Seawater
Oceanography Fundamentals
Course 1 · Chapter 3 · Seawater: Salt, Heat, Density, Light and Sound
The Challenger expedition of the 1870s brought back hundreds of bottled water samples from every ocean. The chemist William Dittmar analysed them in Britain and found something surprising: the amount of salt varied from place to place, but the proportions of the main ingredients hardly varied at all (check). Whether the water came from the Atlantic or the Pacific, chloride was always in the same ratio to sodium, and sodium to magnesium. That result, now called the principle of constant proportions, makes it possible to measure the saltiness of seawater by measuring just one thing. This chapter looks at what seawater contains, how temperature and salt together set its density, why the ocean is layered, and how light and sound behave in it.
What Is in the Water?
Seawater is, on average, about 3.5 per cent dissolved salts by mass, or roughly 35 grams per kilogram (check). Oceanographers call this the salinity. The salt comes from the weathering of rocks on land, carried to the sea by rivers, and from volcanic and hydrothermal activity on the seafloor.
| Ion | Share of the dissolved salt, approximately |
|---|---|
| Chloride | 55 per cent |
| Sodium | 31 per cent |
| Sulphate | 8 per cent |
| Magnesium | 4 per cent |
| Calcium and potassium | about 1 per cent each |
Table salt, sodium chloride, is therefore the biggest part but not all of it (check the percentages). Trace elements, dissolved gases (oxygen, carbon dioxide) and nutrients such as nitrate and phosphate are present in tiny amounts, but they matter greatly for life (Chapter 9).
Where it is saltier
In hot, dry subtropical regions, where evaporation exceeds rainfall. The Red Sea is among the saltiest open seas, at about 40 grams per kilogram (check).
Where it is fresher
Near river mouths, near melting ice and where rain is heavy, such as the tropical belt. The Baltic Sea is brackish, at only a few grams per kilogram in places (check).
Why the open ocean varies little
The open-ocean range is only about 33 to 37 grams per kilogram. The ocean is so large and well mixed that local differences are small.
Temperature
Sea-surface temperature runs from about minus 2 degrees Celsius near the poles to around 30 degrees in the warmest tropical seas (check). Most of the deep ocean, below about a kilometre, is a few degrees above freezing, whatever the latitude.
Seawater does not freeze at 0 degrees. Salt lowers the freezing point to about minus 1.9 degrees at normal salinity (check). When sea ice forms, most of the salt is pushed out into the water below, which makes that water saltier, colder and denser. This detail drives the deep circulation described in Chapter 7.
Density and the Layered Ocean
Seawater has a density of about 1,025 kilograms per cubic metre, a little more than fresh water. Two things control it:
- Temperature. Colder water is denser.
- Salinity. Saltier water is denser.
- Pressure also compresses water slightly at great depth.
Dense water sinks beneath lighter water, so the ocean arranges itself in stable layers.
| Layer or feature | Where | What changes sharply |
|---|---|---|
| Mixed layer | The top tens to a couple of hundred metres | Stirred by wind and waves, so temperature and salinity are nearly uniform |
| Thermocline | Below the mixed layer, to about 1,000 m | Temperature falls rapidly with depth |
| Halocline | Where salinity changes rapidly | Salinity changes with depth |
| Pycnocline | Often coincides with the above | Density increases rapidly with depth, making a barrier to mixing |
| Deep water | Below the thermocline | Cold, dense and little changed |
Light in the Sea
Water absorbs light, and it absorbs different colours at different rates. Red goes first, within the top ten metres or so; orange and yellow follow; blue and green penetrate deepest. This is why the open ocean looks blue, and why a diver's red equipment looks dull and grey at depth (check the depths). The lit layer where photosynthesis is possible, the euphotic zone, reaches about 200 metres in the clearest water but only a few tens of metres in murky coastal water, where sediment and plankton scatter and absorb the light.
Sound in the Sea
Where light fails, sound carries. Sound travels at about 1,500 metres per second in seawater, about four to five times faster than in air (check). Speed rises with temperature, salinity and pressure, so it changes with depth: it falls through the warm upper layers, reaches a minimum at around a kilometre down, and then rises again with pressure (check). Sound waves bend toward the speed minimum and can become trapped in a channel there, a feature known as the SOFAR channel. Whales are thought to use such channels for long-distance calls, and the navy and scientists use them for listening and monitoring (check).
Exercises
A kilogram of seawater has a salinity of 35 g/kg. How many grams of salt, and of chloride in particular (about 55 per cent of the salt), does it contain? How much salt remains if 1 kg of it is evaporated to dryness, ignoring losses?
Two water samples: A is warm (25 degrees) with salinity 36; B is cold (5 degrees) with salinity 34. Which is denser, and what happens if A is poured gently onto B? Explain which factor wins.
A diver in clear tropical water holds a bright red object. Describe what colour it appears at 1 m, at 20 m and at 100 m, and explain why, using what you know about light absorption.
Quick Reference
- Salinity: about 35 g of dissolved salt per kg; open ocean range about 33 to 37
- Constant proportions: the main ions are always in the same ratio, so one measurement gives the salinity
- Freezing point: about minus 1.9 degrees; sea ice rejects salt
- Density: about 1,025 kg/m3; colder and saltier means denser
- Layers: mixed layer, thermocline, halocline, pycnocline, deep water
- Light: red absorbed first, blue and green deepest; euphotic zone up to about 200 m
- Sound: about 1,500 m/s; trapped in a deep channel
- Check: ion percentages, heat-store comparison, sound-speed profile