Transcript The physical and chemical properties of seawater

Sea Water Chemistry
Chapter 7
Sea Water Chemistry
determine many important
oceanographic phenomena including :
 Global patterns of oceanic and
atmospheric circulation, and the growth
and distribution of marine organisms.
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Life on earth probably evolved in
water
 Most animals are 50-65% water
 water exists in all three physical
states of matter: solid, liquid, and
gas
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71% of the Earth Surface - Sea
Water
 regulates the climate, dilute waste
 major habitat for living creatures
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polarity of water molecules results
in hydrogen bonding
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Characteristics of H20
1. has cohesive behavior
 2. resists changes in temperature
 3. has a high heat of vaporization
and cools surfaces as it evaporates
 4. expands when it freezes
 5. is a versatile solvent
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1. Surface tension
– measure of how difficult it is to
stretch or break the surface of a liquid
– water has a greater surface tension
than most liquids
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2-3. Water's high heat of
vaporization:
 moderates the earth's climate.
 solar heat absorbed by tropical
seas dissipates when surface water
evaporates
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4. Oceans and lakes don't freeze
 because of hydrogen bonding,
water is less dense as a solid than
it is as a liquid.
 consequently, ice floats.
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5. water is a versatile solvent
owing it to the polarity of the
water molecule
 ionic compounds dissolve in water
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4/11/2016
Prepared by: Prof. Rodriguez
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Major Components of Seawater
dissolved salts - hydrated anions and
cations (Table 7.1;, f. 7.3)
 dissolved gases - nitrogen, oxygen,
carbon dioxide
 organic and inorganic - dissolved
organic materials suspended particulate
matter
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Major Ions in Typical Seawater
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Ion Parts per thousand by weight ‰ (ppt)(Table
7.1)
Cl18.98
SO4-2 2.649
HCO3- 0.140
Na+
10.556
Mg2+
1.272
Ca+
0.400
K+
0.380
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On average, concentration of dissolved salts,
i.e., the salinity, in seawater is 3.5% or
35‰.
The relative abundances of the ions listed
above does not change, even though salinity
does; are said to be conservative.
Relative abundances of minor and trace
constituents do vary
Determining Salinity
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Evaporation of water to weight the salt is an
imprecise method
Because of the constancy of composition if
we measure one component we can get a
more precise measurement
Salinity ppt = 1.80655 x Chlorinity in ppt
If chlorinity is 19.2 ppt, what is the salinity of
sea water?
34.7 ppt = 35 ppt
Sources of Salt
Rivers (winds and glaciers are a less
important, indirect source)
 Weathering of oceanic crust
 Hydrothermal Vents associated with
Mid-ocean ridges and other submarine
volcanoes
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Sinks
Biologic activity
 Interaction with Particulate matter: clays
and organic matter absorb dissolved
metals
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Direct Precipitation
Hydrothermal Activity: (fig. 7.4)
 Reaction between seawater and new
oceanic crust
 Minerals like magnesium is incorporated
into deposits
 Calcium is added to sea water
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Physical and Chemical
Properties of Water
Heat Capacity - energy added to raise
temperature of 1 gram of substance by
1°C
 adding energy breaks H-bonds,
increases fraction of free water
 important in thermal buffering and heat
transport to higher latitudes
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Latent Heats and Evaporation
heat input or release associated with
phase changes (ice - liquid, liquid vapor)
 changes in water structure, H-bonding
with phase changes
 important in thermal buffering, heat
transport and heat exchange with
atmosphere
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Density - mass per unit volume
(grams/cm3)
 density of water phases (ice, liquid,
vapor) due to structural changes at
molecular level
 density maximum at 4°C in pure water
 Major role in deep ocean circulation and
water column structure and stability
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Light Transmission
transparent in visible part of spectrum
 Absorbed as is goes deeper in the
water column
 strongly absorbs infrared (heat) and
ultraviolet (prevents damage to DNA)
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Dissolving Power
hydration of solutes - interactions
between solutes and free water
 decreases H-bonding, increases order
of free water, increases density
 exclusion of solutes on freezing and
evaporation
 other effects of solutes: freezing point
depression, boiling point elevation
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pH (acidity or alkalinity)
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measure of the dissociation of water
into ions (H+, OH), (fig. 7.9)
 pH
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= - log [H+]
pH effects on biological and
geochemical reactions
Conservative vs. Non
Conservative Properties
Conservative Properties of Seawater
 those properties that can only be altered
at the sea surface: temperature, salinity,
inert gases
 properties not altered by biological or
geochemical reactions
 importance in water mass identification,
tracing and mixing
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Nonconservative Properties of
Seawater
 those properties that can be altered
anywhere in the water column
 properties altered by biological and
geochemical reactions
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Dissolved Gases
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The proportions of gases in the atmosphere
is not the same as their proportions in
seawater, (Table 7.4)
There is less N2 (nitrogen gas) in the ocean
than in the atmosphere,
much more oxygen,
and even more CO2.
All this CO2 in the oceans keeps CO2 from
being in the atmosphere and causing global
warming.
The colder the water, the more gas
can dissolve in it.
 When you leave your can of coke in the
car in the sun, then open it, what
happens?
 Coke sprays all over you.
 That's because the gas has exsolved
(come out of solution); a lot has
accumulated in the little space at the top
of the can.
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Very active fish, such as trout and salmon,
require very cold water to live in because they
have high oxygen requirements.
They literally suffocate when the water gets
too warm, and the oxygen levels drop.
This explain why these fish don't live down
south.
'Thermal pollution' occurs when electric
plants put warm water into streams, lowering
the oxygen level.
CO2 is important because it is needed
by plants so they can photosynthesize.
 O2 is important because animals need it
for respiration.
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Photosynthesis:
 CO2 + H2O + energy [from the sun]  O2 +
sugar (organic matter)
Respiration (the reverse of photosynthesis):
 O2 + sugar  CO2 + H2O + energy
Dissolved Oxygen
seawater\atmosphere exchange at air
water interface only (fig.7.8)
 biological processes that affect O2
concentration: photosynthesis and
respiration
 typical distribution of O2 in water column
and processes that control this
distribution
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Phytoplankton Nutrients
inorganic sources of N, P, S and other
atoms required for phytoplankton
growth
 photosynthesis and respiration
contributes in nutrient distribution
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Especially important, because so much is
needed, are N (nitrogen) and P
(phosphorus).
Si (silica) is also important for all the
siliceous organisms we‘ll discuss: diatoms,
radiolarians, and siliceous sponges.
N is necessary to make proteins.
P is necessary to make new cells (it's part of
the cell wall), and also genetic material, DNA
and RNA.
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N is useful for plants only in these forms:
NO3- nitrate
NO2- nitrite
NH4+ ammonium
N2, the gas, is not usable by most plants.
Only a few bacteria can break this very strong
molecule apart and turn it into nitrate.
These are 'nitrogen-fixing bacteria'.
P is useful in the form of phosphate, PO43-
Thus weathering, sedimentation, and
ocean chemistry are all closely linked.
 Other ions in seawater, such as Cl – and
SO4 , are not derived from weathering,
but from volcanic degassing.
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The Carbonate System in
Seawater
CO2 in seawater is controlled by: ( f.710-11)
 Exchange with the atmosphere
 Photosynthesis/Respiration:
 6CO2 + 6H2 O  C6 H12 O6 + 6O2
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The Carbonate Buffer
Carbon Dioxide: CO2 ,
 Carbonic Acid: H2CO3 ,
 Bicarbonate: HCO3 - ,
 Carbonate: CO32 CO2 + H2O  H2CO3 .
 H2CO3  HCO3 - + H+2.
 HCO3  CO32- + H +3.
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Another important reaction is the dissolution
and precipitation of calcium carbonate:
CO3 + Ca+2  CaCO34..
Importance of these reactions:
Maintain constant pH (seawater is said to be
buffered).
Few marine organisms can tolerate a pH very
different from 8.
Biological Productivity
In general, shells of organisms are likely
to be preserved where their production
rate is high,
 Siliceous shells are preserved only
where the production rate is high.
 Siliceous oozes occur where
productivity rate is high and terrigenous
sedimentation rate low.
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Carbonate shells: the oceans are
supersaturated with respect to CaCO3 at the
surface and become increasingly
undersaturated with depth.
Shells more likely to be preserved at shallow
depth.
Lysocline: depth at which rapid dissolution of
CaCO3 begins.
This is deeper than the depth where ocean
becomes undersaturated.
CCD
Carbonate Compensation Depth (or
‘snow line’):
 depth where dissolution>supply of
CaCO3 and
 below which CaCO3 shells are not
preserved in sediment.
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