Lesson 6: Ocean Layers I Chemical Oceanography

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

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 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 Cl- Na+ Sodium Chloride (A salt) Water Cl- Na+ Chloride ion Sodium 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.

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?

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

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 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 salinity2.  Evaporation - High rates of evaporation will increase salinity3.  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 Photo: NASA

11. A closer look Relatively LOW salinity Relatively HIGH salinity

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