Lecture Notes for Lecture 8
Focus Question: Why is the ocean water layered?
Today we will look at how the ocean water is structured, and some of the important physical characteristics of sea water. Specifically the term THERMOHALINE Circulation is introduced.
1. What is ocean water made of? The “salt” in the sea is actually made up of several types of major ions dissolved in the water. We taste “salt” (sodium chloride) because there is an overwhelming amount of sodium and chloride in the water, but other important major ions are in there, too. They fall into 2 categories-the cations (+ charge), and the anions (- charge).
In order of residence time from longest to shortest
Cations Anions
Sodium Chloride
Magnesium Sulfate
Potassium Carbonate & Bicarbonate
Calcium
2. The supply of these major ions is predominantly from rock, reaching the
sea by way of erosion, river water, volcanic activity, groundwater, and
hydrothermal vents. The interesting part is that most of them are used
for specific purposes, and so most major ions have a distinct residence time.
calcium, bicarbonate & carbonate constituents of shells &
corals
magnesium, potassium, sulfate constituents in clays
sodium and chloride not taken up as much, and so are
the dominant ions
3. What does a typical profile of ocean water look like:
a. For temperature (tends to get much colder with depth)
b. For salinity (tends to get saltier with depth-away from surface where
rainwater can reach it)
c. Density (combination of cold temperature and saltier conditions leads to an
increase in density with depth
4. These characteristics of temperature, salinity, density, are set at the surface, where an ocean water mass forms. As the water mass makes its journey, it will take with it the characteristics that it starts with.
5. The idea of temperature and salinity controlling density leads to a well known diagram in oceanography-the T &S diagram (temperature & salinity diagram). Figure 6.17 and Figure 6.18, pg. 169 (Figure 6.13 and 6.14, pgs. 148-149).
a. water masses have distinct characteristics that can be identified (T
& S)
b. water masses take on their characteristics when they form
c. water masses can be traced as they journey around the ocean by studying
their characteristics
d. When we talk about these water masses, we use the T-S diagram to help us see
them as distinct water masses, and to help us see under what conditions they
will MIX or STRATIFY (LAYER) themselves with other water masses.
6. Water masses move because of the density differences or contrasts between
them and other water masses they encounter. This creates a layered,
dynamic (moving) ocean.
a. the most dense water masses sink
b. the lighter water masses move up
c. those that are cold and less salty can hold more oxygen
d. those that are warm and more salty hold less oxygen
e. All together, they create a layered world of water.
7. One of the most interesting things about this THERMOHALINE
(temperature-salinity) circulation is that plants and animals in the ocean
really see these differences in water masses, much the way plants see different
kinds of soils, or insects see different layers in the canopy of the
rainforest.
Next Lecture
Focus Question How do sound, light, and dissolved gases behave in the
ocean?
Read Garrison: Chapter 6, pgs 167-179, chapter 7, pg 193-198 (Ch.6, pgs.
151-161, Ch.7, pgs. 171-176).