First rainfall from storms off the ocean will be enriched in the heavier 18 isotope. Hence the high 18 O and 2 H near the coast. Progressively lighter rainfalls inland due to the initial loss of the heavier isotopes. Enrichment toward the equator a function of evaporation rate. Highest in the tropics and lowest at the poles.
Figure shows the isotopic composition of various waters.
Meteoric water plots as a line which is latitude dependent as discussed above. Seawater plots as a single point due to the general homogeneity of ocean water. Magmatic water plots as relatively small box. Determined by analyzing the water in hydrous silicates. Metamorphic water plots as a larger box the result of analyzing hydrous minerals which are a mixture from igneous and sedimentary protoliths.