Natural Abundances of the Elements and Isotope Variations

A main concern of geochemistiy is the investigation of the abundance and the distribution of the elements on the surface and in deeper layers of the earth, and of transport processes. The components of the geosphere are the lithosphere, the hydrosphere and the atmosphere. The relative abundance of the elements on the surface of the earth is plotted in Fig. 15.1 as a function of the atomic number. This relative abundance is similar within the solar system. The elements H, O, Si, Ca and Fe exhibit the highest abundances and maxima are observed at the magic numbers Z = 8, 20, 50 and 82. The abundances of the elements and their isotopes are determined by the nuclear reactions by which they have been produced and by their nuclear properties, whereas the chemical properties of the elements are only responsible for distribution and fractionation processes. 

The abundances of the isotopes of the elements in the geosphere show some variations caused by their formation, by isotope effects or by transport processes. A certain isotope ratio IR is taken as standard and the relative deviation from this standard is expressed as the (5-value  

High variations of the isotope ratio are observed for the isotopes of hydrogen, H and D, because of their high relative mass difference. The average isotope ratio D H in the oceans (IR = 1.56 10 ) is taken as standard. In natural waters (5(D) varies between +6 and -200. Evaporation of water leads to appreciable isotope effects and high isotope effects are also found in the water of hydrates. 

Some values of the isotope ratios 0, and are listed in Table 

Because oxygen is the most abundant element on the earth, determination of 

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