Isotopes of the Earth's Hydrosphere

From the physical point of view, all natural waters are solutions of various inorganic and organic compounds. They also contain gases, colloidal and suspended particles of organic and inorganic origin, and many species of microorganisms. Thus, when studying the dynamics and phase transition of natural waters which result in their isotopic fractionation, one should not use the physicochemical constants for distilled water but rather those for solutions of given composition and concentration, which are features of the given type of natural water.

On an average, there are 320 molecules of HDO, 420 molecules of H₂ ¹⁷O, and about 2,000 molecules of H₂ ¹⁸O for each 10⁶ molecules of H₂ ¹⁶O in natural waters of the Earth. The ratio of isotopic abundances of deuterium (D) to protium (H) is D/H = 0.000155 (0.0150 atom.%) and that of oxygen is ¹⁸O/¹⁶O = 0.002 (0.2 atom.%).

The main factor controlling the fractionation of isotopic species in surface waters is the difference in the saturated vapor between various water molecules: \({{P}_{{{\text{H}}_{2}}^{16}\text{O}}}>{{P}_{{{\text{H}}_{2}}^{18}\text{O}}}>{{P}_{\text{HDO}}}\). The fractionation factor in isotopic species of water molecules under equilibrium conditions is determined by the ratio of the saturated vapor pressure of light (p) and heavy (р′) components: α = р/р′. The fractionation factor α at 20°С is 1.08 for HDO and 1.009 for Н₂ ¹⁸О. In this case, the vapor in equilibrium with water will be depleted in deuterium (D) by 80‰ and in oxygen-18 (¹⁸О) by 9‰. The isotopic composition of the vapor is R _v  = R _w /α.

The isotopic composition of natural and artificial surface reservoirs is determined by several factors, the most important of which are (Gat et al. 1968; Merlivat 1970; Fontes 1976):

On the basis of experimental data, it has been pointed out that the isotopic composition of shallow groundwaters, which are replenished by infiltration of atmospheric precipitation through the unsaturated zone, is characterized on average by the heavy isotopes of typical precipitation of a given region. Under certain conditions, there are some differences in isotopic ratios for the two types of water mentioned above. This is explained by the fact that precipitation in the spring–summer season is partially and often completely re-evaporated from the Earth’s surface and from the unsaturated zone both directly and due to transpiration of moisture by plants.

It is observed from the previous chapters that the processes of water evaporation and condensation are of great importance in the fractionation of isotopes of natural waters. At the same time, the evaporation is primarily an attribute of surface conditions. It might occur in shallow underground waters but it is generally agreed by hydrogeologists that groundwater evaporation does not occur on a regional scale (Zaitsev 1967; Stankevich 1968; Smirnov 1971); however, in local zones, the underground evaporation is likely to take place. An example of such phenomenon is the evaporation of groundwater accompanying oil and gaseous deposits (Sultanov 1961). As a rule, these processes of water evaporation occur at elevated temperatures (~80°C). In this case, the isotopic fractionation factors are α _D = 1.032 and \({\alpha}_{18_{{\text{O}}}}=1.0042.\) The vapor phase differs insignificantly in isotopic composition from layer waters to a deposit. The water vapor which has been formed migrates with oil gases. During the migration of the vapor–gaseous mixture through porous layers at lower temperatures, underground fresh water deposits with mineralization less than 1 g/l and δD and δ ¹⁸O values greater than those which are characteristic of meteoric waters might form.

Important information regarding the evolution of the ocean and climatic changes of the Earth during geological history becomes available from investigating oxygen and hydrogen isotope abundances in pore waters and in the minerals of sedimentary rocks.

Groundwaters of modern volcanic regions are of interest to specialists as are the deep groundwaters of sedimentary basins, using isotope techniques in their studies, in order to elucidate the problems of their formation and solve a number of applied problems of interest concerning their utilization. Let us consider the most important results obtained during these investigations.

The results of stable hydrogen and oxygen isotope studies in minerals of magmatogenic rocks and fluid inclusions are in common use in geology mainly when the problem of hydrothermal ore deposition is concerned. Valuable evidence concerning the genesis of ore-forming fluids and mineralization sources of hydrothermal solutions, temperature conditions of the formation of mineral constituents, magmatogenic, metamorphic, and hydrothermally altered rocks and ore minerals may be obtained from isotope investigation (Taylor Econ Geol 69:213–298, 1974; Taylor, Earth Planet Sci Lett 38:177–210, 1978; Hall et al., Econ Geol69:884–901, 1974; Rye, Econ Geol 69:468–481, 1974; Robinson, EconGeol, 69:910–925, 1974; Sheppard and Taylor, Econ Geol 69:926–946,1974; Ohmoto and Rye, Econ Geol 69:947-953, 1974; White, Econ Geol69:954–973, 1974; Javoy, Geol Surv Open-File Rep 701:202, 1978; Taylor andSilver, Geol Surv Open-File Rep, 701:423–426, 1978; Matsuo et al., Geol SurvOpen-file Rep 701:278–280, 1978; Borshchevsky, Geokhimiya11:1650–1661, 1980; Shukolyukov, Geokhimiya 12:1763–1779, 1980;Vlasova et al., Isotopy of Natural Waters, pp. 180–192, 1978).

Besides hydrogen and oxygen-18 other stable isotopes, which are present in the hydrosphere, are of use in studying natural waters. They are the isotopes of helium ³Hе/⁴He, boron ¹⁰B/¹¹B, carbon ¹³C/¹²C, nitrogen ¹⁵N/¹⁴N, sulfur ³⁴S/³²S, chlorine ³⁷Cl/³⁵Cl, and argon ³⁶Ar/³⁸Ar. We discuss here briefly only some aspects of geochemistry and distribution in natural compounds of carbon and sulfur in order to understand dynamics and genesis of natural waters. These isotopes found wide application for solving practical problems in hydrology, hydrogeology, climatology, and oceanography. Information about the other isotopes one can find in the works of (Fritz and Fontes, Handbook of Eenvironmental Isotope Geochemistry,1980; Hoefs, Stable Isotope Geochemistry, 1973), and others.

The Earth’s atmosphere is penetrated by a continuous flux of charged particles, constituting of protons and nuclei of various elements of cosmic origin. Consequently, a great variety of radioisotopes, referred to as cosmogenic, are produced due to the interaction of these particles with the atomic nuclei of elements which constitute the atmosphere. Transported by air masses, radioisotopes are abundant over the whole gaseous sphere of the Earth. Being mixed with atmospheric moisture, a proportion falls over the Earth’s surface, to enter the hydrological cycle as components of surface waters, soil-ground moisture, and groundwaters. Another proportion becomes a component of ocean and inland basin waters through exchange at the surface of water reservoir. Finally, the Earth’s biosphere plays an active role in exchange processes, which are of great importance for some cosmogenic isotopes.

Among the environmental radioisotopes, tritium is the most attractive to those researchers who are studying the principles of water circulation in nature. It is a constituent of water molecules and, therefore, is a perfect water tracer. The interest in application of tritium for hydrological and meteorological purposes was increased greatly during the period of thermonuclear tests in 1953–1962, and also subsequently when a large amount of this artificially produced isotope had been injected into the atmosphere. The bomb-tritium, injected into the atmosphere by installments after each nuclear test, is a kind of fixed time mark of water involved in water cycling.

Carbon has always played an important role in geochemical processes which take place in the upper layers of the Earth and, in the first instance, in the formation of the sedimentary terrestrial layer and the evolution of the biosphere. Radioactive carbon ¹⁴C is often used as a tracer of various natural processes such as the circulation of natural waters, their redistribution between the natural reservoirs, water dynamics of the hydrosphere and its elements, and is applicable for estimating the age of such waters. The age of geological formations and groundwater within the time scale up to 60,000 years is of a great interest for modern geology and hydrogeology. The data of radiocarbon distribution in different carbon-bearing natural objects are used for reconstruction of their paleoclimatic changes and for solving astrophysical problems related to variation with respect to time of the cosmic rays. In this chapter, the main attention is drawn to the distribution and application of radiocarbon with respect to dynamics of natural waters.

In addition to tritium and radiocarbon, other cosmogenic isotopes are used to study the natural waters.

While considering the amounts and production rates of primary cosmic rays and secondary nuclear particles with the three main constituents of the atmosphere such as nitrogen, oxygen, and argon were accounted for, but it has been pointed out that the nuclei of neon, krypton, and xenon, and also those of volcanic and meteoric origin, can make a certain contribution to the production of cosmogenic isotopes.

At present, more than 20 long-lived radioisotopes of heavy elements are known to exist in the Earth’s crust. These are evidence of the gigantic processes which resulted in the formation of chemical elements in our galaxy. The main characteristics of these elements are shown in Table 16.1.

The notion of the age of water is rather ambiguous. The age of water is usually understood to be its residence time in the studied geological object. It is further assumed that either the isotopic composition of the radioactive elements changes only through radioactive decay or that the law which governs their contribution or removal from water with a definite isotopic composition is known. These simple, theoretical suggestions are hard to apply in practice. Therefore, the main criterion of data verification is the comparison of results obtained by different methods, which usually correlate weakly with each other.

The environmental stable and radioactive isotopes are widely applied to solve the practical problems related to the study of different aspects of dynamics of natural waters and the hydrosphere as a whole. The solution of such problems by the isotopes as natural tracers is based on natural regularities in their space and time distribution in the hydrosphere, which were discussed in the previous chapters. Some examples of practical application of the environmental isotopes in hydrology, hydrogeology, meteorology, oceanography, and climatology are presented in this chapter.

The use of the stable and radioactive environmental isotopes in combination with the conventional classical methods appears to be a most productive way to study the regional natural processes which are developed during considerable intervals of time. The example of such a work based on application of the isotope and classical methods is the paleohydrologic investigation in the Aral-Caspian basin which was carried out for a number of years by the initiative and practical action of the authors.

The samples of the Moon rocks brought by the astronauts and space apparatus allowed in obtaining data on their physical-chemical properties and on this basis to make a number of new, in principle, cosmophysical and cosmochemical conclusions. Owing to the use of new methods of matter analysis, the information about chemical, isotope content, and absolute age of meteorite, Moon and Earth’s rocks, water, and gases has substantially grown.