Why is the sea salty?

Professor Rachel Mills explains why water is such a powerful solvent* and how this contributes to making our oceans salty.

Rachel describes how early oceanographers measured the saltiness of the ocean and, with the help of Jon Copley and Dr Verity Nye (a former Postgraduate Research Fellow of the university), demonstrates the modern day method using technology.

What units are used to measure salinity?

Often we think in terms of g per kg of water and we can express this as % or as parts per mille (‰). But when Verity measures conductivity of seawater she actually is using the Practical Salinity Scale and this doesn’t have units (i.e. it is dimensionless). You can read more about salinity here.

You will see in the following sections how temperature and salt content both affect the density of the water in the estuary just like they do in the open ocean. By measuring salinity we track the mixing of the fresh water (zero salinity) with the seawater (salinity of around 35). In general the saltier water is denser so sits below the fresher water which is less dense. The temperature differences depend on the temperature of the river water which is colder than the oceans in the winter but is a similar temperature to the oceans in the summer months. The relationship between saltiness, temperature and density is a complicated, non-linear one but in an estuary, as we have seen in the video, the dominant control on density is the salinity of the water. Colder fresh water flowing in from the land flows over the top of warmer saltier seawater and is mixed up in the estuary as the tide comes in and out.

^{A solvent is something into which a solute will dissolve in order to make a solution. Water is a solvent for anything that is soluble in it, salt is a solute because it can be dissolved in a solvent and salty water is a solution.}

^{Titration is a means of determining what sort of or how much of a solute is dissolved in a solvent.}