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Physical &
Chemical
OceanographyUnit 7
Chemical Composition of Sea Water
� Sea water� Has an average salinity of 35‰ (parts per thousand)
� Salinity� The total amount of salt dissolved in seawater
� Usually expressed in number of grams left behind when 1,000 grams of seawater are evaporated (parts per thousand)
� Due to amount of solutes (dissolved materials), usually ions (charged atoms present in seawater
� Salinity has a huge impact on organisms living in the water� Most marine organisms die in fresh water� Even slight changes in salinity can harm animals
� Salinity varies based on:� Location
� Near river mouths or melting glaciers = less salinity
� In dry, arid areas = higher salinity
INB pg.
45
Chemical Composition of Sea Water
� Regardless of where you take samples, percentage of ions within sea water remains fairly constant� Only 6 ions make up 98% of solids
in seawater!
� Sodium (Na+) & Chloride (Cl-) make up 85% - why seawater tastes “salty”
� This phenomenon is the rule of constant proportions� states that the relative amounts
of the various ions in seawater are always the same
� However, ion composition can be locally changed due to:� Volcanic activity� Runoff� Atmospheric dissolution
55%31%
8%
4% 1%1%
0%
Chloride (Cl-)
Sodium (Na+)
Sulfate (SO4-2)
Magnesium
(Mg+2)Calcium
(Ca+2)
Volcanic Changes� Volcanoes emit the following gases which enter sea
water� Carbon dioxide (CO2)
� Sulphur Dioxide (SO2)
� Hydrogen Sulphide (H2S)
� Hydrogen Chloride (HCl)
� These gases dissolve in atmospheric water (evaporation/condensation in clouds) & enter seawater as precipitation (recall: hydrological cycle)
� Underwater (submerged) volcanoes at tectonic plate boundaries emit the above gases
� Gases from volcanoes are major source of chloride ions in seawater
Changes from runoff
� Runoff = flow of water from land� Comes from rain or melting or snow & ice
� As part of hydrological cycle, most will eventually reach oceans
� Water flowing through soil or urban landscapes picks up pollutants � Pesticides
� Fertilizers
� Oils
� Plastics
� Pollutants may have initially low concentrations, but if taken in by organisms, biomagnification occurs� Pollutant eaten/absorbed by organism at low trophic level
� Passed on to each level within food web
� Increases its concentration at each trophic level
� Ex. Minamata Bay in Japan, from 1932 to 1968
Biomagnification through runoff
Changes through Atmosphere� Gases dissolved in seawater are at equilibrium with
atmosphere due to diffusion� Actual concentration of gases is dependent on:
� Relative solubility
� Temperature of water
� Salinity of water
� Dissolved gases include:� Nitrogen
� Can be “fixed” (changed) by nitrogen fixing bacteria into products usable by other organisms
� Great fertilizer
� Oxygen� Necessary for respiration
� Carbon Dioxide� Used in photosynthesis
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