Lesson 6: Ocean Layers I Chemical Oceanography

Salinity is an 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

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

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.

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.

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.

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: http://aquarius.nasa.gov/science.html click on The Aquarius/SAC-D Mission: Designed to Meet the Salinity Remote-Sensing Challenge Photo: NASA, http://www.nasa.gov/topics/earth/features/Aquarius_final.html; Accessed: November 2010 The Aquarius: NASA satellite that measures ocean salinity

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

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, http://aquarius.nasa.gov/science-ocean.html; Accessed: November 2010 Photo: NASA

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

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

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