Ocean Chemistry Unit 5

Colligative Properties of Seawater   Heat Capacity – –heat required to raise 1 g of substance 1°C – –Heat capacity of water is among the highest of all known substances.   Water can absorb (or release) large amounts of heat with little change in temp. – –The heat capacity of seawater decreases with increasing salinity (saltwater is less able to hang on to heat)

Colligative Properties of Seawater   Salinity – –total quantity of dissolved inorganic solids in water (NOT just salt!) – –Salinity is usually % depending on evaporation, precipitation, and freshwater runoff – –Proportion of Cl to salinity is constant:   Salinity in % = 1.81 x Cl % – –As salinity increases, freezing point decreases   Gives seawater a natural “antifreeze” property (saltwater freezes at a lower temp than fw) – –Salt water evaporates more slowly than fw (salt hangs onto water)

Colligative Properties of Seawater   Osmotic Pressure – –O.P. of organisms increases with increasing salinity (organisms lose more water when salinity is higher)

Water Density   Density of water is a function of salinity and temperature. – –Seawater density increases with increasing salinity, increasing pressure, and decreasing temperature.

Water Density   Freezing & Density – –During the transition from liquid to solid, water expands   This makes ice less dense than liquid water, and thus floats.   Density of ice is.917 g/cm 3   Density of liquid water is.999 g/cm 3. – –Density of water increases as seawater freezes.   Ice crystals are pure water because they exclude the salt.   The left over cold, salty water is very dense.

Water Density   Ocean Layers – –The ocean layers by density stratification.   Surface (mixing) zone – –2% – –least dense zone – –Top of the sea can actually be fw   Pycnocline – –18% – –density increases with depth   Deep zone – –80% – –below 1000m, densest layer

Pycnocline   Thermocline + Halocline = Pycnocline – –Halocline - the area where the salinity changes rapidly. – –Thermocline - the layer that changes in temp rapidly.   Can range in temp from °F   Average temp of ocean being 38°F.   Water masses (having characteristic temp and salinity, density) get trapped at great depths. – –The pycnocline isolates 80% of the ocean from the 20% circulating on the surface.

Dissolved Gases Gases dissolve most readily in cold seawater Plants and animals in the ocean require dissolved gases in order to survive   Nitrogen – –48% of gas in ocean (atmosphere 78%) – –living organisms require N to build proteins, but bottom dwelling bacteria must “fix” the N into a useable form for them   Oxygen – –36% of gas in ocean (atmosphere has 100x more) – –Primary source of O 2 in ocean is from plants – –most of the oxygen is near the surface and diffuses into the atmosphere   Carbon dioxide – –15% of gas in ocean (60x more in ocean than atmosphere) – –used by marine plants- low at surface

pH pH   Acidity (release of H+) and alkalinity (release of OH-) is measured by pH   The ocean contains buffers to prevent large swings in pH when acids or bases are introduced   pH scale acid neutral base (alkaline) Pure water Seawater 7.8