1. Lesson 6: Ocean Layers I Chemical Oceanography

2. We have been learning about ocean chemistry
What are two important cycling nutrients we have learned about?
Name one way human activities affect each of
these nutrients.
What is a Dead Zone?
N & C
Teacher’s Note: For the second bullet, you may want to discuss increasing CO2 in the atmosphere and ocean, or release of nutrients to fresh and saltwater. For the third bullet, remember that dead zones can be created when excess nitrogen is released into the water, causing plankton blooms and eventual reduction in dissolved oxygen.

3. Salinity is another important part of ocean chemistry
Salinity is a measure of the amount of dissolved salts in water
Salinity is not homogenous (uniform) across the Earth’s oceans
Both salinity and temperature affect the density of seawater

4. Seawater is a mixture of pure water and chemical compounds
Cl-
On average,
Seawater is 96.5% pure water…
…and 3.5% compounds including dissolved salts
Remember from chemistry that when salts dissolve they form ions
Na+
Sodium Chloride
(A salt)
Water
Teacher’s Note: Ions are charged atoms or molecules.
Cl-
Na+
Sodium ion
Chloride ion

5. Salinity is a measurement of how much salt is dissolved in the water
Common units of salinity are:
Parts per thousand (PPT)
Practical Salinity Units (PSU)
What does ppt really mean?
Imagine you have a bag of 1000 m&ms
Blue m&ms represent pure water
Red m&ms represent salts
To represent 35ppt (average salinity of seawater), 35 m&ms would be red and 965 would be blue.
Teacher’s Notes:
1. A more comprehensive way to define salinity would be to say “salinity is measured as the total of all the individual dissolved salt ions”
2. Salinity has traditionally been defined as parts per thousand (ppt). This unit typically comes up at the Bowl. More recently, however, salinity is commonly measured in practical salinity units (psu). PSU is technically a more accurate definition, but often psu and ppt are nearly equivalent.

6. What’s in the water?
7 primary chemicals make up almost all (~99%) the salts in seawater:
Chloride (Cl-): 55%
Sodium (Na+):  31%
Sulfate (SO42-):  8%
Magnesium (Mg2+):  4%
Calcium (Ca2+): 1%
Potassium (K+):  1%
Bicarbonate (HCO3-): < 1%
Can you come up with an acronym to remember them all?
Teacher’s Note: Approximate percentages of ions by weight. You may note that the percentages don’t exactly add up to 99% and this is because they are rounded. More exact percentages would be: CL 55.0%, Na 30.6%, SO4 7.7%, Mg 3.7%, Ca 1.2%, K 1.1%, HCO3 .4%.
Source: Pinet, Paul R. (1992). Oceanography: An Introduction to the Planet Oceanus. St. Paul, MN: West Publishing Company.

7. How do scientists figure out how much salt is in the water?
The Rule of Constant Proportions = elements in a compound are present in a fixed proportion
This means that although salinity may vary, the ratio of any one of the 7 primary ocean salts to each other will not change
Therefore if scientists figure out the amount of one primary ocean salt, they can then calculate the amount of the other six, and thereby determine the sample's salinity
Teacher note:  Elements  highly affected by biological (e.g., nitrogen) or geological activity change their proportions over time and are called non-conservative.

8. How do scientists figure out how much salt is in the water?
Scientists have several tools to measure the amount of ions
The newest method is a
NASA satellite that will
actually measure salinity
from space
Photo: NASA
Teacher’s Note: An article about how salinity is measured using satellites can be found here: click on The Aquarius/SAC-D Mission: Designed to Meet the Salinity Remote-Sensing Challenge
Photo:
NASA, Accessed: November 2010
The Aquarius: NASA satellite that measures ocean salinity

9. Factors that influence salinity
What factors do you think might influence salinity?
Three primary factors influence salinity: 1.  Freshwater input - High rates of freshwater input (river inflow to the sea; melting ice) will decrease salinity 2.  Evaporation - High rates of evaporation will increase salinity 3.  Precipitation - High levels of rainfall will decrease salinity

10. Salinity is variable across the ocean
Salinity is highest
in the mid -latitudes and lowest at the equator and high-latitudes
Teacher’s Note:
High salt concentrations are usually in the middle of the ocean basins away from the mouths of rivers, which input fresh water. High concentrations are also in sub-tropical regions because evaporation rates are high and in landlocked seas in arid regions. At high latitudes, salinity is low because the melting of ice dilutes seawater. As a general rule, salinity is low where precipitation is greater than evaporation, mainly in coastal or equatorial regions.
Photo Credit: NASA, Jet Propulsion Laboratory
Photo: NASA, Accessed: November 2010
Photo: NASA

11. A closer look Relatively LOW salinity Relatively HIGH salinity
Teacher’s Note: Low salinity areas are circled in blue, high salinity areas in black.

12. Why is salinity important?
Salinity is one factor that controls the density of ocean water
What happens when water at different depths has different densities?
Layers of water will form
Formation of layers is part of the reason we have ocean currents
We will learn more about ocean currents in another lesson

13. Student activity
We will complete a lab activity to help us determine how salinity and temperature affect water density