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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