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Seawater Chemistry

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Components of Seawater waters ability to dissolve crustal material as it cycles from ocean to atmosphere have added solids and gases to the ocean ~97.2% of 1,370 million cubic kilometers (329 million cubic miles) is salt

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Components of Seawater by mass seawater about 96.5% water about 3.5% dissolved substances if the oceans waters could evaporate, remaining salts would cover the entire planet to a depth of 45 m (150 ft)

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Salinity The total quantity (concentration) of dissolved inorganic solids in water About 3.5-3.7% by mass in oceans Average ~3.5% or 35 ppt or 35 % 0

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How do ions modify the physical properties of water? Heat capacity decreases with increased salinity Less heat is needed to raise the temperature of seawater

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How do ions modify the physical properties of water? As salinity increases, the freezing point of water decreases Dissolved salts disrupt hydrogen bonding Sea ice forms at a lower temperature than freshwater ice

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How do ions modify the physical properties of water? Seawater evaporates more slowly than fresh water Dissolved salts attract water molecules Osmotic pressure increases with increasing salinity

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Components of Seawater About 3.5% of seawater consists of dissolved substances Boiling 100 kg of seawater yields 3.5 kilograms of residue Oceanographers use parts per thousand (o/oo) or ppt

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Major Constituents of Seawater Nearly every element present in the crust & atmosphere is also in oceans Water96.5% total percent by mass Oxygen85.8%(by mass) Hydrogen10.7% Ions 3.4% total percent by mass

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Major Constituents of Seawater at 34.4 ppt Over 99% of seawater salinity comes from 6 ions: Chloride55% Sodium32% Sulfate 8% Magnesium 3% Calcium 1% Potassium 1%

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Sources of Oceans Salts Weathering and erosion of crustal rocks accounts for some (not the only source) Salts in the ocean are different concentration than those in river water

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Sources of Oceans Salts Upper mantle appears to contain more of the substances found in seawater (including water itself) than are found in surface rocks their proportions are about the same as in the ocean

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Sources of Oceans Salts Combination of weathering (ex, sodium) and outgassing (ex, chloride) Differences in expected seawater concentrations may be the result of interactions at mid ocean rifts (hydrothermal vents) All the water in the oceans cycles through the seabed every 1 to 10 million years

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Principle of Constant Proportions The percentage of salts in seawater is the same in samples from many places, regardless of how salty the water is Same proportions for 33 ppt and 37 ppt

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The Ocean is in Chemical Equilibrium The proportion and amounts of dissolved salts per unit volume are nearly constant what goes in must go out

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The Ocean is in Chemical Equilibrium Ions are added to the ocean at the same rate the are removed Additions from the mantle or from weathering are balanced by subtractions being bound into sediments

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Residence Time Concept of helps explain why ocean is not getting saltier Chemically active ions have shorter residence times See Table 7.3 page 169 (Oceanography book) If an ion remains in the ocean longer than the oceans mixing time (~1600 years) it becomes evenly distributed

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Dissolved Gases Seawater also contains dissolved gases Most gases in the air dissolve readily in seawater at the surface Plants and animals need dissolved gases to survive

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Dissolved Gases Major gases nitrogen, oxygen & carbon dioxide Gases dissolve better in cold water Cold polar water contains more gases that warm tropical water

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Dissolved Gases Nitrogen 48% of the gases in ocean (78% in atmosphere) Source diffusion of atmospheric nitrogen Upper layers saturated with nitrogen gas

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Dissolved Gases Living organisms require nitrogen to build proteins Nitrogen gas cant be used by organisms until it is attached to oxygen in a process called nitrogen fixation Blue-green algae convert nitrogen gas to a useable form that animals need for building proteins and amino acids

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Dissolved Gases Oxygen 36% of the gases in the ocean (21% in atmosphere) Average of 6 ppm (6 mg/L) Source photosynthesis and diffusion of atmospheric oxygen Living organisms require oxygen for respiration

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Dissolved Gases Carbon dioxide 15% in ocean (