Salinity in ocean

Salinity is roughly the number of grams of dissolved matter per kilogram of seawater. This was the original definition, and at one time salinity was determined by evaporating the water and weighing the residual. The dissolved matter in seawater affects its density, hence the importance of measuring salinity.

The law of constant proportions (Dittmar, 1884), formalized the observation that the composition of the dissolved matter in seawater does not vary much from place to place.

Why constant proportions?

Salts come from weathering of continents and deep-sea vents, etc - the input is very very slow (order 100.000 years) compared with the mixing rate of the whole ocean (which is order 1000 years). Thus it is possible to measure just one component of the dissolved material and then estimate the total amount of dissolved material (salinity). This approach was used until the 1950's.

The main constituent of sea salt is Cl, the second largest is Na, followed by many other constituents. In actuality, there is a slight variation in the proportions, and recommendations are underway to formulate new definitions of salinity which depend on the actual constituents - this may likely take form of geographically-dependent tables of corrections to the quantity which is measured (usually conductivity).

Goksenin Sen
Marine Educator

Reference:

Properties of seawater. In SIO 210 Lynne Talley Lecture notes. Retrieved from http://sam.ucsd.edu/sio210/lect_2/lecture_2.html in June, 2012.