Within the Earth’s crust most chlorine and bromine is present in solution in water. The single largest of these reservoirs are the interconnected oceans. This reservoir contains roughly one third of all chlorine and bromine that is present in the surface reservoirs and is very well mixed. As a result the chloride and bromide concentrations in the open oceans are constant within narrow limits at about 19,353 ppm Cl and 67 ppm Br, giving a Br/Cl ratio of 3.46*10−3. Only in more restricted parts of the oceans the concentrations significantly deviate from these numbers. In arid regions the concentrations are, due to evaporation of seawater higher, while in colder regions the concentrations may be, due to dilution with rain and river water lower. In the case of evaporation only water molecules are removed from the oceans so that the Br/Cl ratio remains constant during the progress of evaporation. Only when so much water has evaporated that the brine is saturated in NaCl, and halite starts to precipitate, the Br/Cl ratio will change. The reason for this is that bromide is incorporated in halite at a much lower ratio to chloride than present in seawater (Boeke 1908), resulting in a slowly increasing Br/Cl ratio in brine from which salt precipitates.