I. SOURCE OF THE OCEANS: OUTGASSING OF THE EARTH

(Outgassing: the process by which water vapor and other gasses are released from the rocks that held them and then vented them to the surface)

Energy for outgassing supplied by:

• gravitational collapse during differentiation of the Earth
• volcanic outgassing
• meteorite and asteroid bombardment

Was there enough water in the Earth to form the oceans?

If so, much of the water must have come from the mantle.

Mass of mantle = 4.5 x 10²⁷ grams

Mass of oceans = 1.4 x 10²⁴ grams

Therefore, mantle must have lost 0.031% of its water

Is this reasonable? How much water in mantle?

We estimate the water composition of the mantle from that of some meteorites because the mantle has not been sampled directly but is thought to be similar. Based on a meteorite water composition of 0.5%, the mantle contains16 times water needed to form the oceans.

When did the oceans form? -- Early! Oceans by 3.8 B.Y.

• rapid formation because of greater volcanic activity and impact events in early earth history!

Did comets contribute to the formation of the oceans?

Outgassing could also cause addition of:

N, CO₂, He, Ar, and H₂O vapor to atmosphere

II. ATMOSPHERIC EVOLUTION

• Age of earth 4.6 B.Y.
• No rocks > 4.1 B.Y. therefore limits conjecture about early atmosphere and hydrosphere. Recently zircons were found dating to 4.4 B.Y. old, represently the erosional products of rocks. It would appear that some crustal rocks had cooled by this time.
  
• Oldest rocks do indicate presence of atmosphere and hydrosphere.
  

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1st Atmosphere Probably similar to Jupiter which has retained greater share of original gas

• CH₄ methane
• NH₃ ammonia
• H hydrogen
• He helium
• Ne neon

• both assume considerable loss of free H and He because of small atomic mass, what remained oxidized to form sea water
• greatest problem is explaining abundance of free oxygen on earth today!
  

2nd Atmosphere Formed by loss of H and He and chemical modification of original atmosphere.

1. Dissociation of primeval water vapor into hydrogen and oxygen with most hydrogen escaping into space:

2H₂O + uv light energy react to form 2H₂^- + O₂

2. Newly freed oxygen reacted with methane to form carbon dioxide and more water vapor:

CH₄ + 2O₂ react to form  CO₂ + 2H₂O

3. Oxygen would also react with ammonia to form nitrogen and water:

4NH₃ + 3O₂ react to form 2N₂ + 6H₂O

4. After all CH₄ and NH₃ were converted (by oxidation) to CO₂ and N₂ , then free O₂ could accumulate as further dissociation of water vapor occurred.

- Earliest life probably anaerobic (living in oxygen free environment) and heterotropic (requiring external food source)

- Later oxygen-releasing photosynthesizers arose

-probably did not have advanced enzymes to rid self of oxygen

-use ferrous iron as an oxygen acceptor

therefore free oxygen first locked-up in formation of marine iron oxides

-origin of Banded Iron Formations! These are marine deposits of alternating rust red and gray bands. The red is due to ferric iron oxide (Fe₂O₃).

• 3 TO 1.8-2.0 B.Y. ago
  

- Evolution of oxygen and peroxide-mediating enzymes

- Spread of life in oceans

- O₂ accumulates in the atmosphere

- Increased ultraviolet protection by ozone formation

(molecular oxygen O₂ + atomic oxygen O = ozone O₃)

oxidation of iron rich sediments exposed to atmosphere (on land) to formed. .RED BEDS beginning 1.8 to 2.0 B.Y. ago. The great RUST Event!