Ocean science questions ANSWERED! post 9 - Why is the sea salty?

Q:  Why is the ocean salty? And what are the benefits of its saltiness? (Emma, Paul)

A:  Yayyyy!  I love this question – it seems so simple, and then there are all of these nuances that become important and it’s not a simple story.  So here goes:  The ocean is salty because it slowly accumulated salts over the course of its formation, and with the formation of the hydrologic cycle of the earth.  Nowadays, there is a tidy balance between th continuous onslaught of salts being delivered from rivers, and the output (export or sequestration) of those materials.  On land, rocks are weathered through chemical and mechanical weathering that dislodges minerals from the rocks and mobilizes them into runoff waters such as rain, rivulets, groundwater and rivers and streams.  This weathered material can take the form of dissolved salts or suspended particles (sediment).   Regardless, a huge amount of sediment and dissolved minerals are delivered to the ocean at the mouths of rivers.  There are other sources of salt material, as well:  dust mobilized from the great deserts and deposited in the ocean, hydrothermal vents and underwater volcanic eruptions, for example.  It seemed counter-intuitive to me that this bit of salt could make a difference in the enormous ocean,  but we’ve used this idea of salts input and outputs we see today to explain how salty must have formed on early earth.  Some of the ocean’s salts – especially chlorine – are extremely soluble, and extremely unlikely to be removed from the water column once they’ve entered.  The water in the ocean does not mix and turn over very quickly, so the salt eventually accumulated, but it took time.  The commonly held belief now is that salts built up in the ocean over a very long time period, enough to make the water salty, and now the ocean is in a relative steady state with respect to salinity (the latter is not disputed, and there is a huge amount of evidence that shows that salinity has been stable for millions, maybe billions of years).  When ocean water evaporates, the salts are left behind.  Back to present day ocean:  there is a net balance of salt in and salt out, while water continues to flow in and evaporate out, and we experience the salty ancient ocean.   Some of the salts are taken out of the water by organisms (especially calcium carbonate formation), by volcanic activity on the seafloor, and by the subduction of tectonic plates, but these are all slow processes. 

The salinity of the ocean ranges from brackish in the estuaries and some bays, with a salinity of less than 1, to high salinities in a place like the Mediterranean Sea with salinities upwards of 38 (parts per thousand).  Most of the ocean ranges from between 32 – 36.  The units used to describe salinity are interesting, because we don’t use a measure of actual salt per volume water anymore.  We now use the conductivity of the water, which indicates the level of salts, and the measure is a unitless ratio, so sometimes we use “PSU” to denote practical salinity units, but it’s not necessary to use units and sometimes it’s frowned upon. 

The ocean being salty is important, because we wouldn’t have life as we know it without it!  This is because of the hydrologic cycle.  If we didn’t have our hydrologic cycle, we wouldn’t be here.  We rely on the cycle for our water, and everything we eat depends on it too.  The ocean and the land are inextricably connected by the water and biogeochemical cycles of the planet, and we need them all to function.