How did chemisty and oceans produce this?

There is still a problem:

• Not much is available for anerobic processes to use.
• But hey, there is all this that we are floating in so let's do this

  

So, chlorophyll evolves to more complex forms so that it can capture and store enough UV energy to breaking the hydrogen-oxygen bond in water.

Now we have a mechanism to deliver Oxygen to the atmosphere via the aerobic photosynthesis of blue-green algae. But, the oxygen does not initially go into the earth's atmosphere:

Addition of O₂ to the Atmosphere:

Today, the atmosphere is ~21% free oxygen. How did oxygen reach these levels in the atmosphere? Revisit the oxygen cycle:

• Oxygen Production
 
• Photochemical dissociation - breakup of water molecules by ultraviolet
• Produced O₂ levels approx. 1-2% current levels
• At these levels O₃ (Ozone) can form to shield Earth surface from UV
• Photosynthesis - CO₂ + H₂O + sunlight = organic compounds + O₂ - produced by cyanobacteria, and eventually higher plants - supplied the rest of O₂ to atmosphere. Thus plant populations
• Oxygen Consumers
 
• Chemical Weathering - through oxidation of surface materials (early consumer)
• Animal Respiration (much later)
• Burning of Fossil Fuels (much, much later)
Throughout the Archean there was little to no free oxygen in the atmosphere (<1% of presence levels). What little was produced by cyanobacteria, was probably consumed by the weathering process. Once rocks at the surface were sufficiently oxidized, more oxygen could remain free in the atmosphere.

 During the Proterozoic the amount of free O₂ in the atmosphere rose from 1 - 10 %. Most of this was released by cyanobacteria, which increase in abundance in the fossil record 2.3 Ga. Present levels of O₂ were probably not achieved until ~400 Ma.

Evidence from the Rock Record

• Iron (Fe) i s extremely reactive with oxygen. If we look at the oxidation state of Fe in the rock record, we can infer a great deal about atmospheric evolution.
• Archean - Find occurrence of minerals that only form in non-oxidizing environments in Archean sediments: Pyrite (Fools gold; FeS₂), Uraninite (UO₂). These minerals are easily dissolved out of rocks under present atmospheric conditions.
• Banded Iron Formation (BIF) - Deep water deposits in which layers of iron-rich minerals alternate with iron-poor layers, primarily chert. Iron minerals include iron oxide, iron carbonate, iron silicate, iron sulfide. BIF's are a major source of iron ore, b/c they contain magnetite (Fe₃O₄) which has a higher iron-to-oxygen ratio than hematite. These are common in rocks 2.0 - 2.8 B.y. old, but do not form today.
• Red beds (continental siliciclastic deposits) are never found in rocks older than 2.3 B. y., but are common during Phanerozoic time. Red beds are red because of the highly oxidized mineral hematite (Fe₂O₃), that probably forms secondarily by oxidation of other Fe minerals that have accumulated in the sediment.
Conclusion - amount of O₂ in the atmosphere has increased with time, but does not reach its current atmospheric level until 400-600 Million years ago.

Biological Evidence for lack of early oxygen

• Chemical building blocks of life could not have formed in the presence of atmospheric oxygen. Chemical reactions that yield amino acids are inhibited by presence of very small amounts of oxygen.
• Oxygen prevents growth of the most primitive living bacteria such as photosynthetic bacteria, methane-producing bacteria and bacteria that derive energy from fermentation. Conclustion - Since today's most primitive life forms are anaerobic, the first forms of cellular life probably had similar metabolisms.
• Today these anaerobic life forms are restricted to anoxic (low oxygen) habitats such as swamps, ponds, and lagoons.





So we have to do all of this before we have oxygen in the atmosphere as well as an ozone layer:



So now we can crawl out on the Land!