Why is the ocean salty?
The Oceans are saline because of constant weathering of continental rocks by chemical and physical processes and dissolution of rocks over millions and millions of years. These rocks are degraded into their elemental constituents due to weathering and erosion.
Salinity and the main salt ions
The salinity of sea water (usually 3.5%) is made up of all the dissolved salts shown in the above table. Interestingly, their proportions are always the same, which can be understood if salinity differences are caused by either evaporating fresh water or adding fresh water from rivers. Freezing and thawing also matter.
Salinity affects marine organisms because the process of osmosis transports water towards a higher concentration through cell walls. A fish with a cellular salinity of 1.8% will swell in fresh water and dehydrate in salt water. So, saltwater fish drink water copiously while excreting excess salts through their gills. Freshwater fish do the opposite by not drinking but excreting copious amounts of urine while losing little of their body salts.
Marine plants (seaweeds) and many lower organisms have no mechanism to control osmosis, which makes them very sensitive to the salinity of the water in which they live.
The main nutrients for plant growth are nitrogen (N as in nitrate NO3-, nitrite NO2-, ammonia NH4+), phosphorus (P as phosphate PO43-) and potassium (K) followed by Sulfur (S), Magnesium (Mg) and Calcium (Ca). Iron (Fe) is an essential component of enzymes and is copiously available in the soil, but not in sea water (0.0034ppm). This makes iron an essential nutrient for plankton growth. Plankton organisms (like diatoms) that make shells of silicon compounds furthermore need to dissolve silicon salts (SiO2) which at 3ppm can be rather limiting.
The main salt ions that make up 99.9% are the following:
|Elemental Abundances in Seawater|
|Element||Symbol||Abundance||Res. Time (t)|
|Chlorine||Cl||19.5 ppt||>100 Million years|
|Sodium||Na||10.8 ppt||83 Million years|
|Magnesium||Mg||1.3 ppt||16 Million years|
|Sulphur||S||0.9 ppt||13 Million years|
|Calcium||Ca||0.4 ppt||1 Million years|
|Potassium||K||0.4 ppt||12 Million years|
|Nitrogen||N||11 ppm||< 0.6 ppm [NO3–]
in surface waters
The density of freshwater is 1 gram/ml or kg/liter. The saltier the water, the higher it’s density. When the water warms, it expands and becomes less dense. The colder the water, the denser it becomes. So it is possible that warm salty water remains on top of cold, less salty water. The density of 35ppt saline seawater at 15ºC is about 1.0255, or s (sigma)= 25.5. Another word for density is specific gravity.
The relationship between temperature, salinity, and density is shown by the blue isopycnal (of same density) curves in this diagram. In red, green and blue the waters of the major oceans of the planet is shown for depths below -200 meter. The Pacific has most of the lightest water with densities below 26.0, whereas the Atlantic has most of the densest water between 27.5 and 28.0. Antarctic bottom water is indeed densest for Pacific and Indian oceans but not for the Atlantic which has a lot of similarly dense water.