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Chapter 4 - Ocean Chemistry 

Scientists use a variety of instruments to measure salinity, temperature, and oxygen in the world's oceans. This information is then used to generate Hydrographic maps that depict the physical and chemical structure of the world's oceans.

      The temperature of the ocean generally decreases with depth. Why?

      The highest average temperature is about 25 oC at the surface. This temperature can still be found at 50 m depth. However cooler temperatures occur off the west coast of South America at this depth. Why? At about 200 m depth the pools of 25 oC water have almost disappeared. By 600 m there are just traces in the western North Atlantic. One can see that the Pacific is colder than the Atlantic. By 1100 m the Pacific is generally < 5 oC. Interestingly one can see a tongue of warm water in the eastern Atlantic, near the mouth of the Mediterranean. Why? Are there any other pools of warm water? By 1750 m depth the water is generally <4 oC, but one can still see the tongues of warm water. Why? How great is the overall change in temperature with depth in the ocean?

      The pattern for salinity is somewhat different. Take a look at the maps for the surface, 50 m, 200 m, 600 m, and 1100 m. Instead of generally decreasing with depth, salinity increases. Why? How great is the change in salinity? Is it as great as the temperature change? Why is there a tongue of high salinity water in the eastern North Atlantic? Are there any other areas that show a particularly high salinity?


      The physical properties of seawater affect the transmission of sound. The speed of sound decreases with decreasing temperature and increases with increasing pressure. The relationship of these two factors results in a zone of minimum sound velocity in the ocean referred to as the SOFAR channel. This channel is important for a variety of reasons. Sound is used by a variety of marine organisms to communicate and it appears that many marine organisms use the SOFAR channel for this purpose.

      Acoustic Thermometry is the field of study that relies on sound to measure the temperature of the oceans. Scientists have proposed using sound to assess global warming. This field experiment has generated quite a bit of controversy. In order to move forward with the experiment additional studies were needed to determine the potential impact on marine mammals.

Chemical resources

  • The ocean is a source of many natural chemical resources; salt is one of them.

Hydrologic cycle

While 97% of the earth's surface water occurs in the oceans the remainder is found in several other reservoirs. In fact the water moves from reservoir to reservoir fueled by the energy from the sun. Water evaporates, is transported, and eventually returns to the earth's surface as precipitation. The movement between reservoirs is called the Hydrologic Cycle.


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