WHY IS THE SEA SALTY AND DOES IT MATTER?

A mouthful of water while swimming in a lake is unpleasant but nothing compared to the same situation during a swim in the ocean. A sudden mouthful seawater leaves you gasping for a glass of water to wash the salty taste from your mouth. But have you ever stopped to consider why the sea is salty? In this article, we will dive into the realm of ocean salinity (salt concentration) and show that there is more to it than you may have thought. Where does the ocean’s salt come from? What is it made of and how is salinity measured? Finally, why should the saltiness of the ocean interest us at all?


ION
An atom or molecule that has a charge because it has gained or lost electrons.Positively charged ions (Na + ) are called cations; negatively charged ions (Cl − ) are called anions.
The ocean receives most of its salt from a process called chemical rock weathering (Figure A).The combination of water from rain,

Stedmon and Visser
What is in a kilo ( , g) of seawater?Most of it is water ( .g).The remainder (just over g) consists of a collection of salts.Chloride (Cl − ) and sodium (Na + ) are the dominant salts, representing % of the total.Much of the remaining salt is sulfate (SO 2− ), Magnesium (Mg + ), Calcium (Ca + ), and potassium (K + ).plus oxygen (O ) and carbon dioxide (CO ) from the air, acts to react with and dissolve the minerals that rocks are made of.You can see this process in places where rainwater has smoothed rock surfaces, or on statues or stone building decorations that have lost their original shapes.
Water molecules consist of hydrogen and oxygen atoms.The hydrogen end of the water molecule has a slight positive charge, and the oxygen end has a slight negative charge.This makes water an excellent solvent, which means a substance that can dissolve ions.

SOLVENT
A liquid that can dissolve a solid or gas.Water is an excellent solvent.
Rocks and minerals contain a mixture of ions, which can be grouped into those with a negative charge (anions) and those with a positive charge (cations).Since opposite charges attract, water molecules surround the ions and isolate them from each other (Figure A).So, although river water does not taste salty, it actually does contain salt-just a very low concentration.

CONCENTRATION
The amount of a substance in a volume of liquid or gas; e.g., g of salt in L of water has a salt concentration of g/L.

WHY IS SEAWATER SALTY?
Rivers ultimately flow out to the sea, taking the dissolved salts from rock weathering with them (Figure B).When ocean water evaporates falls as rain (or snow) over land.This process repeats and supplies more salt to the sea.
But this must only be part of the story, otherwise the oceans would be gradually increasing in salinity, eventually becoming so salty that they SALINITY Salt concentration; a measure of how salty seawater is.
could not dissolve any more salt.Seawater is salty, but not that salty!Try experimenting yourself: see how much table salt you can dissolve in L of water.It will be much more than the ∼ g/L there is in the ocean.So, there must be other processes at play that slowly remove salt from the ocean.Oceanographers call these processes a salt "sink," just as your kitchen sink removes water that comes from the tap.
Salt is slowly removed from the ocean by several processes.Evaporation of water in shallow coastal lagoons can cause the salt concentrations to increase so much that it precipitates and collects on the seafloor.This is how sea salt can be harvested for use in our food.Sea spray can also slowly move salt from the ocean to land.The water in the spray evaporates and leaves the salt behind on land.Finally, saltwater seeping through cracks in the ocean floor near undersea volcanic ridges also slowly remove salt from the ocean.But, on the whole, salt ions linger in the ocean thousands of times longer (several Stedmon and Visser million years) than water molecules do (thousands of years), making seawater saltier than river water.

MEASURING SALINITY
Worldwide, millions of measurements of ocean salinity are made every day.Let us look at why this is necessary and how it is done.The salinity and temperature of seawater influences the density of

DENSITY
The mass of a specific volume of gas, liquid or solid.The density of seawater is influenced by water, its temperature and the concentration of substances dissolved in it.
seawater.The more salt that is dissolved in water, the denser it is: while L of freshwater at • C weighs , g, L of seawater at the same temperature weighs , g. Di erences in ocean temperature and salinity between depths and locations influence ocean currents (to learn more about this, read this Frontiers for Young Minds article).If we want to understand how the oceans a ect local weather, global climate, and the distribution of resources such as fish, we need to understand ocean circulation, and for that salinity plays a role.
Measuring salinity is no easy task.As mentioned earlier, salt is not one substance but a mixture.In the early days of ocean exploration, precise volumes of seawater were evaporated and the salts left behind were weighed.In the s, the Danish geologist Forchhammer went further and determined the concentrations of individual salts [ ], which is a very time-consuming process even for just one water sample.After carefully measuring samples sent by explorers from all over the world, Forchhammer discovered that the relative amounts of the various salts in ocean water was almost always the same, which made things much simpler.This meant that scientists could measure just one salt, such as the chloride ion (Cl − ), which is present in high concentrations and is easy to measure.Salinity can then be calculated by multiplying by the constant Forchhammer derived: . .This number is remarkably similar to modern estimates ( . ) [ ], which is amazing given that he worked with simple equipment and did not even have electric lighting!In the s, electronic equipment was developed to assess salinity by measuring how well a seawater sample conducts electricity.This is the basis of modern salinity-measuring devices, which can be mounted on drones called Argo floats that are released into the ocean and send data back via satellites (to read more about these, see this Frontiers for Young Minds article).These underwater drones have a collection of sensors that can measure pressure (for depth), temperature, and conductivity (for salinity).They drift with ocean currents and can automatically control how well they float.When they sink and rise, they collect measurements of seawater properties that enable scientists to construct maps of ocean salinity (Figure ).

THE STORY CONTINUES
With all this progress, you would think that the puzzle of ocean salinity has been solved.But this is not the case.While Forchhammer's idea about the constant composition of sea water has been enormously useful, there are actually small but measurable di erences in the salt composition across regions of the ocean.Although these di erences are small, they are important if we want to accurately describe the properties of seawater.So, scientists are now updating how they calculate ocean salinity, taking into account that the salts in seawater are not so constant after all [ ].After over years, the story continues to unfold.There is more to salt than you might think.Keep that in mind next time a wave takes you by surprise and you get a mouthful of salty water!

ACKNOWLEDGMENTS
This publication was supported by a grant from the Smed Foundation to AWV and Independent Research Fund Denmark Grant No.
-B to CS. Figures were designed and created with support from Pernille W. Rasmussen.

Figure
Figure

Figure
Figure (A) The salinity of the surface ocean around the world.Dark red colors show the highest salinity, which is often in the tropics where hot weather leads to more evaporation.(B) A slice through the Atlantic Ocean (red line on map), showing how salinity can change with depth.The highest salinities are in surface waters of the tropics (either side of the equator).In deeper waters (below m), the salinity is slightly lower than at the surface (Data from World Ocean Atlas).