Properties of SeawaterSalinity

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Properties of SeawaterSalinity
Lecture 7
OEAS-306
February 10, 2009

• Outline
• Homework
• Review of unique molecular structure of water
• Water as a powerful solvent
• Salinity
• Impact of salinity on behavior of water
• Global distribution of salinity
• Why is the ocean salty?
• Residence time and mixing time

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Water Is a Powerful Solvent

• What are solutions and mixtures?
• A solution is made of two components, with
  uniform molecular properties throughout
• The solvent, which is usually a liquid, and is
  the more abundant
• component.
• The solute, often a solid or gas, is the less
  abundant component.
• A mixture is different from a solution. In a
  mixture the components retain separate
  identities, so it is NOT uniform throughout.

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Water Is a Powerful Solvent
Salt in solution. When a salt such as NaCl is put
in water, the positively charged hydrogen end of
the polar water molecule is attracted to the
negatively charged Cl- ion, and the negatively
charged oxygen end is attracted to the positively
charged Na ion. The ions are surrounded by water
molecules that are attracted to them and become
solute ions in the solvent.
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Dissolved Salt Changes the Freezing Point of Water

• Presence of salt also
• Decreases heat capacity
• Reduces evaporation (increases boiling point)

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The Ocean Is Stratified by Density
The complex relationship between temperature,
salinity and density of seawater. Note that two
samples of water can have the same density at
different combinations of temperature and
salinity.
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Density of Seawater is a complicated function of
Pressure, Temperature and Salinity
Equation of State
Determined empirically
As a general rule of thumb a change in density of
1 kg/m3 results from 1) A temperature change of
5C 2) A salinity change of 1.2 ppt 3) A
pressure change of 200 decibars (200 meters depth)
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The Ocean Is Stratified into Three Density Zones
by Temperature and Salinity
Density stratification in the ocean. (a) In most
of the ocean, a surface zone (or mixed layer) or
relatively warm, low-density water overlies a
layer called the pycnocline. Density increases
rapidly with depth in the pycnocline. Below the
pycnocline lies the deep zone of cold, dense
water about 80 of total ocean volume. (b) The
rapid density increase in the pycnocline is
mainly due to a decrease in temperature with
depth in this area the thermocline. (c) In
some regions, especially in shallow water near
rivers, a pycnocline may develop in which the
density increase with depth is due to vertical
variations in salinity. In this case, the
pycnocline is a halocline.
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A Few Ions Account for Most of the Oceans
Salinity

• A representation of the most abundant components
  of a kilogram of seawater at 35 salinity. Note
  that the specific ions are represented in grams
  per kilogram, equivalent to parts per thousand
  ().

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Forchhammers principle or the principle of
constant proportions
Chemist Georg Forchhammer noted that while the
total amount of dissolved solids (salinity) may
change, the ratio of major salts is constant
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Minor Components of Seawater
These elements do not follow the principle of
constant proportions.
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SalinityThe total quantity (concentration) of
dissolved inorganic solids in water.
This is difficult to measure exactly ( some salts
become gases when heated )
1) Definition based on Chlorinity Salinity in
parts per thousand 1.80655 Chlorinity in
parts per thousand
Chlorinity is easier to measure and principle of
constant proportions allows this to be converted
directly to salinity.
2) Definition based on Conductivity Electrically
conductivity of seawater is proportional to
amount of salt in solution.

• Practical Salinity Scale (1978) official
  definition
• Defined Salinity based on the conductivity ratio
  relative to a KCl standard.
• Because it is a ratio, it is dimensionless.
  Reported as psu (practical salinity units)

In oceanography, salinity is most typically
measured using a CTDConductivity, Temperature
and Depth sensor.
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Simplified Global Hydrologic Cycle
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Global Ocean Water and Salinity Balance
Conservation of Mass
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• Salinity follows patterns of Evaporation-Precipita
  tion in tropics and subtropics.
• Lower salinity near the poles is due to
  glacial/polar ice melt.
• Atlantic is generally saltier than Pacific or
  Indian Oceans.

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Why is Atlantic Saltier?
Evaporation
N.E. Trade Winds
Precipitation
Westerilies
Evaporation from the Atlantic falls as
precipitation in Pacific. So the Atlantic is
saltier. However, Atlantic is in steady state
and water and salt must balance. This
accomplished by global ocean circulation.
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Why is Ocean Salty
Most obvious source of salt is the input by river
water.
Rainwater contains dissolved gases (mainly CO2
and SO2) which form acidic solutions in water.
This leads to chemical weathering of continental
rock
Weathering of sedimentary rocks
CaCO3(s) CO2(gas) H2O Ca2(aq) 2HCO3-(aq)
Calicite, common mineral in sedimentary rocks
From rainwater
In solution
Weathering of igneous or metamorphic rocks
2NaAlSi3O8(s) 2CO2(gas) 3H2O
Al2Si2O5(OH)4(s) 2Na(aq) 2HCO3-(aq)
4SiO2(aq,s)
Silica, partly in solution
From rainwater
In solution
Kaolinite, clay mineral
Albite, common mineral in igneous and metamorphic
rocks
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Comparison of River Water and Seawater Composition
Salts are put into the ocean by rivers.
Evaporation removes water from the oceans. But,
oceans are NOT getting saltier and composition of
oceans is very different from river water.
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For Ocean to maintain a constant concentration of
salt, it must be in steady state.

• Seawater does not have the same composition as
  river water, so there must be other sources of
  salt precipitation and hydrothermal activity.
• Ocean is not getting saltier, so salts must be
  removed at the rate they are supplied
  sedimentation and biologic activity.

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There is significant circulation through the
seafloor.
It is estimated that the entire ocean cycles
through the seabed at rift zones every 1 to 2
million years
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Amount of element in the ocean
Residence Time
Rate at which the element is added to (or removed
from) the ocean
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Amount of element in the ocean
Residence Time
Rate at which the element is added to (or removed
from) the ocean
Estimate residence time for the Global Ocean
Total ocean volume 1.37 109 km3
Total evaporation 3.34 105 km3 per year
Residence time 4,100 years
Because ocean is in steady state, loss of water
by evaporation must be balanced by inputs by
precipitation, glacial melt, and river runoff.
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Mixing Time
Time it takes for a substance to become uniformly
distributed.
t 0
t tmix
Accomplished by molecular diffusion, which is
enhanced by turbulence
Mixing time for the global ocean is estimated to
be 1,600 years. This mixing is driven by the
large scale circulation in the ocean.
When the residence time is much longer than the
mixing time of the ocean, materials will behave
conservatively and will maintain constant
proportionality.
Short residence times lead to nonconservative
behavior Chemically and biologically reactive
materials often exhibit nonconservative behavior
in the ocean.
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Summary

• The polar structure of the water molecule makes
  it a very powerful solvent.
• Salts and other ionic compounds go into solution
  easily.
• The presence of dissolved material in seawater
  increases its density, lowers its freezing point,
  and lowers its specific heat.
• Average ocean salinity is 35 ppt (or officially
  psu) and is composed primarily of Sodium and
  Chloride.
• The major elements in seawater maintain constant
  proportionality even if the overall salinity
  changes.
• The easiest way to measure salinity is using its
  electric conductivity.
• Global differences in surface salinity largely
  reflect differences in evaporation, precipitation
  and ice melt.
• Ocean salts come from the weathering of rocks and
  are input into the ocean by rivers.
• However, the composition of seawater and river
  water are different because there are other
  sources such as precipitation and hydrothermal
  activity.
• Ocean is not getting saltier because salts are
  removed by sedimentation and biological processes.