Seawater Chemistry

Components of Seawater

• water’s 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

Components of Seawater

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

Salinity

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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

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

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

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

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

Major Constituents of Seawater • Nearly every element present in the crust & atmosphere is also in oceans • 

Water 96.5% total percent by mass

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Oxygen 85.8% (by mass)

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Hydrogen 10.7% Ions 3.4% total percent by mass

Major Constituents of Seawater at 34.4 ppt 

Over 99% of seawater salinity comes from 6 ions:

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Chloride 55%

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Sodium

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Sulfate

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Magnesium 32% 8% 3%

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Calcium

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Potassium 1% 1%

Sources of Ocean’s 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

Sources of Ocean’s 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

Sources of Ocean’s 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

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

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

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

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 ocean’s mixing time (~1600 years) it becomes evenly distributed

Dissolved Gases

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Seawater also contains dissolved gases

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Most gases in the air dissolve readily in seawater at the surface

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Plants and animals need dissolved gases to survive

Dissolved Gases

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Major gases – nitrogen, oxygen & carbon dioxide

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Gases dissolve better in cold water

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Cold polar water contains more gases that warm tropical water

Dissolved Gases

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Nitrogen – 48% of the gases in ocean (78% in atmosphere)

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Source – diffusion of atmospheric nitrogen

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Upper layers saturated with nitrogen gas

Dissolved Gases

Living organisms require nitrogen to build proteins

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Nitrogen gas can’t be used by organisms until it is attached to oxygen in a process called nitrogen fixation

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Blue-green algae convert nitrogen gas to a useable form that animals need for building proteins and amino acids

Dissolved Gases

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Oxygen – 36% of the gases in the ocean (21% in atmosphere)

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Average of 6 ppm (6 mg/L)

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Source – photosynthesis and diffusion of atmospheric oxygen

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Living organisms require oxygen for respiration

Dissolved Gases

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Carbon dioxide – 15% in ocean (<<1% in atmosphere Sea water CO 2 and 54 ml/L Source – levels range between 45

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Respiration of animals

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Very soluble in water - moves quickly from atmosphere to ocean, slowly from ocean to atmosphere

Dissolved Gases

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Some CO 2 forms carbonate ions that combine with calcium to form limestone (a sedimentary rock)

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Most of earths surface carbon – 10,000 times that in mass of all living things – is stored in sediments

Acid-Base Balance

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Seawater is slightly alkaline ~8.0 pH

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Water + carbon dioxide makes carbonic acid, which lowers the pH

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If acid is added to the ocean chemical reactions take place to remove the excess H + (less acidic)

Acid-Base Balance

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Seawater is slightly alkaline ~8.0 pH

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Carbonic acid disassociates into bicarbonate (a base) and hydrogen which raises the pH (more alkaline)

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If alkaline is added to the ocean, chemical reactions take place to remove excess OH (less alkaline)

Acid-Base Balance

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This behavior acts to buffer the water preventing broad swings of pH when acids or bases are added Enzyme activities and the shapes of vital proteins require a stable pH Since mollusk’s shells are calcium carbonate, a decrease in pH could dissolve shells