The Meteoric Isotope Line

Harmon Craig published (1961a) a 5D and 5lsO diagram, based on about 400 water samples of rivers, lakes, and precipitation from various countries (Fig. 9.4). An impressive lining of the data along the best-fit line of 

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The data in Fig. 9.4 lie on a straight line in spite of the very wide range of values 5D of -300%o to +50%o, and r)lxO of -42%o to +6%o. This line, called the meteoric line, has been found, with some local variations, to be valid over large parts of the world. 

The meteoric line is a convenient reference line for the understanding and tracing of local groundwater origins and movements. Hence, in each hydrochemical investigation the local meteoric line has to be established from samples of individual rain events or monthly means of precipitation. A specific example of a local meteoric line, from northeastern Brazil, is given in Fig. 9.5. A local meteoric line is obtained 5D = 6.4 5180 + 5.5 (Salati et al., 1980). Examples of equations of local meteoric lines reported from various parts of the world are given in Table 9.1. 

In Fig. 9.7 the groundwater data fall distinctly below the relevant meteoric line, indicating that secondary fractionation has occurred, or that the waters are ancient and were recharged in a different climatic regime that 

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