Ocean Chemistry and Composition

Brian Schuster

Chemical Properties of Sea Water• polar: unequal sharing of electrons• hydrogen bonding: intermolecular dipole-dipole

interaction between H2O molecules• cohesion: sticking of H2O molecules to each other

resulting from hydrogen bonding; causes surface tension• adhesion: sticking of H2O molecules to other polar

materials• capillarity: movement of water up a small tube• latent heat: energy stored in water that doesn’t change

it’s temperature

Acidity/Alkalinity

• ocean: pH 7.5 to 8.5 due to carbonate (CO3-2)

from dissociation of calcium carbonate• freshwater: pH 6.5 to 7.5 due to carbonic acid

(H2CO3)

• bicarbonate buffering, resistance to pH change– H2O + CO2 H2CO3 (carbonic acid=weak acid)

– H2CO3 H+ + HCO3- (bicarbonate=buffer)

– CaCO3 Ca+2 + CO3-2 (carbonate=base)

Temperature

• depth dependent• thermocline: rapid

change in temperature around a certain depth; more defined near equator; less prominent at poles

• isotherm: line of constant temperature

Global isotherms display currents

Salinity

• isohaline: line of constant salinity• residence time: average time a molecule spends in

a certain reservoir• salinometers: determine salinity through

conductivity• halocline: rapid change in salinity• constancy of composition: major ion constituents

exist in constant proportions• water is the “universal solvent”• ocean salinity: 35ppt, brackish: 17ppt, brine: >50ppt• saltiest sea: Dead Sea

Salinity (continued)• Long-term sources:

– hydrothermal vents & volcanoes– weathering of rocks

• cation: positively charged ion• anion: negatively charged ion• salinity effected by:

– precipitation, evaporation– freezing, thawing– river input

• Top six constituents– chlorine (Cl-) 55%– sodium (Na+) 31%– sulfate (SO4

-2) 8%– magnesium (Mg+2) 4%– calcium (Ca+2) 1%– potassium (K+) 1%

Desalination

1. reverse osmosis

2. freezing & thawing

3. ion exchange

4. distillation (evaporation & condensation)

Density

• pycnocline: rapid change in density• freshwater most dense at 4 °C, saltwater most

dense just before freezing• saltwater freezes at -2 °C (freezing point

depression)• density affected by:

– temperature– salinity– depth, pressure: minimally

• isopycnal: surface of constant density

Pressure and Gases

• rises by 1 atm (14.7 psi, 101.3 kPa) for every 10 m (33 ft) depth

• absolute pressure at 20 m is 3 atm

• gauge pressure at 20 m is 2 atm

• isobar: line of constant pressure

Dissolved Gases

• Henry’s Law: more gas can be dissolved under high pressure

• more gas is dissolved in deep, cold water

• oxygen minimum zone: depth depends on productivity and aerobic respiration

• atmospheric gases resemble ocean’s due to mixing

Calcium Carbonate (CaCO3)• used in shells, compound of limestone and

calcite, base of coral reef• sources and sinks

– chemical precipitation, dissolution– weathering of limestone and calcite– organisms

• solubility increased by:– greater depths– higher acidity (lower pH)

• carbonate compensation depth (CCD): dissolution rate of CaCO3 equals supply rate

Nutrients• eutrophication: excessive

addition of nutrients

• oligotrophication: excessive removal of nutrients

• iron is the limiting nutrient for most algae

South Atlantic phytoplankton bloom

Heat Transfer

• conduction: molecules speed each other up by physically bumping

• convection: carried by movement of fluid

• radiation: electromagnetic radiation is absorbed and raises temperature

• heat is conducted faster in water than in air, so divers can get cold faster

Sound

• in water, speed of sound = ~1500 m/s (3500 mph), 5x speed in air

• speed changes with density• sound travels farther in water than light, so

it is good for cetacean communication• SOFAR (sound fixing and ranging)

channel: sound travels slower around 1000 m, and can go farther; sound gets stuck in this channel

Light• penetration

– long wavelengths (red) absorbed first– mid-range wavelengths (green, blue) go farthest

• attenuation: decrease in light intensity due to absorption and scattering by suspended particles

• turbidity increases attenuation• index of refraction (n) = 1.33

Other• conservative property: mostly affected by mixing and

diffusion (ex: salinity)• non-conservative property: affected mostly by

processes other than mixing and diffusion (ex: dissolved oxygen relating to productivity)

• temperature-salinity diagram: unique to different bodies of water; shows lines of constant density in sigma units