Meteoric Water General Considerations

The International Atomic Energy Agency (IAEA) has conducted a world-wide survey of the isotope composition of monthly precipitation for more than 35 years. The global distribution of D and in rain has been monitored since 1961 through 

A compilation of studies throughout the world s mountain belts has revealed a consistent and linear relationship between change in the isotopic composition of precipitation and change in elevation (Poage and Chamberlain 2001). The isotopic composition of precipitation decreases linearly with increasing elevation by about 0.28%c/100 m in most regions of the world except in the Himalayas and at elevations above 5,000 m. 

Dansgaard (1964) introduced the concept of deuterium excess , d defined as c = 5D -8 5 0. Neither the numerical coefficient, 8, nor the deuterium excess, d, are really constant, both depend on local climatic processes. The long-term arithmetic mean for all analyzed stations of the IAEA network (Rozanski et al. 1993) is  

Relatively large deviations from the general equation are evident when monthly data for individual stations are considered (Table 3.1). In an extreme situation, represented by the St. Helena station, a very poor correlation between 8D and 5 0 exists. At this station, it appears that all precipitation comes from nearby sources and represents the first stage of the rain-out process. Thus, the generally weaker correlations for the marine stations (Table 3.1) may reflect varying contributions of air masses with different source characteristics and a low degree of rain-out. 

The imprint of local conditions can also be seen at other coastal and continental stations. The examples in Table 3.1 demonstrate that varying influences of different sources of vapor with different isotope characteristics, different air mass trajectories, or evaporation and isotope exchange processes below the cloud base, may often lead to much more complex relationships at the local level between 8D and 8 0 than suggested for the regional or continental scale by the global Meteoric Water Line equation. 

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