Skip to 0 minutes and 7 secondsWILL HOMOKY: Rain falling on the land dissolves the rocks that build up our continents. And this chemical weathering occurs on the surface of the land and underground, where water percolates through the cracks and the gaps in the soils and rocks. Eventually this water emerges and it flows to the ocean as rivers, carrying with it a cocktail of chemical constituents that enter the oceans over many millions of years. Now, the composition of rocks that dissolve on land are not all the same. In fact, rocks vary between regions in their composition and have done throughout Earth history. And so it's difficult for us to quantify exactly what an average river composition is.
Skip to 0 minutes and 46 secondsWe do know that rivers contain only a tiny amount of salt, just a pinch compared to ocean waters. In fact, they contain just 120 parts per million in these waters. And these salts are slightly different to ocean waters too. River waters are dominated by calcium, magnesium, carbonate, and silicon. And these proportions may vary slightly depending on the rocks that have been dissolved and where these rivers have come from. So sea water, we know, is largely a sodium and a chloride salty composition. But the rivers entering the oceans contain largely calcium, magnesium, carbonate, and silicon.
Skip to 1 minute and 25 secondsSo over many millions of years of these rivers flowing into the oceans, why is it the oceans' composition is different to that of the river water entering them? Something else must be going on to account for the difference between the river water and the sea water's composition. So now you've considered where the salt is coming from and how it's entering the oceans. What happens next? Where does the salt go in the oceans and where does it end up over time.
Where does the salt come from?
In this step, Dr Will Homoky (University of Oxford) talks about the process of chemical weathering and the transport of water by rivers into the ocean. But rivers contain just a tiny amount of salt compared to ocean waters so there must be something else that accounts for the difference.
The salt comes from weathering and volcanic activity. The ocean formed very early on in Earth history, as soon as water comes into contact with rock then weathering processes start - these leach (dissolve) the soluble elements preferentially out of the rock (sodium, calcium, magnesium, potassium etc). There isn’t very much chlorine or sulphur in rocks but there is lots in volcanic gases and it readily dissolves in water in the atmosphere to form chloride and sulphate that rains into the ocean. If these processes go on for billions of years we get a salty ocean.
Where evaporation outweighs precipitation, surface seawater will become more salty. Local seawater salinity is also enhanced if there aren’t many rivers nearby, and/or if the basin is restricted, as the high salinity seawater can’t mix very well with seawater of normal salinity in this case. The Red Sea is a good example and has an average salinity of 40. Eventually high salinity water from restricted basins does escape and enter global circulation. Higher and lower salinity waters are still recognisable a long way from their source due to slow ocean mixing.
The reason river water is fresh is also due to evaporation. When water evaporates from the ocean surface, the salts don’t evaporate with it. It’s this freshwater that eventually ends up in rivers. That’s why we end up with a totally different balance of salts in rivers and the ocean.
What is the difference between land and sea salt?
Salt that is found on the land can come from evaporated ancient oceans, for instance, in the south of Spain today, there are large deposits of gypsum and salt that formed during a period of time known as the Messinian Salinity Crisis. During this period, several kilometres of seawater evaporated from cut-off basins, and left behind a series of minerals, including halite (rock salt) in thick layers. This salt would have been in similar concentrations to the salt in the ocean today.
However, there are other sources of salt on the land. The photographs of volcanoes on this site are incredible. It is run by a team of volcanologists who travel the world and photograph volcanic activity. In Dallol, there are vast salt lakes that are formed in a completely different way. Here, the groundwater is heated from below, as the plates are pulling apart in the region, and the magma is close to the surface, hence the series of Great Lakes and volcanoes throughout the Rift Valley.
When this water is heated, it dissolves a wide range of minerals in the pore waters, and brings them up to the surface. When they reach the surface they cool, and the salt in the waters returns to its crystalline form. Because this salty water is effectively “erupted” at high temperatures, it forms a variety of unusual, and often very short-lived geological features, such as the “hornitoes” seen in some of these photos. The end result is a region very rich in salt, which has been mined for several centuries. The colours in the photographs come from a range of accessory minerals that are also dissolved by the hot waters.
© University of Southampton