Chapter 6: Water and Seawater

Fig. 6-19

Atomic structure Nucleus Protons and neutrons Electrons Ions are charged atoms

Water molecule H2O Two hydrogen, one oxygen Bonded by sharing electrons Bend in geometry creates polarity Dipolar molecule

Dipolar molecule Weak negative charge at O end Weak positive charge at H end Hydrogen bonds Weak bonds between water molecules and ions Explains unusual properties of water

Fig. 6-3

Two unusual properties High surface tension

Hydrogen bonding creates “skin” Important for living organisms

Capillarity Universal solvent

Electrostatic bond between dipolar water and ions

Ocean is salty

Fig. 6.4

Fig. 6-5b

Thermal properties of water Solid, liquid, gas on Earth’s surface Water has high freezing point Water has high boiling point Water has high heat capacity Water has high latent heats

Fig. 6-7

Heat capacity Heat absorbed or released with

changes in state Latent heats of

Melting; freezing Vaporization, evaporation Condensation

Global thermostatic effects Moderate global temperature Evaporation removes heat from

oceans Condensation adds heat to

atmosphere Heat re-distributed globally

Differences in day and night temperatures

Water density Maximum density at 4oC Ice less dense than liquid water

Atomic structure of ice Ice floats

Increased salinity decreases temperature of maximum density

Fig. 6-10

Fig. 6-8

Seawater Salinity=total amount of solid material

dissolved in water (g/1000g) Typical salinity is 35 o/oo or ppt Brackish (hyposaline) < 33 ppt Hypersaline > 38 ppt

Measuring salinity Evaporation Chemical analysis

Principle of Constant Proportions Chlorinity

Electrical conductivity (salinometer)

Dissolved substances Added to oceans

River input (primarily) Circulation through mid-ocean ridges

Removed from oceans Salt spray Recycling through mid-ocean ridges Biogenic sediments (hard parts and fecal pellets) Evaporites

Residence time Average length of time a substance remains dissolved in

seawater Long residence time = unreactive

Higher concentration in seawater Short residence time = reactive

Smaller concentration in seawater Steady state

Ocean salinity nearly constant through time

Dissolved gases Solubility depends on temperature, pressure, and ability

of gas to escape Gases diffuse from atmosphere to ocean

Wave agitation increases amount of gas Cooler seawater holds more gas Deeper seawater holds more gas

Conservative vs. nonconservative constituents

Conservative constituents change slowly through time Major ions in seawater

Nonconservative constituents change quickly due to biological and chemical processes Gases in seawater

Oxygen and carbon dioxide in seawater Nonconservative O2 high in surface ocean due to

photosynthesis O2 low below photic zone because of

decomposition O2 high in deep ocean because source is

polar (very cold) ocean

CO2 low in surface ocean due to photosynthesis

CO2 higher below photic zone because of decomposition

Deeper seawater high CO2 due to source region and decomposition

Acidity and alkalinity Acid releases H+ when dissolved in water Alkaline (or base) releases OH- pH scale measures acidity/alkalinity

Low pH value, acid High pH value, alkaline (basic) pH 7 = neutral

Carbonate buffering Keeps ocean pH about same (8.1) pH too high, carbonic acid releases H+ pH too low, bicarbonate combines with

H+ Precipitation/dissolution of calcium

carbonate CaCO3 buffers ocean pH Oceans can absorb CO2 from

atmosphere without much change in pH

Fig. 6-17

How salinity changes Salinity changes by adding or removing

water Salinity decreases by

Precipitation (rain/snow) River runoff Melting snow

Salinity increases by Evaporation Formation of sea ice

Hydrologic cycle describes recycling of water

Hydrologic cycleFig. 6-19

Horizontal variations of salinity Polar regions: salinity is lower, lots of

rain/snow and runoff Mid-latitudes: salinity is high, high rate of

evaporation Equator: salinity is lower, lots of rain Thus, salinity at surface varies primarily with

latitude

Fig. 6-20

Vertical variations of salinity Surface ocean salinity is variable Deeper ocean salinity is nearly the same

(polar source regions for deeper ocean water)

Halocline, rapid change of salinity with depth

Density of seawater 1.022 to 1.030 g/cm3

Ocean layered according to density Density of seawater controlled by temperature,

salinity, and pressure Most important influence is temperature Density increases with decreasing temperature

Salinity greatest influence on density in polar oceans

Pycnocline, rapid change of density with depth

Thermocline, rapid change of temperature with depth

Polar ocean is isothermal

Layers of ocean Mixed surface

layer Pycnocline Deep ocean

End of Chapter 6: Water and Seawater