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Topic 9 Salt Water

Topic 9 Salt Water. GEOL 2503 Introduction to Oceanography. What is in seawater?. Hydrogen + Oxygen make up only 96.5% Not 100% as in pure water In 1,000 grams of seawater: 965 grams water 35 grams salt. Salinity. Salt content of water is called salinity Measured in units of:

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Topic 9 Salt Water

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  1. Topic 9 Salt Water GEOL 2503 Introduction to Oceanography

  2. What is in seawater? • Hydrogen + Oxygen make up only 96.5% • Not 100% as in pure water • In 1,000 grams of seawater: • 965 grams water • 35 grams salt

  3. Salinity • Salt content of water is called salinity • Measured in units of: • grams salt per kilogram of seawater (g/kg) • parts per thousand (‰) • A typical open ocean salinity is around 35‰ or 35 g/kg

  4. Major Constituents(not just salt, which is NaCl) • Sodium (Na) + Chlorine (Cl) = 86% • Sulfur (S) • Magnesium (Mg) • Calcium (Ca) • Potassium (K) • Bring total to 99.36% • Everything else—trace elements

  5. Major Constituents

  6. chloride sodium sulfate E--magnesium D--calcium

  7. Table Salt • Table salt is sodium and chlorine • Each salt molecule: Na+ Cl- (ions) • Water forms spheres around ions • Salt ions are surrounded and separated by water molecules • Sodium Chloride is the chemical name • Halite is the mineral name

  8. Atoms arranged in a crystal lattice (framework)

  9. NaCl molecule dissociates in water into individual atoms of Na+ and Cl-

  10. Dissolving Ability of Water • Substances dissolved from land are carried to sea by rivers, streams, underground water • Supply “salt” to oceans • That is why oceans are salty

  11. Salt Inputs Volcanoes Rivers Rainfall Hydrothermal vents Salt Outputs Sea spray Bottom sediments Biologic processes Adsorption Chemical precipitation But why aren’t oceans getting saltier?

  12. Adsorption—solute accumulates on the surface of sediments

  13. Principle of Constant Proportions • William Dittmar analyzed the 77 water samples from the Challenger Expedition • He found that, regardless of the actual salinity, the ratios of the major constituents remained constant • Applies to major constituents in open-ocean water only

  14. The Principle of Constant Proportions Forchhammer’s principle states that although the salinity of various samples of seawater may vary, the ratio of major salts is constant. Forchhammer’s principle is also known as the principle of constant proportions.

  15. Determining Salinity • Salinometer • Measures electrical conductivity, which reflects dissolved material • See http://www.salinometry.com for a history of measuring salinity

  16. Gases in Seawater Numbers are percents of total gases

  17. Dissolving Gases in Water • Cold water holds more gases than warm • Water under pressure holds more gases than water under less pressure

  18. Oxygen • Produced by plants (photosynthesis) • Plants only live in upper 100 meters (more or less) of ocean • Also mixed into ocean from atmosphere at surface • Used by animals and plants at night for respiration, and in decomposition

  19. Carbon Dioxide (CO2) • Used by plants in photosynthesis • Produced by respiration and decomposition • Also enters oceans from atmosphere at surface, but produced and available at all depths

  20. Oxygen Concentrations • Surface—high concentration because of photosynthesis plus mixing from atmosphere • Below surface layer—concentration decreases because of respiration and decomposition • Oxygen minimum layer—around 800 meters

  21. Why does Oxygen increase below 800 meters?

  22. Why does Oxygen increase below 800 meters? • Oxygen falls from the surface layer in sinking water • Rate of removal decreases because there are fewer animals, less matter to decompose

  23. Carbon Dioxide Concentrations • Opposite of Oxygen • Low in surface—used in photosynthesis • Increases throughout the rest of the water column—animal respiration and decomposition

  24. Salt as a Resource • 30% of world’s salt is extracted from seawater (here we mean NaCl) • Primarily in evaporating ponds in warm, dry climates • Southern France, Puerto Rico, Mexico, California

  25. Desalinization • Any of several methods of obtaining fresh water from salt water

  26. Why Desalinate? • There is no absolute shortage of fresh water in terms of overall supply, but mostly it’s not where the people are. Why? • Distribution—not where we need it • Mismanagement—pollution, unwise use • Population increase—more users • Increased usage—per person per day

  27. Solar Still • Simplest method of desalinization • Use Sun’s energy to evaporate water • Evaporated water is fresh • Trap water on plastic dome cap • Water condenses, rolls along cap to collectors • Slow production

  28. Osmosis • Water molecules move across a semipermeable membrane from region of high concentration of water to region of low concentration (gradient-driven) • We’ll see this again in biological oceanography • Creates pressure

  29. Reverse Osmosis • We supply the pressure • Force salt water through a semi-permeable membrane • Dissolved substances can’t pass • Fresh water is produced

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