Marine Chemistry
Chemical oceanography
Objectives
• Definitions.
• Properties of sea water
• Physical Properties of sea water
• Temperature
• Salinity
• Density
• Water circulations
• Ocean
• Temperature distribution in ocean
• Salinity distribution in ocean
• Sea water composition
• Major Ions
Conservative elements
• Trace elements
Dissolved gases
• Micronutrients
Organic matter
Definition
• Marine chemistry/ Chemical Oceanography is the study
of Sea/ocean water chemistry
• The ocean is unique (Why?)
• It contains nearly every element in the periodic table.
Behavior of
Chemical
elements within
Ocean
Marine Chemistry studies
Describe cycles of
elements
Biogeochemical
Cycle
Behavior of isotopes
and their role as
tracers for past and
present
Cycle of
Biologically
active elements
C, N, P, & trace
Fe
• Some Isotopes' applications
• 1-The incidence of 18O (the heavy isotope of oxygen)
can be used as an indicator of polar ice sheet extent.
and
• 2- Boron isotopes are key indicators of the pH and
CO2 content of oceans in the geologic past.
Properties Of Sea water
• Water is the universal solvent because of its ability to
dissolve at least a little of every substance.
• Water is a particularly good solvent for substances
held together by different bonds.
Density Decreased
• Activity #1
• What happened when we add salts to water?
• Think about the following items?
• 1- Density
3- Boiling point
2- Freezing point
4- Conductivity
Density is increased.
The freezing point is depressed
The boiling point is elevated
The conductivity is increased
• The density is increased:
• Any substance dissolved in a liquid has the effect of
increasing the density of that liquid.
• The greater the amount of solute, the greater the effect
• As a result of the salt content in seawater, the density of
seawater increases continuously with decreasing
temperature.
• There is no density maximum like for freshwater.
• The colder the water temperature, the (heavier- lightermoderate) the water density. (Choose between brackets)
•
• The freezing point is
depressed:
• Salts lower the temperature
at which water freezes
• (This is why salt is spread
on frozen roads).
• That is because dissolved
salts inhibit the tendency of
water molecules to form
direct bonds with other
water molecules.
• The freezing point of salty
water is (higher than- lower
than-equal to) that of pure
water. (Choose between
brackets
• The boiling point is
elevated:
• The salts have the effect of
making the water molecules
cluster and become more
“ordered” thus harder to pull
apart and evaporate.
• Activity #2
• Ice melts and mixes with seawater of salinity 35 g/kg.
Will this have the effect of raising or lowering the
freezing point of seawater?
• How would this effect the formation of more sea ice
when temperatures fell once more?
• This will add fresh water, dilute seawater, make it less
saline and raise the freezing point thus facilitate the
formation of further sea ice.
• The conductivity is increased:
• The transport of electrons causing and electric current
to flow is enhanced by the strong electrolyte nature of
salts.
• If an electromagnetic field is applied to a solution of
strong electrolytes, the ions will migrate, producing
an electric current (e.g. conductivity is increased).
Temperature, Salinity, Density and Ocean Circulation
Temperature
Control Density
That
Governs
Vertical
Movement
Of Ocean water
Salinity
Temperature and salinity are together two of the most important
characters of sea water because …..(Complete).
Temperature Distribution in the Ocean
At the sea surface
temperature is fixed due to exchange with atmospheric heat
Incoming energy from the sun at the earth's surface
is about 4 times at the equator> than at the poles
Infrared radiation heat loss to space
Is more constant with latitude
There is a net input of heat to the earth's surface into the tropical
regions, where we find the warmest waters
Heat is transferred from low to high latitudes
by winds in the atmosphere and by currents in the ocean.
Salinity Distribution in the Ocean
At the sea surface, salinity is controlled by
Evaporation
Precipitation
The highest S‰ are found in sub-tropical regions centered at
20º to 3º North and South
Evaporatio
n
>
Rainfall
Red Sea has
The highest surface salinity
Sea water
Composition
Major
ions
Dissolved
gases
Trace
elements
Diagram showing concentrations of various salt ions in seawater:
Cl− 55%, Na+30.6%, SO2−4 7.7%, Mg2+ 3.7%, Ca2+ 1.2%,K+ 1.1%
, Other 0.7%. Note that the diagram is only correct in units of
wt/wt, not wt/vol or vol/vol.
Major Ions
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Definition:
Those elements whose concentration is greater than 1 ppm.
Why 1ppm?
Because Salinity is reported to ± 0.001 or 1 ppm.
What are their effect?
They contribute significantly to the salinity.
How many?
Eleven (11) major ions.
What are names and ranking?
Sodium> Magnesium> Calcium> Potassium> Strontium>
chloride> Sulfate> Bicarbonate> Bromide> Borate>
Flouride
• Activity #2:
• There are 11 major ions contribute significantly
to salinity although of this Na+ and Cl- are the
most dominant. (Explain)
• The major ions are conservative. (why?)
The major ions are conservative.
This means that they have
constant ratios, to one another
and to salinity, in almost all
ocean water.
Sea salts have constant composition.
They almost always consist of 55%
sodium ion, 31% chloride, 8% sulfate,
4% magnesium ion, 1% calcium ion,
and 1% potassium ion.
The main exception is where freshwater is mixing with seawater.
River water has a different composition than seawater, it contains
more calcium ion.
At a salinity of S = 35.000 seawater, The major ions concentration
is >1mg/kg seawater.
Na+ and Cl- account for >86% of the salt content.
The order of dominance of the other cations is
Mg2+ > Ca2+ > K+ > Sr2+.
The anion Cl- is approximately equal to the sum of the cations.
The other anions are barely significant in the charge balance of
seawater.
What are the synonym of other elements that their
concentration are < than 1mg/kg seawater?
Sea water composition Via river water
Properties
Sea water
River water
Amount/ Number
Huge number of dissolved
elements
Low number of dissolved
elements
Sea water has 300 times more elements than river water
Mixture
Heterogeneous
nearly Homogenous
Different from sea water.
Source
Terrestrial
Biogenic
Cosmogenic
Hydrogenic
chemical weathering of
rocks.
recycled sea salts via
rainfall
Contribution
Na+ and Cl- account for
>86% of the salt content
A minute fraction of river
Cl- content (0.01%) from
rock weathering.
Most from recycled salts.
Gases and Gas Exchange
The ocean is a sink for anthropogenic
CO2.
The major transfer modes of CO2 to
the ocean from the atmosphere is by
gas exchange.
Oxygen is a chemical tracer for
photosynthesis. The gas exchange flux
of O2 is an important parameter for
calculating net biological production.
Some gases can act as tracers for ocean
circulation.
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Trace Elements in Seawater:
Definition:
Those elements that do not contribute to the salinity.
Concentrations:
All elements are present in concentrations less than
1mg kg-1. Many are present at very low
concentrations (>10-21 M).
like 1 drop of ink in 100,000 liters of swimming pool.
Restrictions
This presents analytical challenges to measure and
avoid contamination.
Importance
Many are nutrients and required to sustain life (e.g. P,
N, Fe)
Others are toxic (e.g. Cu, Hg)
Some are tracers for re-dox conditions (Cr, I, Mn,).
Some are tracers for pollution (Pb).
Some form economic deposits such as manganese
nodules (e.g. Cu, Co, Ni, Cd)
• Macronutrients
• Definition:
• Macronutrients are those elements that are believed to be limiting
to plant growth in the surface ocean.
• Concentration
• They have concentrations in the range of micromoles kg-1.
• Components
• Phosphate, nitrate and silicate are the important macronutrients.
• Distribution
• Po4-4 and NO3-3 have maximum concentrations shallower than
silicate (Why?).
• 1- Because maximum degradation of organic debris happens
shallow in the thermocline.
• 2- SiO2 increases with depth because of the dissolution of siliceous
tests of diatoms, which dissolve deeper in the water column and on
the sea floor.
Dissolve Organic Matter (DOM)
• Any material with a carbon atoms joined to each other, and often to
H, O, N and P.
• We study organic matter in seawater because:
• 1. OM is the principle chemical form in which solar energy is
made, stored, and used on Earth (source of fossil fuels).
• 2. About 80% of the total particulate carbon flux through the
thermocline is in the form of organic matter.
• 3. About 20% of the total carbon buried in marine sediments is
organic.
• 4. Over geological time scales OM burial in marine sediments is a
major source of atmospheric O2.
Summary