Water and the Hydrosphere

Patricia C. Henshaw, Robert J. Charlson, and Stephen J. Burges 

It is the hydrogen bonds of water that give it unique physical and chemical properties, characteristics that set it apart from all of the other molecules formed from elements near the top of the periodic table. Table 6-1 compares several key properties of water to selected 

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Still further, water has large latent heats of evaporation and freezing (J/mol), all because of 

Humidity is a general term used to refer to water vapor in the air. The actual amount of water vapor that the air can hold increases with the temperature the higher the temperature, the greater the amount of water vapor that the air can hold. Various measures are used to express humidity absolute humidity and relative humidity are the two most widely used. Absolute humidity is the amount (by weight) of water vapor in the air at a particular temperature and pressure and it is usually measured in grams per cubic meter. 

For most practical purposes, however, humidity is expressed as the relative humidity (RH), the percentage of moisture in the air at a given temperature in relation to the amount of moisture that the air could hold at that temperature before condensing as dew. Since the latter amount is dependent on the temperature, the relative humidity is a function of both moisture content and temperature. A value of 50% RH, for example, means that the air holds half the water vapor it can hold at the prevailing temperature. At 100% RH, moisture condenses and falls as rain. The relative humidity is measured with instruments known as a hygrometers. 

The hydrosphere (the Greek prefix hydro means water) is the great mass of water that surrounds the crust of the earth. Water is one of a few substances that, at the temperatures normal on the surface of the earth (which range between about -50 and 50°C), exists in three different states liquid, gas, and solid. Liquid water makes up the oceans, seas, and lakes, flows in rivers, and underground streams. Solid water (ice) occurs in the polar masses, in glaciers, and at high altitudes, and gaseous water (moisture) is part of the atmosphere (O Toole 1995). Liquid and solid water cover over 70% of the surface of the earth. 

Water is the most abundant substance on the surface of the earth and the major constituent of all living organisms. Life on earth would not be possible and the biosphere would not exist without water. Water plays a key role in the sustenance of living organisms, since it is essential for all living processes, transporting nutrients to wherever they are required in their bodies, and removing from them their waste products. Thus, since it plays 

Pure water is colorless, odorless, and tasteless. The earth is pretty much a closed system, neither gaining nor losing much water, with very little of the earth s water escaping Into outer space thus, the same water that existed on the planet millions of years ago Is still here. Water Is, however, continually changing Its form between water vapor, liquid water and ice, and moving around through, below, and above the surface of the earth (see Fig. 86). 

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Henshaw, P.C., Charlson, R.J., and Burges, S.J. (2000) Water and the hydrosphere. In Earth System Science—From Biogeochemical Cycles to Global Change (Jacobson, M.C., Charlson, R.J., Rodhe, H., and Orians, GH., eds.), pp. 109-131, Academic Press, New York. 

Water and the hydrosphere are practically synonymous, but not completely so. The hydrosphere is the sum total of water on earth, except for that portion in the atmosphere. The hydrosphere combines all water underground, which constitutes the vast majority of water on the planet, as well as all freshwater in streams, rivers, and lakes saltwater in seas and oceans and frozen water in icebergs, glaciers, and other forms of ice. 

The carbon cycle is complicated by several reactions that involve CO2. These reactions transfer carbon between the atmosphere, the hydrosphere (Earth s surface waters), and the lithosphere (Earth s crastal solids). The processes that move carbon from one sphere to another are illustrated schematically in the figure below. 

Carbon dioxide is divided between the atmosphere and the hydrosphere. This division strongly favors the hydrosphere, because the large volume of water in the oceans has an immense capacity to absorb CO2. 

The second important source for the hydrosphere and the oceans are asteroids and comets. Estimating the amount of water which was brought to Earth from outer space is not easy. Until 20 years ago, it was believed that the only source of water for the hydrosphere was gas emission from volcanoes. The amount of water involved was, however, unknown (Rubey, 1964). First estimates of the enormous magnitude of the bombardment to which the Earth and the other planets were subjected caused researchers to look more closely at the comets and asteroids. New hypotheses on the possible sources of water in the hydrosphere now exist the astronomer A. H. Delsemme from the University of Toledo, Ohio, considers it likely that the primeval Earth was formed from material in a dust cloud containing anhydrous silicate. If this is correct, all the water in today s oceans must be of exogenic origin (Delsemme, 1992). 

As the planet acquires a volatile molecule inventory it begins to develop an atmosphere, and in the case of the Earth this also includes the extensive circulation of water in the hydrosphere. The weight of the volatiles trapped in the atmosphere of a planet leads to a mean surface pressure, po, given by ... 

Air and water are the two most important fluids on earth. The atmosphere and the hydrosphere are complementary in their role of transporting and transforming chemicals. The atmosphere is the fastest and most efficient global conveyor belt. Yet, certain chemicals prefer the aquatic milieu which is of global dimension, as well. In the hydrosphere typical transport velocities are significantly smaller than in the atmosphere. Therefore, residence times of chemicals in the water are usually much larger than in the atmosphere. 

Only 2.7% of water in the hydrosphere is fresh water. This is found in snow, glaciers, lakes, rivers and clouds. 

Horne, R.A. (1969) Marine Chemistry. The Structure of Water and the Chemistry of the Hydrosphere. Wiley-Interscience, New York. 

Fig. 22 shows the residence times tR of waters in the hydrosphere and the half-life ti/2 of various reactions. If ti/2 tR then it can be assumed that the system is roughly in equilibrium and thermodynamic models can be used. If, on the other hand, tR ti/2 kinetic models must be applied. 

Fig. 22 Schematic comparison between the residence times tR of waters in the hydrosphere (ocean to rainfall), the dissolution of various minerals in unsaturated solutions at pH 5 (quartz to gypsum) and the half-life tm of chemical processes (recristallization to solid-solid reactions) (data after Langmuir 1997, Drever 1997)...

Many human activities interfere with the normal conditions of the upper crust, the hydrosphere and the atmosphere. Geochemistry can monitor and possibly direct the development of more environment-conscious attitudes. For decades geochemists have been busy collecting data on sediment cover, soil, and the hydrosphere, atmosphere and biosphere of our planet. Early observations demonstrated an almost perfect equilibrium system today atmosphere, hydrosphere, and soil in many places are far out of balance (cf. Banar, Forstner and Muller, 1972 a, b, a report on river waters and sediments in Germany with a good reference list). All biological activity is seriously affected (cf. Muller and Forstner, 1973), not only in freshwater systems but also in parts of the oceans, where disequilibrium conditions have been observed locally. The German Science... 

The world s oceans hold 1.37x10 of water (97.2% of the total amount of water of the hydrosphere). They cover 71% of the earth s surface, are actually the biggest reservoir on our planet, and contain many important minerals. The overall content of mineral matter in the oceans is estimated to be about 5 x 10 tons [1,2]. The seas contain virtually all of the naturally occurring elements and are the only universal source of mineral wealth that is available to most nations. For some of them it is the only source. Yet, most of the elements, the microelements, are available in very low concentrations, i.e., in parts per billion (ppb). The products being extracted from seawater with economic profit at present are sodium chloride, magnesium compounds, and bromine [2-4]. During the last two decades there has been growing interest in the possibility of commercial recovery of additional minerals from seawater [5] and brines [6]. 

For special geochemical investigations, isotope ratios of other elements, such as B, N, Si, K and Se are also determined. The measurement of the distribution of the natural radioelements U and Th and their daughter nuclides in minerals, sediments, oil, water and the air gives information about the genesis of the minerals, sediments and oils, and about the processes taking place in the lithosphere, the hydrosphere and the atmosphere. The nuclear methods of dating will be discussed in chapter 16. 

H2O in MORE and the upper mantle. Water is not so readily lost by vesiculation, with measurements of unaltered MORE glasses of 0.12% to 0.33 wt.% H2O (Pineau and Javoy, 1994 Jambon, 1994 Sobolev and Chaussidon, 1996). Water behaves incompatibly during melting (see Jambon and Zimmermann, 1990), and so a concentration of 120-330 ppm is obtained for the mantle source region (see also Thompson, 1992). The upper mantle, therefore, contains at least as much water as the hydrosphere. The flux at ridges totals (1-3) X 10 " g The hydrogen isotopic composition is distinctive from that of the surface, with 5D = —l %c to —91%o (Craig and Lupton, 1976 Kyser and O Neil, 1984 Poreda et al, 1986). 

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