Properties of and Transfers between the Key Reservoirs

Part Two of this book focuses on the major "spheres" that deliver and receive the chemical constituents moving through the Earth system. This part is loosely organized in terms of the speed at which material is processed by a particular sphere. However, since the hydrologic cycle has traditionally served as a paradigm for considering mass balance in the elemental biogeo-chemical cycles, we begin by providing basic information on how the hydrosphere works in Chapter 6.

Earth is a unique planet in that it contains such a large amount of liquid water. The hydrosphere, which comprises this water, is both a reservoir (sphere) for material and a conduit of material between the various spheres (cycle). Its importance to the other cycles and spheres make it a logical starting point. Without this liquid phase and without its ability to evaporate into the gas phase, the Earth would be unlikely to have developed and maintained a biosphere. The polar nature of the water molecule, with its high boiling point determines the character of all aspects of the hydrosphere. We discuss its properties as a solvent, which allows it to participate in the fluxes of other material. Since water is necessary for life, we explore the variation of moisture on the planet, and the fluxes of water in and out of the reservoirs that contain it. The Earth's climate is also closely tied to the presence of water in different reservoirs and phases, especially in the control of the reflectivity of sunlight off of the planet. The hydrosphere and hydrologic cycle affect the cycling of all of the other biogeochemical cycles on Earth. Since the hydrologic cycle has itself undergone human modification, it has indirectly been a vehicle for which the other cycles have been anthropogeni-cally affected.

Closely connected to the hydrosphere and qualitatively similar to it (because it is highly mobile) is the atmosphere, covered in Chapter 7. The atmosphere is the least massive of the geo-spheres, the fastest moving and the one that is most sensitive to perturbations. Far from being a simple body of homogeneously mixed gases, the atmosphere contains a large amount of water in three different phases (vapor, liquid in the form of cloud droplets, and solid in the form of ice crystals in high clouds). In addition, the amounts of water in air are enormously variable in space and time. Other condensed-phase substances also exist (aerosol particles), ranging from supermicrometer dust particles down to molecular clusters of a few tens or hundreds of Angstrom units in dimension. As might be expected from its small mass, the atmosphere presents rapid and extensive variability of composition.

Just as in the case for the hydrosphere, the atmosphere participates in all of the major biogeochemical cycles (except for phosphorus). In turn, the chemical composition of the atmosphere dictates its physical and optical properties, the latter being of great importance for the heat balance of Earth and its climate. Both major constituents (02, H20) and minor ones (C02, sulfur, nitrogen, and other carbon compounds) are involved in mediating the amounts and characteristics of both incoming solar and outgoing infrared radiation.

Chapter 8 considers the pedosphere (literally,

Earth System Science ISBN 0-12-379370-X

Copyright i' 2000 Academic Press Limited All rights of reproduction in any form reserved that upon which we walk) which can be described as the interface where the lithosphere (rocks), the atmosphere, the hydrosphere, and the biosphere intersect. Like these three spheres, soils have a solid phase (made up of weathered rocks from the lithosphere and decayed matter from the biosphere), a liquid phase of pore water from the hydrosphere, and a gas phase of air trapped in spaces between the solid and liquid parts. The pedosphere deserves our attention because of its many roles in communicating with these other spheres, and the processes that provide living organisms soil nutrients for growing edible vegetation. Sometimes denoted as a separate reservoir, sediments formed from the chemical weathering and physical degradation of rocks exist both on land and under the hydrosphere. The solid phases existing in the pedosphere, soils and sediments have relatively low solubility and change much more slowly in composition and physical makeup than either the hydrosphere or atmosphere. However, the chemical reactions and transport of material that takes place in these parts of the Earth system assert long-term control on the chemical makeup of other systems.

The lithosphere - rocks - are the fundamental starting material for the biogeochemical development of the pedosphere. It is the slowest of the major spheres to change via tectonics and all forms of erosion. Chapter 9 surveys the factors -largely geological - that govern the biogeochemical functioning of this most massive of the geospheres. Although it is massive and slow moving, it plays extremely important roles in the Earth system. For example, in the bigger picture, the atmosphere and its condensed water yield acidic species (e.g., H2C03, H2S04, and HN03), the biosphere amplifies the acidity of carbon dioxide through its enhancement of dissolved C02 in groundwater, while the lithosphere provides the basic materials that react with the acidity via chemical weathering reactions. The upshot of this global acid-base interaction is the production and control of alkalinity in the global hydrosphere, mainly in the oceans.

To finish Part Two, Chapter 10 revisits the largest part of the hydrosphere - the oceans -again illustrating the central role of liquid water in the functioning of the Earth system. While water and the hydrosphere were the logical first topic of Part Two, they are also the logical last topic because the oceans are the sink for virtually all of the rapid portions of biogeochemical cycles. Some of the water evaporated from the oceans falls from the atmosphere as rain or snow, where it interacts with intervening and mediating entities - the lithosphere, biosphere, soils, and sediments. The consequence is a flow of dissolved and suspended matter to the oceans. These substances circulate within the marine biosphere, are carried around the globe in the great thermohaline circulation and in wind-driven currents, ultimately to be sedimen-ted or rejoined into the tectonic cycle. The oceans therefore are both the beginning and the end of the hydrologic cycle, completing the loop for many of the elemental cycles as well.

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