Oceanography 100
P Anderson
Chapter 5
Water and Seawater
Atomic structure
• Atoms are the building blocks of all matter
• Nucleus contains:
– Neutrons (no charge)
– Protons (+ charge)
• Outer shell(s) contain:
– Electrons (– charge)
The water molecule
• Composed of 1 oxygen and 2 hydrogen atoms (H20)
• Contains strong (covalent) bonds between atoms
• Unusual bend in geometry
• Has polarity (oppositely charged ends)
Interconnections of water molecules
• Polarity causes water molecules to form weak (hydrogen) bonds between water
molecules
• Water sticks to itself and to other substances
• Allows water to be the universal solvent
Water as a solvent
• Water dissolves table salt (NaCl) by attracting oppositely charged particles
• Pulls particles out of NaCl structure to dissolve it
Water in the 3 states of matter
• Latent (hidden) heat = energy that is either absorbed or released as water changes
state
The ocean moderates coastal temperatures
• Water has high heat capacity, so it can absorb (or release) large quantities of heat
without changing temperature
• Moderates coastal temperatures
Hydrogen bonds in H2O
The formation of ice
• As water cools to 4°C:
– Molecules slow
– Water contracts
– Density increases
• Below 4°C:
– Hydrogen bonds form
– Water expands
• As water freezes:
– Expands by 9%
Snowflake geometry
• All snowflakes have 6-sided geometry
• Caused by water’s polarity and ability to form hydrogen bonds
Salinity
• Salinity = total amount of solid material dissolved in water
• Can be determined by measuring water conductivity
• Typically expressed in parts per thousand (‰)
Constituents of ocean salinity
• Average seawater salinity = 35‰
• Main constituents of ocean salinity:
– Chloride (Cl–)
– Sodium (Na+)
– Sulfate (SO42–)
– Magnesium (Mg2+)
Salinity variations
Ocean buffering
• Ocean pH = 8.1 (slightly basic)
• Buffering protects the ocean from experiencing large pH changes
Processes affecting seawater salinity
• Processes that decrease seawater salinity:
– Precipitation
– Runoff
– Icebergs melting
– Sea ice melting
• Processes that increase seawater salinity:
– Sea ice forming
– Evaporation
The hydrologic cycle
Surface salinity variation
• Pattern of surface salinity:
– Lowest in high latitudes
– Highest in the tropics
– Dips at the Equator
• Surface processes help explain pattern
Surface salinity variation
• High latitudes have low surface salinity
– High precipitation and runoff
– Low evaporation
• Tropics have high surface salinity
– High evaporation
– Low precipitation
• Equator has a dip in surface salinity
– High precipitation partially offsets high evaporation
Global surface salinity
Salinity variation with depth
• Curves for high and low latitudes begin at different surface salinities
• Halocline = layer of rapidly changing salinity
• At depth, salinity is uniform
Seawater density
• Factors affecting seawater density:
– Temperature
– Salinity
– Pressure
• Temperature has the greatest influence on surface seawater density
Density and temperature variations with depth
Pycnocline and thermocline
• Pycnocline = layer of rapidly changing density
• Thermocline = layer of rapidly changing temperature
• Present only in low latitude regions
• Barrier to vertical mixing of water and migration of marine life
Ocean layering based on density
• Mixed surface layer (surface to 300 meters)
– Low density; well mixed by waves, currents, tides
• Upper water (300 to 1000 meters)
– Intermediate density water containing thermocline, pycnocline, and halocline (if
present)
• Deep water (below 1000 meters)
– Cold, high density water involved in deep current movement
Seawater desalination
• Desalination methods:
– Distillation
• Solar
• Heat
– Electrolysis
– Reverse osmosis
– Freeze separation