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Chemistry of Seawater
Aquatic Science 2011
K. Schneider
The Salty Sea
• The salt in the ocean exists
in the form of charged
particles, called ions.
• Sodium and Chloride ions
make up 85% of all the salt
in the sea.
• The rest is made up of
Sulfate, magnesium,
calcium, potassium &
several others present in
smaller quantities.
Principle of Constant Proportions
• Alexander Marcet (1770-1822)
• The Swiss chemist and doctor carried
out some of the earliest research in
marine chemistry.
• 1819 he discovered that all the main
chemical ions (sodium, chloride, &
magnesium ions) in seawater are
present in exactly the same
proportions throughout the world’s
oceans.
Sources of Salt
• Some were dissolved out of rocks on land by
the action of rainwater and carried to the sea
in rivers.
• Others enter through Hydrothermal vents, in
dust blown off the land, or came from volcanic
ash.
Sinks
• Processes that remove salts
from seawater
• Salt spray onto land
• Precipitations of various ions
onto the seafloor as mineral
deposits
Salinity
• The amount of salt in a fixed mass of
seawater.
• It is determined by measuring a seawater
sample’s electrical conductivity and averages
about ½ oz of salt/lb of seawater.
• The salinity depends on what processes or
factors that are operating at that location to
either add or remove water.
Factors that effect Salinity
• Add water, lower salinity:
– High rainfall
– River input
– Melting of sea ice
• Remove water, increase salinity:
– High evaporative losses
– Sea-ice formation
Salinity
• At depth, salinity is near constant throughout
the ocean
• Between the surface and deep water is in a
region called Halocline, where salinity
gradually increases or decreases with depth.
How does the Sea smell and Taste?
1.
2.
3.
4.
Have 2 members stay at their desks.
Use a pencil to label the bottom of 3 cups: F, B, S
Fill one cup with (F) freshwater (tap water)
Fill one cup with freshwater and add a pinch of salt
(B). Mix.
5. Fill one cup with freshwater and add 3 pinches of
salt (S). Mix.
6. Have one student try to differentiate by tasting the
water.
7. Next, have another student try to differentiate by
smelling the water.
Nutrients in Seawater
• Nutrients – A chemical an organism needs to
live and grow
• Nearly constant ratio of N to P in much of the
ocean 16:1
• Surface waters with little N or P, while deep
waters much higher
• The N:P ratio is the same in plankton as it is in
water, reflecting the linkage between life and
chemistry in the ocean.
Nutrients in Seawater
• At the base of the food chain:
Phytoplankton-microscopic floating
life-forms that obtain energy by
photosynthesis.
– Need nitrates, iron, & phosphates in
order to grow
– No nutrients = no growth
– Too much nutrients = blooms (rapid
growth phase)
Phytoplankton
Bloom in the
Gulf of Mexico
Phytoplankton blooms usually occur
where cold water rushes up from the
bottom of the ocean carrying nutrients
to sunlit waters. In this case, the
bloom may be related to recent
flooding along the Mississippi River
and its tributaries. Heavy rains early in
December triggered floods across the
southeastern United States. The
draining flood water carried
agricultural run-off into the Mississippi
River and the Gulf of Mexico, and that
may have provided the nutrients the
microscopic ocean plants needed to
thrive.
http://eoimages.gsfc.nasa.gov/ve/19761/Mississippi_Sea_2004348.jpg
Gases in Seawater
• The main gases are nitrogen (N), oxygen (O2),
& carbon dioxide (CO2).
• The levels of O2 & CO2 vary in response to the
activities of photosynthesizing organisms.
Gases in Seawater
• The level of O2 is generally
highest near the surface,
where the gas is absorbed
from the air and
photosynthesizers.
• CO2 sinks! Concentrations
are the highest at greater
depths and lower at the
surface.
• “Soda on a warm day
effect” – Higher capacity to
hold a gas at a lower
temperature
http://eesc.columbia.edu/courses/ees/slides/climate/gas_exch.gif
Qruote from: Agassiz professor of biological oceanography James McCarthy
from - http://harvardmagazine.com/2002/11/the-ocean-carbon-cycle.html
Carbon in the Ocean
• The ocean contains the world’s largest store of CO2.
• Biological and chemical processes turn some of this
CO2 into the calcium carbonate shells and skeletons
of organisms, other organic matter, & carbonate
sediments.
• However, the CO2 concentration is beginning to
acidify the oceans, threatening shell and skeleton
formation in marine organisms, which then
threatens the food chain
Ocean’s Carbon Cycle
• Single-celled marine plants (phytoplankton and
other marine microalgae) take in carbon dioxide
(CO2) and convert it into biomass.
• By converting carbon dioxide into more complex
carbon compounds, the phytoplankton
effectively make atmospheric carbon available
to other marine organisms
Ocean’s Carbon Cycle
• Bacteria eat dissolved organic C compounds
secreted by the phytoplanktonphytoplankton
are eaten by protozoa protozoa &
phytoplankton are eaten by zooplankton  eaten
by fish passing the carbon through the food chain
and into animals like seals & polar bears
• When any of these organisms die without then
being consumed, or when they defecate, the
carbon locked away in their bodies gradually
settles to the sea floor.
Ocean’s Carbon Cycle
• However most of the carbon captured from
the atmosphere by phytoplankton never
reaches a polar bear. Some will be lost back to
the water and atmosphere as the different
plankton species respire.
• And a vast amount will be retained in the
microscopic community of phytoplankton,
bacteria, and viruses living near the sea
surface
10
13
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Organic matter – Comes from a living
Nitrogen Cycle
organism, is capable of decaying, and is
• Nitrogen gas (N2)
made of carbon-based –compounds.
Nitrous oxide (N O)
2
– Nitric oxide (NO)
• “Fixed” Nitrogen
Organic:
•N has to be taken
– Inorganic nitrogen:
from the atmosphere
•DNA
• Nitrate (NO )
and converted
intosucrose
a
• Nitriteplants),
(NO )
•table
sugar
or
(from
usable form, either
• Ammonium (NH )
C12H
through
lightning
22O11 or
3
2
+
4
“nitrogen fixing”
bacteria.
• Organic nitrogen:
– Detritus and Living biomass
– Dissolved organic matter (DOM)
Inorganic:
• Proteins/Amino Acids (ammonia)
•table salt or sodium chloride,
NaCl
• Urea
•carbon dioxide, CO2
• Nucleic Acids
HOW?
Ammonia
HOW?
HOW?
Ammonium
Nitrogen Fixation
Nitrogen used to form ammonium
N2 + 6 e- + 8H+ ---> 2 NH4+ (ammonium ion)
Another way by which ammonia may be formed is by the
process called nitrification. In this process nitrates and
nitrites, released by decaying organic matter are converted to
ammonium ions by nitrifying bacteria
NO3- (nitrate ion) + 2e- + 2H+ -----------> NO2- + H2O
(nitrite ion)NO2- + 6e- + 2H+ ----------> NH4+ + 2 H2O
BACK
http://library.kcc.hawaii.edu/external/
chemistry/everyday_nitrogen.html
Nitrogen Fixation
• Another way in which molecular nitrogen is
modified is via the discharge of lightning. The
tremendous energy released by the electrical
discharges in our atmosphere breaks the
rather strong bonds between nitrogen atoms,
causing them to react with oxygen.
lightning
N2 + O2 --------------> 2 NO (nitric oxide)
http://library.kcc.hawaii.edu/external/chemi
stry/everyday_nitrogen.html
Marine Nitrogen Cycle
Courtesy of Karen Casciotti
Nitrogen Cycle
The role of bacteria:
 Convert harmful ammonia into non-toxic
nutrients.
○Nitrosomonas – convert ammonia (NH4)
into nitrite (NO2).
○Nitrobacteria – convert nitrite (NO2) into
nitrate (NO3).
○These processes together are called
nitrification.
Nitrogen Cycle
What happens to the nitrate?
 Absorbed by algae
 Converted to nitrogen gas
Courtesy of Karen Casciotti
• Basic
Ocean
Nitrogen
Cycle
Temperature
• Temperature
varies
depending on
location and
depth.
Temperature in the Tropics &
Subtropics
• In the Tropics and Subtropics,
solar heating keeps the ocean
surface warm throughout the
year.
• Below the surface, the temp
declines steeply to about 4650° F at a depth of 3,300ft.
– The boundary that separates the
surface layers from the deep parts
of the ocean is called Thermocline
• 36° F on the sea floor
http://www.windows2universe.org/earth/Water/tem
p.html
Temperature in Mid-latitudes &
Polar Oceans
• In mid-latitudes there
is much more marked
seasonal variations in
surface temp.
• In high latitudes and
polar oceans, the
water is constantly
cold, sometimes
below 32 °F.
Density & Buoyancy
• Density is the mass of a substance per unit
volume (usually measured in grams per
milliliters, g/ml).
• Buoyancy is the upward force that a fluid exerts
on an object less dense than itself.
• Depends on temperature and salinity.
• Decrease in temp makes seawater denser, unless
the water is below 39 F (4 C), than it is a little less
dense.
Density
• Processes that change the density of seawater
cause it to either rise or sink, and drive largescale circulation in the oceans between the
surface and deep water.
Q: How do Ships Float?
A: A greater force is pushing up on the
ship than the weight force pushing down.
• This supportive force is called buoyant force.
Density & Buoyancy
• If the buoyant force is equal
to the object’s weight, it will
float.
• If the buoyant force is less
than the object’s weight, it
will sink.
Archimedes’ Principle
• Bouyant force was explained by
Archimedes, a Greek mathematician
around 3rd century B.C., and it
became known as Archimedes’
Principle.
• Archimedes’ Principle states that an
objects weight will cause the object
to sink while at the same time
displacing the fluid.
Archimedes’ Principle
• If the weight of the water displaced becomes equal
to weight of the object, it floats.
• If the weight of the water displaced becomes less
than the weight of the object, it sinks.
• Archimedes’ Principle is important b/c: Properties
of fluids ultimately determine the design of ships,
airplanes, cars, and hydraulic machines.
Nutrient Upwelling &
Turnovers
• An upwelling is an oceanographic
phenomenon that involves wind-driven
motion of dense, cooler, and usually nutrientrich water towards the ocean surface,
replacing the warmer, usually nutrientdepleted surface water
Pressure
• Scientists measure pressure in units called bars.
• At sea level, the atmosphere exerts a pressure of
about 1 bar.
• Underwater, pressure increases by 1 bar every
33ft.
• Divers must breathe pressurized air or other gas
mixtures.
Boyle’s Law
• If the temperature of a gas
does not change, Its volume
decreases as pressure
increases and vice versa
•Fish have a swim bladder, gas filled space for
buoyancy. Swim bladder expands or explodes if
brought to the surface to quickly.
Pressure sicknesses
• Nitrogen Narcosis
– Nitrogen gas will dissolve better under higher
pressure and therefore will be forced into body
tissues
– “rapture of the deep” – divers feel intoxicated by
excess nitrogen
– Feeling subsides as diver returns to surface
Pressure sicknesses continued
• Decompression sickness “The Bends”
– As diver ascends to the surface, bubbles
form in the blood and body tissues.
– Small bubbles are of no danger (slow
ascension)
– Medium bubbles block smaller vessels,
causing tingling and slight bruising.
– Larger bubbles block blood flow to vital
organs or cause nerve damage to
joints(the bends).
Overcoming Pressure
• If diver gets decompression sickness, they must be
put into a recompression chamber to relieve
symptoms.
• Underwater habitats – aquanauts live for several
days at the depth and pressure at which they are
working in specialized housing.