lecture 7

a solvent (e.g. water- usually a liquid) a solute (e.g. salt- usually a solid or gas)

Precipitation occurs when the constituent ions get back together and form the salt. A solid falls out of the solution.

Precipitation occurs when the concentration of ions exceeds the capacity of the polar water molecules to keep the ions apart. This is called saturation.

Salinity is the total quantity of dissolved inorganic solids in water (we don‟t consider biological cell matter for calculating salinity). (usually measured in parts per hundred (per cent), parts per thousand (‰), or parts per million (ppm).

A Few Ions Account for Most of the Ocean’s Salinity

i) Erosion of continents via rain, glaciers, rivers ii) From Earth‟s interior (mantle)

Water‟s great ability to dissolve solids allows it to carry ions from the continents in runoff to the oceans.

Look at samples of river OR look at salts precipitated in large inland lakes like the Great Salt Lake or the Dead Sea:

1) Negligible chloride (Cl-), sulfate (SO42-)in rivers but oceans have lots!

2) Oceans are deficient in calcium, silicon, magnesium

Biological processes remove calcium and silica from seawater. They are nonconservative constituents.

Volcanos produce gases that contain chlorine and sulfur, and these get carried by rain into the ocean. But not enough!

Undersea volcanic activity at ocean ridges adds chlorine, sulfur, carbon dioxide, hydrogen, fluorine, and strips out magnesium.

Mg2+ stripped out of seawater by fresh ocean crust formed at spreading centers:

the current chemical composition of the ocean is a combination of processes that link the atmosphere, lithosphere, and hydrosphere:

The ocean is in chemical equilibrium.

This does NOT mean static (non-reactive), but rather steady state.

. Measure all ion concentrations. Sum. OR

2. Use Principle of Constant Proportions and measure only a few ion ( mostly chloride*, Cl-).

Ions occur in seawater in constant ratios, or proportions. Total concentation may vary but ratio does not.

This is because the ocean is in chemical equilibrium.

Chloride ion is, on average, 55.04% of total dissolved inorganic solids. Measured Chlorinity also includes Br-, I-, F- [All these comprise 55.354% of salinity]

Salinity = 1.80655 x chlorinity

At some point in the past, In > Out!

So, a chemical ion comes into the ocean from weathering or mantle outgassing, stays for a time in the seawater, and then leaves via sedimentation and Biological processes.

Residence time =

Amount of an element/ion in ocean/

Rate at which this element/ion is added or removed

Conservative elements - do not change much over time where as Nonconservative elements - change

over short time (usually tied to biological or seasonal processes)

Ocean mixing time is around 1,600 years

1,370,000,000 cubic kilometers (km3)

334,000 km3/year

Residence time =

1,370,000,000 km3/ 334,000 km3/year

~4100 yr

recycled almost 1,000,000 times!

nitrogen (N2)

carbon dioxide (O2)

oxygen (O2)

Nitrogen, oxygen,and carbon dioxide are the principal gases in the atmosphere, but proportions in water and air differ:

absolute amounts of N2 and O2 are much higher in atmosphere (by about a factor of 100) than the oceans

Plants make oxygen and sugars from water and carbon dioxide:

respire, breaking down sugars with oxygen and generating energy, water, and carbon dioxide.

made by photosynthesizing plants, used by animals, plants to respire

made by respiration, used by plants

dissolved from atmosphere

Solubility of gases in seawater are higher at lower temperature, Opposite effect seen for salts

“Dissolved carbon dioxide in the oceans and solid carbonate minerals in sediments play a KEY role in the carbon cycle and stablizing climate. There is far more carbon stored in sediments and dissolved in the ocean than is present in all life on Earth.”

CO2 is a greenhouse gas: abundance in the atmosphere has risen by 33% in 300 years.

BUT....60 times more CO2 in oceans than in atmosphere.

40,000 times as much carbon stored in sediments (as CaCO3 ) than is present in ALL life on Earth.

ocean: FAST

atmosphere: SLOW

Oceans tend to act as a SINK for atmospheric carbon dioxide

Carbonic acid acts as a buffer to maintain pH in the ocean.

1. deep water is cold, (also high pressure), so can dissolve more CO .

2.the ocean acts as a buffer and

adjusts its chemistry to accommodate more or less CO2.

3. the oceans hold much more CO2 than does the atmosphere.

Calcareous ooze is NOT found everywhere because deep seawater contains more carbon dioxide (CO2) which makes the water slightly acidic. Below the CCD, calcium carbonate dissolves.

Precipitation/dissolution of carbonate sediments: deep water is slightly acidic (higher CO2 and thus carbonic acid), so carbonate sediments from plankton shells slowly dissolve, adding carbonate ion to deep water. The result is an ocean with a remarkably stable pH: