Download Density

Chapter 5: Dirt
Chapter 6: Water
Density
• Density simply refers to how much mass
something has for a constant volume.
• Units: weight/volume: grams/cm3, etc.
• Intuitively, a block of aluminum weighs less
(is less dense) than an equal-sized block of
lead.
• Similarly, salt water is more dense than
fresh water, because it has salt added to it!
http://video.google.com/videoplay?docid=-316104399201484335
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Fig. 6.10
Fig. 6.1
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Thermal Properties of Water
•
Heat = total kinetic (moving) energy of
molecules
–
•
Calorie : amount of heat required to raise
temperature of 1 gram of water by 1 ˚C
Temperature = direct measure of kinetic
energy of molecules
–
Degrees Fahrenheit (˚F) or Degrees Celsius
(Centigrade) (˚C)
Heat capacity
Water has High Heat Capacity
Takes a lot of energy to get water to
change its temperature.
• Water can ‘hold’ a lot of energy
• It slowly heats up or cools down
• It keeps ‘systems’ from dramatically
changing.
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Three Phases of H20
Phase Changes Require Energy
•
2 types of heat energy
–
Sensible heat - changes in temperature
•
•
–
Molecules change their speed
You can “sense” it
Latent heat - hidden heat
•
•
Molecules don’t change speed
Energy required (produced) to
break (form) hydrogen bonds
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Phase changes require large amounts of heat
Latent (hidden) heat = energy that is either absorbed or released
as water changes state
• Melting: latent heat required to break intermolecular bonds in ice
• Evaporation: latent heat required to break hydrogen bonds to form gas
• Condensation: latent heat is released when gas condenses
• Freezing: latent heat is released when water freezes
Phase Changes
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Temperature
effect on
Density
•
•
•
As water cools to 4°C: Molecules slow, Water contracts, Density
increases
Below 4°C: Water becomes highly structured and expands, density
decreases
As water freezes: Expands by 9%
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Temperature, Salinity,
Density, and Depth
Imagine the ocean is a glass of beer.
We could take an imaginary column
from the glass, and measured the
temperature, salinity, and density as
a function of the height of the glass
(z, the “depth”)
Temperature, Salinity,
Density, and Depth
Depth (z)
Temperature (° C)
Salinity (ppt)
Density (g/mL)
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A sharp change with depth is
called a CLINE…
Pycnocline
Increasing Density
Depth (z)
Density (g/mL)
To increase density, we can increase salinity, decrease
temperature, or both. In the oceans, it is usually a
combination of both.
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T-S Diagrams--An Ocean
“Fingerprint”
So what, exactly, is “salinity”?
•
Salinity = total quantity of
dissolved inorganic solids
and gases in seawater
•
Law of Constant
Proportions
–
Though total salinity may
differ, the relative ratios of
each ion stay the same
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Law of Constant Proportions:
If we measure one element,
we (more or less) know them
all!
We don’t actually measure
salinity (concentration of solids
and gases)--instead, we
measure conductivity, or the
amount of ions in seawater.
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Seawater salinity
•
Typically about 3.5% - (3.5% of the weight
of seawater is from salts, 96.5% is pure
water)
•
Expressed as parts per thousand (‰ or ppt)
–
•
Percent (%) is parts per hundred
35 ‰ means 35 grams of salt are dissolved
in 1000 grams of water (1 liter)
Salts come from weathering (via rivers),
volcanoes, atmosphere, hydrothermal vents
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What changes Salinity?
• Salt is added/removed
on LONG time scales
(millions of years)
• Water moves in the
ocean MUCH fast
(thousands of years)
• So salinity changes
are mostly due to
changes in the amount
of water!
Processes affecting seawater
salinity
•
Processes that add fresh water to surface ocean and
decrease salinity:
–
–
–
–
•
Precipitation
Runoff
Icebergs melting
Sea ice melting
Processes that remove fresh water from the surface
ocean and increase salinity:
–
–
Evaporation
Sea ice forming
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More energy means more
evaporation, which means less
water, more salt… equals higher
salinity
Evaporation occurs at low latitudes (lots of energy from the sun)
Water transported in atmosphere to higher latitudes
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Surface salinity
variation
•
Low in High latitudes
–
–
•
High in Tropics and subtropics (0 -35˚ latitude)
–
–
•
High precipitation and runoff
Low evaporation
High evaporation
Low precipitation
Dip at Equator
–
High precipitation partially
offsets high evaporation
Hydrologic cycle - powered by the sun
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Salinity of World’s Oceans
Salinity + Temperature = Density
• To make a layered
ocean (or beer), we
need to change the
density.
• TEMPERATURE is
much more important
than salinity!
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Starting from the same place, increasing temperature
means crossing the density lines faster than increasing
salinity….
Thermocline
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Ocean “layering” based on density
• Mixed surface layer (surface to ~200 meters)
– Low density
– Well mixed by waves, currents, tides
• Upper water (200 to 1000 meters)
– Intermediate density water containing thermocline, pycnocline,
and halocline
• Deep water (below 1000 meters)
– Cold, high density water involved in deep current movement
Nutrients: A special type of “salt”
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Nutrients
• Nutrients = essential substances for growth
– Primary producers feed all other organisms, so
inorganic nutrients are important!
• Main inorganic bio-limiting nutrients:
– Nitrogen (nitrate, ammonium)
– Phosphorus (phosphate)
– Silicon (silicic acid)
– Iron, manganese, etc.
Bio-limiting Nutrients
Nitrogen
– Used in DNA, proteins (enzymes)
– Nitrate, ammonium, dissolved amino acids
Phosphorus
– Important in DNA, ATP (energy)
– Phosphate
Silicon
– Used in shells of diatoms
– Silicic acid
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Macro-Nutrients
• Nitrogen
• Phosphorus
• Silicon
• Micro-nutrients = iron, manganese, zinc
– Present in low abundance (trace
constituents of seawater)
Nutrients are LOW in the surface, while Oxygen is HIGH...
Oxygen DECREASES at ~ 1000 m, while Nutrients INCREASE
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Why are nutrient profiles related
to oxygen profiles?
Photosynthesis: produces oxygen
Respiration: consumes oxygen
CO2 + H2O  sugar + O2
• Surface water (< 200 m): photosynthesis,
therefore low CO2, high O2
• Deep water (> 1000 m): no photosynthesis,
therefore high CO2, low O2
Organic matter DECOMPOSES as it sinks--releasing CO2,
consuming oxygen…
Sinking organic matter
OXIDATION
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Fig. 6.16
Fig. 6.17
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Ocean Chemistry
• The ocean is layered--lighter (less dense)
water on top.
• HEAT is much more important than salinity,
but both change density
• Surface waters have lower nutrients (a
special type of salt), higher oxygen, and lower
carbon dioxide
• Intermediate and deep waters have
increasing nutrients and carbon dioxide, and
decreasing oxygen
• The oceans will keep this in balance, but it
takes thousands of years--short term changes
(humans) can interrupt the balance
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