No substance exemplifies the principles of isotope distribution better than water. Water is practically ubiquitous at the Earth's surface, where it undergoes phase transitions, interacts with minerals and the atmosphere, and participates in complex metabolic processes essential to life. The isotopes of hydrogen and oxygen undergo large fractionations during these processes, providing a multiple isotopic tracer record of diverse phenomena. In the hydrologic cycle, hydrogen and oxygen isotope ratios provide conservative tracers, uniquely intrinsic to the water molecule, that elucidate the origin, phase transitions, and transport of H2O. In particular, the isotope data associated with these processes are amenable to theoretical modeling using the laws of physical chemistry. The characteristics of the principal reservoirs of natural waters on Earth are provided in the following sections. The distinct characters of these different reservoirs are very clearly shown on graphs where the δD values are plotted against those of δ18O. The oceans constitute 97.25% of the hydrosphere, cover 70% of the Earth's surface to a mean depth of 3.8 km, and have an enormous total volume of 1.37 × 109 km3. This large reservoir has strikingly uniform isotopic concentrations, with almost all samples havingδ18O = 0 ± 1 and δD = 0 ± 5 per mil relative to SMOW (Craig and Gordon, 1965). Values outside these ranges are almost invariably confined to surface waters that have salinities that differ from the normal value of 3.5 wt. %. These varations are generally attributable to evaporation, formation of sea ice, or addition of meteoric precipitation that may occur by direct rainfall, by river inflow, or by melting of icebergs. The latter effect was clearly documented by Epstein and Mayeda (1953) in the surface waters of the North Atlantic, where the isotopic variations were strongly correlated with variations in salinity. In detail, the deep waters of different ocean basins have distinct values of δ18O and salinity. Thus, the δ18O values of deep waters from the North Atlantic (ca. +0.05%o), Pacific (-0.15%o), and Antarctic (-0.40%o) oceans are distinct, and careful measurements can be used to infer details of oceanic circulation patterns (Craig and Gordon, 1965).
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