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