Chap 5-6: Seawater
SPECIAL DATES:
MPA meeting…6 Jul
R/V Pt Sur Cruise…14 Jul
R/V Pt Sur Cruise…25 Jul
Exam-1 (definite)...2 Aug
Exam-2 (Tentative)…1 Sep
Labor Day Holiday...5 Sep
Final Exam...19 Sep
(Sp-226, 1300-1450)
OC3230-Paduan
OC3230 Calendar, Summer 2005
version 13 July 2005
EX 1
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Chap 5-6: Seawater
(+)
Water molecules are unique:
105˚ angle produces polar molecule
act as tiny magnets-good dissolver
Hydrogen bonds change the energy requirements for temperature and phase changes
(-)
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Chap 5-6: Seawater
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Chap 5-6: Seawater
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Chap 5-6: Seawater
680 cal
Water exists in all three phases at the Earth’s surface temperature
range; Energy is required to change phase
Units: 1 cal = 4.18 Joule
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Chap 5-6: Seawater
Water has a very high heat capacity compared with most other
substances
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Chap 5-6: Seawater
Note variations
for water in
different phases
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Chap 5-6: Seawater
Fresh water is unique
in that its maximum
density is not at the
coldest temperature
Also, the phase
change from water to
ice leads to an
unusual decrease in -30 0.9200
-60 0.9224
-100
0.9257
density due to change
in molecular packing
4˚ C water has max density(ice has a more
implications for freshwater lakes and
regular but less dense
winter-time ecosystems
tetrahedral structure)
Ice floats!-also important for
ecosystems
look ahead: freezing problem
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Chap 5-6: Seawater
Again, ice floats and 4˚ C water sinks (previous density
versus temperature figure is more useful/important)
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Chap 5-6: Seawater
What about salt water?
Dissolving ability of water: molecules can surround and isolate
charged ions as a function of the polar nature of the molecule
Seawater has nearly all known elements dissolved
within it
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Chap 5-6: Seawater
Salinity:
“The total amount of solid materials in grams contained in
one kilogram of seawater when all the carbonate has been
converted to oxide, the bromine and iodine replaced by
chlorine, and all organic matter completely oxidized.”
This is an operational definition related to the practical
methods used to compute salinity
Units: g/kg; parts per thousand, ppt (‰)
Open Ocean Range: 34.0–35.5‰
Open Ocean Average: 35‰
Measurement Methods: 1) Evaporation, 2) Absolute
Salinity, SA, from Chlorinity, and 3) Practical Salinity from
conductivity
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Chap 5-6: Seawater
Why are the oceans salty?
1) Rivers dump sediments into the oceans
No evidence that salinity has been increasing
over time but, rather, it has been about the
same as it is now
Composition of dissolved salts is not like typical
river sediments
2) Mid-ocean ridges provide dissolved material to
the oceans
Composition of dissolved salts is more like
material in ocean crust than river sediments
Material from ridges are more soluble
Chemical equilibrium keeps salinity constant
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Chap 5-6: Seawater
A few major ions account for most of the dissolved material
The six most common are: Cl-, Na+, SO42-, Mg2+, Ca2+, K+
Two diagrams illustrating fraction of major ions dissolved in
seawater (nearly all other elements are also contained in trace
amounts):
?
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Chap 5-6: Seawater
Major ions in seawater
are always found in the
same ratio one to
another
This is known as the
“Law of Constant
Proportions”
SA (‰) = 1.80655 x Cl
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Chap 5-6: Seawater
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Chap 5-6: Seawater
Plants in the ocean
require nutrients for
cell growth and other
chemosynthetic
activities
Major nutrients are present in the form of
Nitrate (NO3-), Phosphate (PO43-), and
Silicate (SiO44-) ions
Availability of these nutrients can control
productivity (e.g., upwelling supplies
nutrients and supports growth)
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Chap 5-6: Seawater
Plants in the ocean
require nutrients for
cell growth and other
chemosynthetic
activities
?
Some trace elements, in particular iron (Fe), are now believed to
also be necessary for growth
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Chap 5-6: Seawater
Back to density of
water…
What happens when
you add salt?
Properties of the
liquid that change
when the
concentration of
dissolved materials
changes are known as “Colligative Properties”
Examples:
Freezing point depression, boiling point elevation, and changes
in the temperature of maximum density
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Chap 5-6: Seawater
At typical ocean
salinity values
(>24.70‰), liquid
water does not have
a temperature of
maximum density
This has large
repercussions for
freezing of ocean
water versus fresh
water (as seen in
the related
fresh water
homework
values
changes with increasing salinity
problem)
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Chap 5-6: Seawater
At typical ocean
salinity values
(>24.70‰), liquid
water does not have
a temperature of
maximum density
Ocean ranges of
temperature and
salinity are relatively
narrow
Look ahead: this is an example of a T-S diagram
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Chap 5-6: Seawater
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Chap 5-6: Seawater
Homework Problem
related to water density,
freezing points, and heat
capacity
Problem set and related
MATLAB library of
seawater density routines
can be downloaded from
the course web site
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Chap 5-6: Seawater
How is salinity measured?
1) Absolute Salinity: Silver Nitrate
Titration method to determine
Chlorinity:
SA (‰) = 1.80655 x Cl
2) Practical Salinity
Method based
on conductivity
(CTD=cond.temperaturedepth)
Must continuously check
CTD calibration against
conductivity of “Wormly
Water”
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Must measure T very
accurately along with
conductivity to get S
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Chap 5-6: Seawater
Dissolved Gases
Mixing with atmos. +
photosynthesis
Animal respiration
Sinking from above +
decreased respiration
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Chap 5-6: Seawater
Dissolved Gases
Deep basins with little physical
mixing, due to (or resulting in)
strong vertical density stratification,
can lead to anoxic conditions
e.g., Black Sea, some fjords, some
trenches
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Mixing with atmos. +
photosynthesis
Animal respiration
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Chap 5-6: Seawater
The Equation of State
The relationship between Temperature, Salinity, Pressure
and the resulting density, i.e.,  = f(T,S,P)
Recall in the atmosphere (in an ideal gas): P = RT
P 
also, P T
Related: Potential Temperature = that temperature a water
parcel would have if it were raised from its actual depth to
some reference depth adiabatically (without exchanging
heat)
Computation requires knowledge of “adiabatic lapse rate”
Several online calculators are available to give
 = f(T,S,P,Pref); usually  = f(T,S,P,0)
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Chap 5-6: Seawater
Potential Temperature,
, is less than or equal
to the in situ (i.e.,
observed)
temperature, T
Difference is not
significant in shallow
waters (above ~500m)
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Chap 5-6: Seawater
What about the equation of state for seawater?
It is very complicated and cannot be expressed by
a simple formula
An empirical formula has been developed and
agreed upon by the international community
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Chap 5-6: Seawater
Empirical
Equation of
State for
sea water
(EOS90);
Expressed
in terms of
the surface
density and
K
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Chap 5-6: Seawater
Conclusion: Equation of State for sea water is complex but
empirical formulae give give us consistent results given T,S,P
Next: Sound in the Ocean
T

S

P

The secant bulk modulus, K, is related to the compressibility
of sea water and, thereby, to the speed of sound
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Chap 5-6: Seawater
Sound in the Ocean
The speed of sound, c, is related to the density and the
compressibility, , of the medium according to:
where, K = 1/ is the bulk modulus
c
1





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Chap 5-6: Seawater
Sound in the Ocean
The speed of sound, c, is related to the density and the
compressibility, , of the medium according to:
where, K = 1/ is the bulk modulus
c
1




Sound travels as a “Longitudinal” or compression wave:
y

x
To be compared with “Transverse” waves (i.e., surface waves):
z
x
Animations courtesy of Dr. Dan Russell,
OC3230-Paduan Kettering University
images Copyright © McGraw Hill
Chap 5-6: Seawater
Sound in the Ocean
The speed of sound, c, is related to the density and the
compressibility, , of the medium according to:
where, K = 1/ is the bulk modulus
c
1




Note that c is inversely proportional to compressibility;
While water is much less compressible than air, sound travels
through water much more quickly than it does through air
water << air
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
cwater >> cair
cwater ~ 1500 m/sec
cair ~ 330 m/sec
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Chap 5-6: Seawater
Sound in the Ocean
The speed of sound, c, is related to the density and the
compressibility, , of the medium according to:
where, K = 1/ is the bulk modulus
c
1




Note that c is inversely proportional to compressibility;
While water is much less compressible than air, sound travels
through water much more quickly than it does through air
water << air

cwater >> cair
cwater ~ 1500 m/sec
cair ~ 330 m/sec
Given c, the wavelength, , and frequency, f, of sound waves
(seawater)
f

are related by the wave equation:
Low Frequency
30Hz
50m
High Frequency
1.5MHZ
1mm
c = f
Instruments:
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10-100kHz 14-1.4cm
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Chap 5-6: Seawater
Sound in the Ocean
Sound speeds in the ocean are ~1500 m/sec but they vary by
about 5% as a function of T,S, and P:
T
c
S
c
P
c
Variations in c lead to bending (turning)
of sound waves according to Snell’s Law
sin 1 c1

sin 2 c 2
Snell’s Law applies equally to
surface waves and light waves;

Animation courtesy of Dr. Dan Russell,
OC3230-Paduan Kettering University
If c changes in the medium,
wave crests will bend toward
the region of slower speed
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Chap 5-6: Seawater
Sound in the Ocean
Sound waves spreading out
uniformly for c = constant
Bending upward toward lower c
Bending downward toward lower c
Animations courtesy of Dr. Dan Russell,
OC3230-Paduan Kettering University
images Copyright © McGraw Hill
Chap 5-6: Seawater
Sound in the Ocean
The distribution of c
can lead to
shadow
zones where
sound energy
does not
penetrate
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Chap 5-6: Seawater
Sound in the Ocean
A more complicated distribution of c(z) and resulting shadow
zones
The process of bending or turning of waves due to variations in
their speed is called “refraction”
SOFAR: Sound Fixing and Ranging channel; energy is trapped
by the effect of minimum c around 1000m
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Chap 5-6: Seawater
Sound in the Ocean
SOFAR: Sound Fixing and Ranging channel; energy is trapped
by the effect of minimum c around 1000m
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Chap 5-6: Seawater
Sound in the Ocean
There is no middepth minimum or
sound channel at
high latitudes
Overview:
Temperature and Pressure are most critical to c
Salinity is important at high latitudes
Look Ahead: Global distributions of T(z), S(z)
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Chap 5-6: Seawater
Sound in the Ocean
Match the
sound speed
profiles on the
left with the
refraction and
shadow zone
results on the
right
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Chap 5-6: Seawater
Sound in the Ocean
Match the
sound speed
profiles on the
left with the
refraction and
shadow zone
results on the
right
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Chap 5-6: Seawater
Sound in the Ocean
Absorption Coefficient
versus frequency
Attenuation:
1) Spreading (Spherical vs
Cylindrical)
“Relaxation”
2) Absorption, I = Ioe-ax (energy
transfer to medium;
a=absorption coefficient)
3) Scattering (redirection by
particles)
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f

a
I
Lower frequencies travel further
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Chap 5-6: Seawater
Sound in the Ocean
Target Strength (active acoustic applications)
Related to effectiveness of the reflection of sound
Measured by the Acoustic Impedance, Z = c
The Reflectivity, R, determines how much acoustic energy is
reflected from a discontinuity and how much is transmitted
through it; R = (Z1 - Z2)/(Z1 + Z2) x 100%
closely
matched
impedance
values
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Chap 5-6: Seawater
Sound in the Ocean
Ambient Noise (passive acoustic applications)
Related to background
sound
Environmental processes make
distinctive noises
Can cause a detection
problem for some
applications
Can also be inverted to
remotely monitor the
environment (e.g., rain
rate, wind speed)
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Chap 5-6: Seawater
Sound in the Ocean
Acoustic Thermometry of Ocean Climate (ATOC)
Exploits SOFAR channel to send sound great distances
Travel time is tied to the average T along the path
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