All the chemical elements in the periodic table that can be found on Earth (Appendix A) must be present to some extent in seawater, although not quite all have been detected there yet. For a variety of reasons it is convenient to consider separately the major and minor constituents. In this chapter only the major constituents will be considered. In a continuum of concentrations, any separation is arbitrary, but it is convenient to pick a value of one part in one million (=1 ppm or 1 mg/kg) as a lower limit for the concentrations of major constituents. Substances present above this concentration may have a detectable influence on the density, for example, while those present in lesser concentrations will, individually at least, generally not. Most of the substances that occur in concentrations greater than 1 mg/kg are conservative – that is, they are found in nearly constant proportions to each other and to the salinity. Most of the substances present in concentrations of less than 1 mg/kg are not conservative.

As defined above, there are 11 substances included among the major constituents, and there are a couple more that could be included but conventionally have not been.