Introduction: Biogeochemical Cycles As Fundamental Constructs for Studying Earth System Science and Global Change
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1 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 This includes all of the atoms in your own body and in all other living things, which have also The latter part of the 20th century has seen been permanent residents of the Earth through remarkable advances in science and technology. the eons. This means that the Earth is an essen- Accomplishments in biochemistry and medi- tially closed system with respect to atomic cine, computer technology, and telecommunica- matter, and is therefore governed by the law of tions have benefited nearly everyone on Earth to conservation of mass. This law dictates that all one degree or another. Along with these of the Earth's molecules must be made of the advances that have improved our quality of same aggregation of atoms even though molec- life, scientific research into the study of the ular forms may vary, evolve, and be transported Earth has revealed a planetary system that is within and around the planetary system. Pollu- more complex and dynamic than anyone would tion, therefore, is a human-induced change in have imagined even 50 years ago. The Earth and the distribution of atoms from one place on the environment have become one of society's Earth to another. greatest concerns, perhaps as the result of these In order to understand the impact of pollution discoveries combined with the quick dissemina- on Earth, we must realize that the planet itself is tion of information that is now possible with not stagnant, but continually moving material modern telecommunications. around the system naturally. Any human The basis of most environmental issues is (anthropogenic) redistribution in the elements is pollution. But what is pollution? Keep in mind superimposed on these continuous natural that with very minor exceptions, virtually all of events. Energy from the sun and radioactive the atoms in the solid, liquid, and gaseous parts decay from the Earth's interior drive these pro- of the Earth have been a part of the planet for all cesses, which are often cyclic in nature. As a of its approximately 4.5 billion years of exis- result, almost all of the rocks composing the tence. Very few of these atoms have changed continents have been processed at least once (i.e., by radioactive decay) or departed to space. through a chemical and physical cycle involving Earth System Science Copyright © 2000 Academic Press Limited ISBN 0-12-379370-X All rights of reproduction in any form reserved 4 Michael C. Jacobson, Robert J. Charlson, and Henning Rodhe weathering, formation of sediments, and sub- of the biota (all living things) on the planet. The duction, being subjected to great heat and pres- interfaces between the geospheres are often sure to produce new igneous rocks. The water in fuzzy and difficult to define. For example, the oceans has been evaporated, rained out, and ocean sediments contain water as well as rock returned via rivers and groundwater flow many and organic material; it is difficult to say exactly tens of thousands of times. The main gases in the where the hydrosphere ends and the lithosphere atmosphere (nitrogen and oxygen) are cycled starts. Part of the hydrosphere exists in the frequently through living organisms. The com- atmosphere as rain and cloud droplets. bined effect of these dynamic transports and The constant transport of material within and transformations is a planet that is in a state of through the geospheres is powered by the sun continual physical, chemical and biological evo- and by the heat of the Earth's interior. A simple lution. A bird's eye, cartoon view of the diagram of these geospheric concepts and the dynamic Earth system is shown in Plate 1. This energy that moves material within them is pre- book is about putting together all of the different sented in Fig. 1-1. The result of the interactions dynamic parts of this figure into an understand- shown in Plate 1 and Fig. 1-1 is an Earth system able, coordinated picture. In the last chapter of that is complex, coupled, and evolving. the book, we will revisit the topic of human In addition to the natural evolution of the modification of the system in detail. interacting geospheres, human activities have brought about an entirely new set of perturba- tions to the system. Because many political and T. T. 1 Biogeochemical Cycles and Geospheres social issues surround the problem of human induced global change, there are both basic and Aside from the cyclic systems listed above, applied scientific motivations to study biogeo- there is a complementary set of chemical cycles chemical cycles and their roles in the Earth that we can describe for each of the most system. The need for development and applica- important biological elements (carbon, nitro- tion of basic science to the broad policy issues of gen, oxygen, sulfur, and the trace metals). dealing with global change have inspired the These biogeochemical cycles are descriptions of formation of a new integrative scientific disci- the transport and transformation of the ele- pline. Earth system science (NASA, 1986). ments through various segments of the Earth The subject offers a number of challenges that system, called geospheres. We use these con- are important for the scientific community to structs to compartmentalize the larger Earth address. Probably the largest challenge is inte- system into more manageable, chemically grating knowledge and material from many definable parts. disciplines. This is a major theme of this chapter What are the geospheres? One of them is and of this book, as will be seen in the sections easily definable and requires no special intro- that follow. If the scientific community is not duction: the atmosphere is the gas-phase enve- able to integrate the science necessary to lope surrounding the globe of the Earth. describe biogeochemical systems, it seems un- Another geosphere is the hydrosphere, which likely that it will be easy for society to derive includes all of the oceans, and freshwater solutions for the problems raised by global bodies of water on the planet. The lithosphere is change. the entirety of rocks on Earth, including rocks The principal obstacles facing us as scientists exposed to the atmosphere, under the waters of studying Earth system science are the finite the hydrosphere, and the entire interior parts of resources of most educational institutions. the planet. The pedosphere (literally that upon Development of this subject requires that we which we walk) comprises the soils of the Earth. think of novel ways to do interdisciplinary work The geospheres listed thus far are more or less in a setting dominated by traditional disciplines. geographically definable, but there is a geo- Although we can draw heavily on work being sphere that can exist within all of the other done in recently formed disciplines such as geospheres: the biosphere, which is the collection chemical oceanography, stable isotope geo- Introduction Solar Radiation ^oooooooo Saline Hydrosphere C/) Sedimentary Rock CD Magma < ^ O O O ^ Radioactive Decay Fig. 1-1 Diagram of cyclic processes and fluxes between the major reservoirs on Earth. chemistry, and atmospheric chemistry, we may functioning of the planet was by James Hutton be able to glean some clues in how to accom- (1788), who viewed the Earth as a "superorgan- plish our goal of integrating the disciplines by ism, and that its proper study should be by examining the history of Earth system study. physiology." More than 100 years later, a classic Indeed, the recognition that biogeochemical paper by Svante Arrhenius (1896) appeared, issues may be significant for mankind goes called "On the influence of carbonic acid in the back several hundred years. air on the temperature of the ground." This work provided a paradigm for quantitatively connecting the greenhouse effect of carbon diox- 1.2 History ide to climate, as well as to the global biogeo- chemical cycle of carbon. A truly meaningful Som^e of the earliest work in the study of biogeo- study of these issues clearly requires input from chemical cycles and their role in the physical nearly all of the natural sciences (chemistry. 6 Michael C. Jacobson, Robert J. Charlson, and Henning Rodhe physics, biology, geology, meteorology, etc.). back-based integrative science geophysiology These scientific disciplines, which were still in and the system itself Gaia, which is a Greek their early formative stages when Arrhenius' word meaning Mother Earth. pioneering work emerged, have since evolved This view of the coupled nature of the Earth over the past century into highly refined and system has not dominated the historical devel- useful, but largely separated entities. Accord- opment of the key disciplines of the natural ingly, most scientists have adopted a reductionist sciences. Many evolutionary biologists have approach to biogeochemistry (i.e., simplifying viewed the changing physical climate of the large scientific problems into smaller parts to be Earth as an externally imposed factor to which examined by an individual discipline). In con- the biosphere must adapt. Likewise, many trast, as we stated above, one of the goals of geologists and geophysicists have viewed the Earth system science (referred to early on as evolution of the planet as being governed natural philosophy) is to integrate the natural primarily or exclusively by chemical and phys- sciences to strive towards understanding the ical processes. Under this perspective, free entire system. oxygen in the atmosphere is viewed as a con- In addition to the work of Arrhenius, several stant factor. The disparate views of the biologi- other scientists have made notable contribu- cal and geologic communities have coexisted tions that have helped to mold the approach for nearly a century, and (with few exceptions) and content of Earth system science (and of this their merging has been controversial (see, e.g., book).