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

• 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 massOxygen 85.8% (by mass)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:Chloride 55%Sodium 32%Sulfate 8%Magnesium 3%Calcium 1%Potassium 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• 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

Dissolved Gases• Major gases – nitrogen, oxygen &

carbon dioxide

• Gases dissolve better in cold water–Cold polar water contains more

gases that warm tropical water

Dissolved Gases

• Nitrogen – 48% of the gases in ocean (78% in atmosphere)–Source – diffusion of atmospheric

nitrogen–Upper layers saturated with nitrogen

gas

Dissolved Gases

Living organisms require nitrogen to build proteins– Nitrogen gas can’t 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

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

Dissolved Gases• Carbon dioxide – 15% in ocean (<<1%

in atmosphere• Sea water CO2 levels range between 45

and 54 ml/L• Source –

–Respiration of animals–Very soluble in water - moves quickly

from atmosphere to ocean, slowly from ocean to atmosphere

Dissolved Gases–Some CO2 forms carbonate ions that

combine with calcium to form limestone (a sedimentary rock)

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

Acid-Base Balance

• Seawater is slightly alkaline ~8.0 pH–Water + carbon dioxide makes

carbonic acid, which lowers the pH– If acid is added to the ocean

chemical reactions take place to remove the excess H+ (less acidic)

Acid-Base Balance

• Seawater is slightly alkaline ~8.0 pH–Carbonic acid disassociates into

bicarbonate (a base) and hydrogen which raises the pH (more alkaline)

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

Acid-Base Balance

• 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