Topic 9 Salt Water

GEOL 2503Introduction 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

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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

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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

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Major Constituents

ElementGrams per kilogram of sea water

Percent of dissolved

solidsCl 19.35 55.07

Na 10.76 30.62

S, Mg, Ca, K 4.80 13.67

Total 34.91 99.36

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D--calcium

E--magnesium

sulfate

sodium

chloride

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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

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Atoms arranged in a crystal lattice (framework)9

NaCl molecule dissociates in water into individual atoms of Na+ and Cl-10

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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

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But why aren’t oceans getting saltier?

• Salt Inputs– Volcanoes– Rivers– Rainfall– Hydrothermal vents

• Salt Outputs– Sea spray– Bottom sediments– Biologic processes– Adsorption – Chemical precipitation

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Adsorption—solute accumulates on the surface of sediments14

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

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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.

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Determining Salinity

• Salinometer• Measures electrical conductivity, which

reflects dissolved material

• See http://www.salinometry.com for a history of measuring salinity

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Gases in SeawaterGas Atmos. Surface water Ocean total

Nitrogen 78 48 11

Oxygen 21 36 6

Carbon Dioxide 0.03 15 83

Numbers are percents of total gases

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Dissolving Gases in Water

• Cold water holds more gases than warm• Water under pressure holds more gases than

water under less pressure

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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

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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

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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

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Why does Oxygen increase below 800 meters?

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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

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Carbon Dioxide Concentrations

• Opposite of Oxygen• Low in surface—used in photosynthesis• Increases throughout the rest of the water

column—animal respiration and decomposition

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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

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Desalinization

• Any of several methods of obtaining fresh water from salt water

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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

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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

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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

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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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