Chapter 6
OCEAN CHEMISTRY AND COMPOSITION
I. Water Properties and Terms
A. polar molecule: oxygen has slight negative charge; hydrogen has slight positive
charge
B. hydrogen bonding: intermolecular bonds (dipole-dipole) formed between
hydrogen atoms of one water molecule and oxygen atoms of the other
C. surface tension: resistance of the thin outer layer of water to break or stretch
D. Van der Waals force: any attractive intermolecular force; includes dipole-dipole
forces, hydrogen bonds, and London dispersion forces
E. cohesion: water molecules stick together because of hydrogen bonding
F. adhesion: water molecules stick to containers (ex. glass)
G. capillarity: due to adhesion and cohesion, water pulls itself to great heights in
small-diameter tubes
H. viscosity: the resistance of a fluid to flow; colder liquids are more viscous than
warm ones; salty water is more viscous than freshwater
1. Reynolds number: gives a measure of the ratio of inertial forces to viscous
forces for a particular object in a certain fluid
2. viscometer: an instrument that measures viscosity by turning a wheel and
measuring the torque required to turn it at a certain speed
I. thermal terms
1. heat energy: the energy of moving particles
2. temperature: a measure of the average kinetic energy of particles
3. heat capacity [specific heat]: amount of heat energy required to raise the
temperature of 1 gram of a substance by 1 degree Celsius
a. calorie: (by definition) the amount of heat required to raise the
temperature of 1 gram of pure water by 1°C
b. seawater has a lower specific heat than pure water
c. responsible for mild coastal climates
d. takes lots of energy to change ocean’s temperatures in comparison to land,
so land heats and cools faster than water
4. latent heat: energy required to break bonds and change the state of the
substance, which is not accompanied by a change in temperature; latent heat
of fusion/melting is for melting or freezing; latent heat of
vaporization/condensation is for condensation or vaporization
5. evaporation: the change of phase of water from liquid to gas without reaching
its boiling point; this occurs when some water molecules get an above-average
amount of energy; latent heat of evaporation, the amount of heat required for
this process to occur, is greater than the latent heat of vaporization
a. water’s high latent heat of evaporation is responsible for taking heat from
low-latitude areas where lots of evaporation occurs and putting it in highlatitude areas, which reduces Earth’s temperature extremes
J. solvation: process of the solvent molecules surrounding the dissolved particles,
resulting in dissociation of the solute, which causes there to be a solution; this is
called hydration when water dissolves a solute
K. colligative properties: physical properties different for a solution than for a pure
substance
1. freezing point depression: seawater freezes at -2°C
2. boiling point elevation: boiling point is higher with salt
3. vapor pressure decrease: lower for saltwater than for freshwater
4. osmotic pressure: freshwater flows through a membrane into saltwater,
pushing against gravity
L. diffusion: the movement of a substance from high concentration to low
concentration; osmosis is the diffusion of water
1. hypoosmotic, hyperosmotic, hypotonic, hypertonic: words describing the
concentration of a solution relative to its surroundings
M. isotopes: elements with different numbers of neutrons
1. deuterium: hydrogen isotope with a proton and a neutron; results in “heavy
water”; used for tracking water
2. tritium: hydrogen isotope with a proton and 2 neutrons; results in “heavy
water”; used for tracking water
II. pH = -log[H+]
A. terms
1. acid: donates protons [H+], which form hydronium ions [H3O+] with water
2. base: accepts protons or donates hydroxide ions [OH-]
3. buffer: maintains pH because it has the abilities of both acids and bases
B. ocean
1. 7.5 - 8.5 (average 8.1); basic
2. basic due to calcium carbonate [CaCO3] and ammonia [NH3]
a. CaCO3 + H2O  HCO3- + Ca+2 + OHb. NH3 + H2O  NH4+ + OH3. held stable by buffers, mostly bicarbonate [HCO3-1], which is the most
common carbon-containing compound in the ocean
a. H2O + CO2  H2CO3 (carbonic acid=weak acid)
b. H2CO3  H+ + HCO3- (bicarbonate=buffer)
c. CaCO3  Ca+2 + CO3-2 (carbonate=base)
C. freshwater (streams, rivers, lakes)
1. pH is between 6.5 and 7.5
2. slightly acidic due to carbonic acid [H2CO3]
III. Temperature
A. depth dependent
B. warmest at the equator
C. temperature remains relatively vertically constant at high latitudes
D. thermocline (300-1000 m): layer of rapid temperature change
E. isotherm: line or surface of constant temperature
F. heat transfer
1. conduction: molecules mechanically bump each other
2. convection: heat is carried to new location by the movement of a fluid
3. radiation: carried by infrared light, electromagnetic waves that do not require
a medium to travel through
4. heat is conducted in water 25 times faster than in air; thus, a person gets
colder swimming in water below body temperature
IV. Salinity: total amount of dissolved ions/salts/electrolytes in water; measured in parts
per thousand [ppt, o/oo], which is equivalent to grams per kilogram [g/kg];
electrolytes are compounds that break into separate ions when they dissolve in water
A. residence time: average amount of time a particular type of molecule stays in a
reservoir, such as the oceans; it is shorter for more reactive substances
1. Mixing time of the ocean is around 1000 years, so if residence time is less
than 1000 years then there will be pockets. If residence time is greater than
1000 years, the substance will be evenly distributed in ocean. Residence time
for water in the oceans is 3500 years.
B. isohaline: line or surface of constant salinity
C. salinity is highest near the tropics (23.5°N/S) because evaporation here is very
high (green line is salinity)
D. constancy of composition [law of constant proportions]: major ion constituents of
seawater are distributed in constant proportions throughout the oceans
1. to find the salinity in an area, the only thing that needs to be measured is
chlorinity (1.81*chlorinity = salinity)
2. to find the dissolved gas concentrations in an area, only argon’s concentration
needs to be measured
3. Alexander Marcet: Swiss chemist who discovered in 1819 that the main
chemical ions are in the same proportions throughout the oceans
4. William Dittmar: firmly establish the principle based on data he obtained
during the HMS Challenger expedition
E. water is the “universal solvent” because it dissolves many compounds
F. salinometers: determine salinity from the electrical conductivity produced by the
dissolved salts; saltier water conducts electricity better (pure water doesn’t
conduct at all)
G. sources of salt
1. hydrothermal vents
2. volcanoes
3. weathering of rocks
H. top six constituents (approximations)
1. Chloride [Cl-]: 55%
2.
3.
4.
5.
6.
Sodium [Na+]: 31%
Sulfate [SO4-2]: 8%
Magnesium [Mg+2]: 4%
Calcium [Ca+2]: 1%
Potassium [K+]: 1%
I. common methods of desalination (making potable freshwater from saltwater)
1. reverse osmosis: pushing water through a porous membrane to extract salts
2. distillation (evaporation/condensation): salts are not vaporized with steam;
expensive and inefficient; one form of this method is solar humidification,
which uses solar energy to heat water
3. freezing/thawing: salts do not stay in ice upon freezing
4. ion exchange [electrodialysis]: ions are pulled by electrodes (anode and
cathode) between solutions through membranes
J. halocline: depth zone where salinity changes rapidly; salinity fluctuates above the
halocline
K. salt is collected for human consumption in pools where the seawater sits and
evaporates, leaving behind salt, which is later refined (similar process to solar
humidification, but the freshwater is not collected)
L. Red Sea is the saltiest (40 ppt) marginal sea in the world; Dead Sea is the saltiest
body of water but is a lake
M. common water salinities
1. ocean: 35 ppt (3.5%)
2. brackish Water (river-ocean interface): 17 ppt (1.7%)
3. brine: greater than 50 ppt
N. salinity is increased by:
1. evaporation
2. freezing
O. salinity is decreased by:
1. precipitation
2. melting of ice
3. river input and runoff
P. saltwater exclusion: salt does not freeze with the ice; instead, it is separated from
the ice and forms brine, liquid water with very high salinity
Q. hypersalinity: high salinity due to high evaporation rates; common in lagoons
V. Density
A. pycnocline (300-1000 m): layer where density changes rapidly; water doesn’t
easily cross this layer; almost non-existent at high latitudes, but prominent
between the tropics
B. pure water has a density of 1 g/cm3 (g/mL) at 4°C; seawater has a density of 1.025
g/mL
C. density affected by:
1. temperature
2. salinity
3. depth/pressure: very minimally; water is mostly incompressible
D. in situ [“in place”] density: the density of water at a certain depth
E. freshwater is most dense at 4°C; ice, formed at 0°C is less dense, so it floats;
thermal expansion occurs during freezing because the crystal lattice structure of
ice has the water molecules more spread out than they are in liquid water;
seawater is most dense at its freezing point (-2°C) unless its salinity is less than
24.7 ppt
F. specific gravity: density of a liquid (g/mL) divided by the density of pure water (1
g/mL); ratio with no units
G. isopycnal: surface or line of constant density
H. Temperature-Salinity Diagram [TS diagram]: a contour map showing lines of
constant density (in sigma units) with respect to temperature and salinity
1. water mass: water with similar temperature and salinity characteristics, such
as the Antarctic bottom water, which is shown above
I. density layering
1. mixed layer: (2% of volume) surface to 300 m; relatively constant temperature
and density
2. pycnocline [thermocline, transition zone]: (18% of volume) rapid change in
density and temperature; 300-1000 m
3. deep layer: (80% of volume) below 1000 m; relatively constant temperature &
density
VI. Hydrostatic Pressure
A. increases with depth; for every 10m (33ft), pressure rises one atmosphere
B. absolute pressure: real pressure at a certain depth, which includes atmospheric
pressure (1atm) and pressure due to ocean
C. gauge pressure: pressure measured by gauges, which doesn’t include atmospheric
pressure (ex: gauge pressure at 50m is 5atm)
D. 1 atm = 101,325 Pa = 101.325 kPa = 14.7 psi = ~1 bar = 760 torr = 760 mm Hg
VII. Dissolved Gases: more gases can be dissolved in cold water
A. Henry’s Law: more gases can be dissolved under high pressures (in deep water)
B. oxygen [O2]: most abundant in surface layer and deep layer; oxygen minimum
layer occurs between 150 and 1500 meters (oxygen concentrations are greater
below this depth because there are fewer consumers)
1. anoxic water: has no oxygen due to excessive decomposition (usually of
algae), so anaerobes live in this type of water
2. hypoxic water: has low amounts of oxygen due to decomposition (usually of
algae), which is bad for fish, so dead zones form, where there is almost no
aerobic life
C. carbon dioxide [CO2]: lowers pH by forming carbonic acid
D. nitrogen [N2]: necessary for nitrogen fixation, which is the reaction of
atmospheric nitrogen turning into ammonia [NH3]
VIII. Calcium Carbonate: is more soluble at greater depths and in acidic environments
A. lysocline (4000 m): depth zone where calcium carbonate solubility increases
rapidly with depth
B. calcite compensation depth [CCD] (4500 m): the depth at which calcium
carbonate is dissolved at the same rate that it is replenished from above; affected
by CO2 concentrations, pressure, temperature, pH, and productivity of calcareousshelled organisms; below this depth, there are no calcium carbonate deposits
C. sources and sinks
1. precipitation from water or dissolving in water
2. limestone and calcite
3. biological sources: shells/tests of foraminiferans, coccolithophores, and coral
skeletons
IX. Nutrients
A. carbon [CO2 carbon dioxide, HCO3- bicarbonate]: most plentiful
B. nitrogen [NO3- nitrate]
C. silicon [SiO4 silicon tetroxide]
D. phosphorus [PO4-3 phosphate]
E. iron [Fe+3]: least plentiful
X. Sound
A. sound travels at ~1500 m/s (3500 mph) in water; this is 5x faster than in air; high
pressure, high density, and warm water all cause the speed to increase
B. sound goes much farther than light in water, so it is used by cetaceans to
communicate
C. SOFAR [sound fixing and ranging] channel (1000 m): depth where sound travels
slowest and gets trapped, so it travels great distances
D. acoustic thermometry of ocean climate [ATOC]: experiment to measure the
temperature of ocean water by measuring sound speeds
XI. Light
A. penetration: light traveling through water
1. light spectrum: roygbv
2. long-wavelength light (red) is absorbed first
3. medium-wavelength light (blue & green) goes the farthest and is attenuated
the least, so deep objects appear blue-green
4. depth-energy ratios: percentage of light energy remaining at a depth in
comparison to how much was at 0 m; changes based on local properties
a. 40% at 1 m
b. 20% at 10 m
c. 1% at 150 m
d. 0% at 1000 m
5. turbidity (opposite of clarity): the amount of substances suspended in the
water; greater turbidity decreases light transmission [clarity]
6. attenuation: decrease in light intensity due to absorption and scattering by
particles; intensity decreases exponentially in seawater
B. photometer: measures transmission of visible light
C. water color: based on the light wavelength that is scattered most
1. blue in tropical/equatorial regions due to low productivity
2. yellow-green in coastal high latitude regions due to high productivity
D. light refracts in water; the index of refraction is 1.33 (n=1.33); this means it bends
at the interface between the air and the water and causes an image coming from
the water to being distorted if viewed from the air
E. electromagnetic radiation: energy waves or photons, depending on how you look
at it; from highest to lowest energy (shortest to longest wavelength; highest to
lowest frequency), the spectrum consists of gamma rays, x-rays, ultraviolet light,
visible light [photosynthetically active radiation, PAR], infrared light,
microwaves, radio waves
F. diel variation: difference in incident radiation between day and night
G. diurnal variation: changes in incident radiation during the day (ex: clouds)
XII. Pollution
A. plastic waste: strangles animals or looks like prey, which is choked on by
predators like sea turtles that eat plastic bags
1. nurdles: little pieces of plastic
2. photodegradation: the breakdown of plastics into many small pieces by
sunlight
B. oil spills: 47% of oil in the sea comes from natural oil seeps; 72% of unnatural oil
in the oceans is the result of petroleum consumption rather than transport or
extraction
1. Exxon Valdez crashed into rocks in Prince William Sound, Alaska
2. Santa Barbara, California: offshore blowout
3. Persian Gulf War (1991): intentional release of oil that was the largest
manmade release of oil into the sea
4. Ixtoc #1, Gulf of Mexico: oil blowout that is the largest accidental oil spill
C. radioactive waste: Irish Sea is the most radioactive area in the world
D. PCBs and DDT: industrial chemicals in water that are persistent and harmful
E. Minamata Bay, Japan: place where a chemical manufacturing plant discharged
lots of mercury in the 1940s and 50s; “Minamata disease” is a name given to
mercury poisoning
F. types of pollution
1. non-point-source pollution: doesn't come from pipes, so the source is a wide
area or is hard to identify
2. point-source pollution: comes from an easily identifiable source such as a pipe
3. flotsam/jetsam: floating waste or debris in the ocean; jetsam is intentionally
jettisoned or thrown into the ocean
XIII. Other
A. hydrological [water] cycle: has affected the global climate because water in the
atmosphere has the biggest impact on the greenhouse effect
B. conservative and non-conservative properties
1. conservative: substance concentration is mostly affected by mixing and
diffusion (ex: salinity), so the substance exists in constant proportions
throughout the sea (law of constant proportions)
2. non-conservative: affected mostly by process(es) other than mixing and
diffusion (ex: dissolved oxygen changes based on biological productivity)
C. isopleth: a level surface or contour that shows a constant value of some function
dependent on location; could be an isohaline, isotherm, isobar, etc.
D. isobath: line or surface of constant depth