Chapter 6/7Chapter 6/7Water, Ocean Structure & Water, Ocean Structure &

ChemestryChemestry

22http://www.ospo.noaa.gov/Products/ocean/sst/anomaly/index.html

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http://arctic.atmos.uiuc.edu/cryosphere/

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Hydrogen bonds form when the positive end of one water molecule bonds to the negative end of another water molecule.

Two important properties of water molecules:

Cohesion – the ability of water molecules to stick to each other, creating surface tension.

Adhesion – the tendency of water molecules to stick to other substances

Hydrogen Bonds

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Temperature, Heat, Heat Capacity, Calories, etc.

Temperature

• Measure of av. kinetic energy (motion) of molecules (KE=1/2mv2)

• unit is degrees C, F or K (Kelvin)

Heat

• Measure of the total kinetic energy of the molecules in a substance

• Unit is the calorie

* Heat Capacity = is a measure of the heat required to raise the temperature of 1g of a substance by 1C.

* Calorie = amount of heat to raise temperature of 1 gram

of pure water by 1°C (from 14.5 °C to 15.5 °C)

* Latent Heat

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Not All Substances Have the Same Heat Capacity

Water has a very high heat capacity, which means it resists changing temperature when heat is added or removed – large thermal inertia

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Remember from Chapter 3?

Density is a key concept for understanding the structure of Earth – differences in density lead to stratification (layers).

Density measures the mass per unit volume of a substance.

Density = _Mass_ Volume

Density is expressed as grams per cubic centimeter.

(pure) Water has a density of 1 g/cm3

Granite Rock is about 2.7 times more dense

just about everything in this course!

Temperature affects water’s density

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The relationship of density and temperature for pure water.

Note that points C and D both represent 0°C (32°F) but different densities and thus different states of water. Ice floats because the density of ice is lower than the density of liquid water.

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Water Becomes Less Dense When It FreezesThe space taken by 24 water molecules in the solid lattice could be occupied by 27 water molecules in liquid state, so water expands about 9% as the crystal forms. Because molecules of liquid water are packed less efficiently, ice is less dense than liquid water and floats.

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Changes of State-due to addition or loss of heat (breaks H bonds)

The amount of energy required to break the bonds is termed the latent heat of vaporization. Water has the highest latent heat of vaporization of any known substance.

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Things to remember:

1. Can have liquid water at 0°C and below (supercooled water) 2. Can change directly solid to gas - sublimation3. Can boil water at temperature below 100°C (if pressure decreases as when at the top of a high mountain)4. Evaporation removes heat from Earth’s surface (it is a cooling mechanism)5. Condensation in atmosphere releases heat that will drive Earth’s weather cycle

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Dissolved salts in water

(a) raise T (temperature) of boiling point(b) lower T of freezing point

(a) not so important to oceanography but(b) is, as T around 0°C are common overmany areas of the oceans

freezing point of seawater ~ -2°C (-1.91 °C)

The effect of salt on water’s properties:

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• The ability of a substance to take in or give up a certain amount of heat and undergo small or large changes in temperature

• Water has high heat capacity = 1 cal/g/˚C

• Water can gain or lose large quantities of heat without large changes in temperature

• Salt does not significantly change water’s heat capacity: heat capacity of seawater = 0.96 cal/g/˚C (4% change)

Heat Capacity

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• density = mass per unit volume• measured in grams per cubic centimeters

density of pure water = 1 g/cm3

(determined at ~ 4°C)• density increases as temperature drops

to 4°C and then decreases as temperature goes to 0°C

• ice is less dense than water

• salt increases water’s densitydensity of sea water > density of pure water

~ 1.03 g/cm3 at 4°C

Density

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http://www.ospo.noaa.gov/Products/ocean/sst/contour/index.html

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•Water nearly incompressible•P increases by 14.7lb/in2 (1

Atmosphere) for every 10 m increase in depth of sea

•1 cm3 will lose 1.7% of its volume at 4000m

•Thus, sea level is 37 m lower due to compression!

Pressure

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San Francisco Norfolk

Tem

per

atu

re (

°F)

San Francisco

Norfolk

Tem

per

atu

re (

°C)

Surface Water Moderates Global Temperature

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Tropic of Cancer Tropic of Cancer

Equator Equator

Tropic of Capricorn Tropic of Capricorn

Salinity

Salinity

Temperature

Temperature

Lat

itu

de

No

rth

So

uth

Ocean-Surface Conditions Depend on Latitude, Temperature, and Salinity

2020Table 6-3, p. 166

2121Fig. 6-16, p. 168

Sea-surface average salinities in parts per thousand (‰).

Sea-surface temperatures during Northern Hemisphere summer

2222Fig. 6-17, p. 169

The Ocean Is Stratified by Density

two samples of water can have the same density at different combinations of temperature and salinity!

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The Ocean Is Stratified into Three Density Zones by Temperature and Salinity

a.The surface zone or surface layer or mixed layerb.The pycnocline, or thermocline or haloclinec.The deep ocean (~ 80% of the ocean is below the surface zone

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5 10 15 20 25

Polar

Tropical 2,000

Temperate1,000

4,000

6,0002,000

Dep

th (

m)

8,000 Dep

th (

ft)

3,000 10,000

40 50 60 70

Temperature (°F)

Temperature (°C)

Typical temperature profiles at polar, tropical, and middle (temperate) latitudes. Note that polar waters lack a thermocline.

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Sound and light in Seawater• Sound and light both travel in waves

• Refraction is the bending of waves, which occurs when

waves travel from one medium to another

• Refraction Can Bend the Paths of Light and Sound

through Water

• Light may be absorbed, scattered, reflected,

refracted and attenuated (decrease in intensity over

distance)

• Sunlight does not travel well in the ocean. Scattering

and absorption weaken light

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• Form of electromagnetic radiation• Seawater transmits visible portion of the electromagnetic spectrum (water transmits

blue light more efficiently than red)• 60% is absorbed by 1 m depth• 80% absorbed by 10 m depth• No light penetration below 1000 m• Shorter wavelengths (blues) are transmitted to deeper

depths

Light

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Sound Travels Much Farther Than Light in the Ocean

On average:ss in Air = 334 m/sss in Water = 1500 m/s

ss increases as temperature and pressure increase: sound travels faster in warm surface waters and then again in deep (cold) waters where pressures are higher

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The sofar layer, in which sound waves travel at minimum speed.

Sound transmission is particularly efficient - that is, sounds can be heard for great distances - because refraction tends to keep sound waves within the layer.

The so(sound)f(fixing)a(and)r(ranging) zone

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Ocean ChemistryOcean Chemistry

The Ocean is considered a well mixed solution

• Salinity is the total quantity of dissolved inorganic solids in water.

• 3.5% salt on average

• measured in g/kg (ppt = parts per thousand)• ocean salinities vary in space • evaporation, precipitation, runoff, freezing, and thawing

And recall that:

The heat capacity of water decreases with increasing salinityAs salinity increases, freezing point decreasesAs salinity increases, evaporation slows (boiling point increases)

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Tropic of Cancer Tropic of Cancer

Equator Equator

Tropic of Capricorn Tropic of Capricorn

Salinity

Salinity

Temperature

Temperature

Lat

itu

de

No

rth

So

uth

Ocean-Surface Conditions Depend on Latitude, Temperature, and Salinity

3131Fig. 7-3, p. 189

One kilogram of seawater

Water 965.6 g

Most abundant ions producing salinity

Other components (salinity) 34.4 g

Sodium (Na+) 10.556 g

Chloride (Cl–) 18.980 g

Magnesium (Mg2+) 1.272 gBicarbonate (HCO3

−) 0.140 gOtherCalcium (Ca2+) 0.400 g

Potassium (K+) 0.380 g

Sulfate (SO42−) 2.649 g

Dissolved SaltsMajor constituents = [] > 1 ppmAccount for 99.8% of all dissolved salts

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Sources of salt:

• Positive ions: weathering and erosion• Negative ions: gases from volcanic eruptions• Hydrothermal activity supply and remove salt from the deep ocean

Balance of salt:Input: rivers, volcanic activity, groundwater, hydrothermal vents and cold springs, and the decay of once-living organisms.Output: sea spray, uptake by living organisms, incorporation into sediments, and ultimately by subduction.

Regulating the Major Constituents in seawater

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• Distribution with depth

Photosynthesis removes CO2 and produces O2 at the surface

Respiration produces CO2 and removes O2 at all depthsCompensation depth (Photosynthesis = Respiration)

CO2 O2

Gases

photosynthesis

respiration

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Oxygen and CO2 profiles

CO2 Concentrations

Direct solution of gas from the atmosphere

Respiration of marine organisms

Oxidation (decomposition) of organic matter

O2 Concentrations

Photosynthesis

Bottom water enrichment

oxygen minimum

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metric tons C (106)

The Carbon/Carbon Dioxide Cycle -

numbers in black = rates of exchange

numbers in green = total amounts stored in reservoirs

numbers in parenthesis = net annual changes

Ocean uptake from atmosphere Depends on: pH, temperature, salinity, chemistry Biological pump

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Seawater

• Alkaline, pH from 7.5-8.5• Average pH=7.8• pH relatively constant due to buffering action of

CO2

• Buffer = substance that prevents sudden or large changes in the acidity or alkalinity of a solution

• Important for biological processes• pH inversely proportional to the concentration of

CO2

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CO2 combines readily with seawater to form carbonic acid (H2CO3). Carbonic acid can then lose a H+ ion to become a bicarbonate ion (HCO3-), or two H+ ions to become a carbonate ion (CO3

2-). Some bicarbonate ions dissociate to form carbonate ions, which combine with calcium ions in seawater to form calcium carbonate (CaCO3), used by some organisms to form hard shells and skeletons. When their builders die, these structures may fall to the seabed as carbonate sediments, eventually to be redissolved. As the double arrows indicate, all these reactions may move in either direction.

CO2 Buffer

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• pH: acidity of seawater 7.5 - 8.5

• Carbon dioxide acts as a buffer that prevents large variations in pH

• Major salt ions are in constant proportions except in coastal areas

• Ocean is a net source of oxygen to atmosphere

• Biological processes pump CO2 into the deep ocean

Chapter 7 - Summary Chapter 7 - Summary