The problem with this 'icon' is that despite the fact that it is now generally agreed that oxygen was present in the early atmosphere
Im sorry but you'll have to provide evidence for early Earth having oxygen.
However, heres evidence for the complete opposite of that from Columbia U.,
Page not found | Department of Earth and Environmental Sciences
Here's another,
The atmosphere - origin and structure
Here's a quote from it,
quote:
First Atmosphere:
Composition - Probably H2, He
These gases are relatively rare on Earth compared to other places in the universe and were probably lost to space early in Earth's history because
Earth's gravity is not strong enough to hold lighter gases
Earth still did not have a differentiated core (solid inner/liquid outer core) which creates Earth's magnetic field (magnetosphere = Van Allen Belt) which deflects solar winds.
Once the core differentiated the heavier gases could be retained
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Second Atmosphere:
Produced by volcanic out gassing.
Gases produced were probably similar to those created by modern volcanoes (H2O, CO2, SO2, CO, S2, Cl2, N2, H2) and NH3 (ammonia) and CH4 (methane)
No free O2 at this time (not found in volcanic gases).
Ocean Formation - As the Earth cooled, H2O produced by out gassing could exist as liquid in the Early Archean, allowing oceans to form.
Evidence - pillow basalts, deep marine seds in greenstone belts.
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:
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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
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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.
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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.
*Can you provide evidence against any of this to prove that oxygen was present?
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