system naturally. Any human (anthropogenic) redistribution in the elements is superimposed on these continuous natural events. Energy from the sun and radioactive decay from the Earth's interior drive these processes, which are often cyclic in nature. As a result, almost all of the rocks composing the continents have been processed at least once through a chemical and physical cycle involving

Earth System Science ISBN 0-12-379370-X

Copyright r 2000 Academic Press Limited All rights of reproduction in any form reserved weathering, formation of sediments, and subduction, being subjected to great heat and pressure to produce new igneous rocks. The water in the oceans has been evaporated, rained out, and returned via rivers and groundwater flow many tens of thousands of times. The main gases in the atmosphere (nitrogen and oxygen) are cycled frequently through living organisms. The combined effect of these dynamic transports and transformations is a planet that is in a state of continual physical, chemical and biological evolution. A bird's eye, cartoon view of the dynamic Earth system is shown in Plate 1. This book is about putting together all of the different dynamic parts of this figure into an understandable, coordinated picture. In the last chapter of the book, we will revisit the topic of human modification of the system in detail.

1.1.1 Biogeochemical Cycles and Geospheres

Aside from the cyclic systems listed above, there is a complementary set of chemical cycles that we can describe for each of the most important biological elements (carbon, nitrogen, oxygen, sulfur, and the trace metals). These biogeochemical cycles are descriptions of the transport and transformation of the elements through various segments of the Earth system, called geospheres. We use these constructs to compartmentalize the larger Earth system into more manageable, chemically definable parts.

What are the geospheres? One of them is easily definable and requires no special introduction: the atmosphere is the gas-phase envelope surrounding the globe of the Earth. Another geosphere is the hydrosphere, which includes all of the oceans, and freshwater bodies of water on the planet. The lithosphere is the entirety of rocks on Earth, including rocks exposed to the atmosphere, under the waters of the hydrosphere, and the entire interior parts of the planet. The pedosphere (literally that upon which we walk) comprises the soils of the Earth. The geospheres listed thus far are more or less geographically definable, but there is a geosphere that can exist within all of the other geospheres: the biosphere, which is the collection of the biota (all living things) on the planet. The interfaces between the geospheres are often fuzzy and difficult to define. For example, ocean sediments contain water as well as rock and organic material; it is difficult to say exactly where the hydrosphere ends and the lithosphere starts. Part of the hydrosphere exists in the atmosphere as rain and cloud droplets.

The constant transport of material within and through the geospheres is powered by the sun and by the heat of the Earth's interior. A simple diagram of these geospheric concepts and the energy that moves material within them is presented in Fig. 1-1. The result of the interactions shown in Plate 1 and Fig. 1-1 is an Earth system that is complex, coupled, and evolving.

In addition to the natural evolution of the interacting geospheres, human activities have brought about an entirely new set of perturbations to the system. Because many political and social issues surround the problem of human induced global change, there are both basic and applied scientific motivations to study biogeochemical cycles and their roles in the Earth system. The need for development and application of basic science to the broad policy issues of dealing with global change have inspired the formation of a new integrative scientific discipline, Earth system science (NASA, 1986).

The subject offers a number of challenges that are important for the scientific community to address. Probably the largest challenge is integrating knowledge and material from many disciplines. This is a major theme of this chapter and of this book, as will be seen in the sections that follow. If the scientific community is not able to integrate the science necessary to describe biogeochemical systems, it seems unlikely that it will be easy for society to derive solutions for the problems raised by global change.

The principal obstacles facing us as scientists studying Earth system science are the finite resources of most educational institutions. Development of this subject requires that we think of novel ways to do interdisciplinary work in a setting dominated by traditional disciplines. Although we can draw heavily on work being done in recently formed disciplines such as chemical oceanography, stable isotope geo-

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