C-The Rise of Oxygen Flashcards
What are the 3 other names for the Great Oxidation Event (GOE)?
Oxygen Catastrophe
Oxygen Crisis
Oxygen Holocaust
What was the GOE related to?
Evolution in an anaerobic world
When did the GOE occur?
2.4-2.1 Ga (2400-2100mya)
What happened in the GOE?
Biologically induced appearance of O2 in Earth’s atmosphere (know when it happened but controversial as to why)
What are proxies?
Indirect evidence
How did we arrive at the GOE Model? (2)
Proxies preserved physical characteristics that track changes in variables of interest (eg. oxygen levels)
Many proxies available to examine ancient-ocean atmosphere system
What 4 ways illustrate the GOE?
1) Distribution through time of detrital pyrite
2) First occurrence of terrestrial red beds
3) Fossil record of cyanobacteria and stromatolites
4) Banded iron formations
What is the importance of the presence of detrital pyrite?
Pyrite altered in presence of oxygen
Generates Ferrous Iron (Fe2+) (Iron Sulphide)
Ferrous Iron is soluble in water and reacts in presence of water and oxygen to form iron hydroxides (rust) insoluble residues (Fe3+)
Colour turns to reds, yellows and oranges
Chemically stable
What is the chemical formula for oxidation (ferrous iron to ferric iron (rust))?
4FeO + O2 = 2Fe2O3 (Fe2+ to Fe3+)
How can we track atmospheric oxygen?
Based on 1st appearance of continental/terrestrial red beds
When did red beds first appear?
Approx 2.3 Ga
What was the only significant source of free oxygen on Earth’s Surface?
Cyanobacteria (oxygenic photosynthesis)
What is the geological evidence of cyanobacteria (and therefore oxygen?)?
Fossilised?
They build distinctive biosedimentary structures (stromatolites)
Laminated sediments = good preservation potential
What is the problem with using cyanobacteria as a source of info for GOE?
Cyanobacteria not the only microbes that form stromatolites
Some stromatolites may be abiogenic (purely physical processes) which means no oxygenic photosynthesis
What is the estimated time frame for the origin of oxygenic photosynthesis?
3.8 to 2.35 Ga
What if you assume older ages for oxygenic photosynthesis are correct? Why is oxygenation delayed until 2.4/2.1 Ga?
Banded Iron Formations
What are Banded Iron Formations?
Alternating laminae of chert (type of quartz: silicon dioxide) and iron oxides
How are Banded Iron Formations formed?
Episodic reaction between ferrous iron (Fe2+ soluble in water)(sourced from hydrothermal systems and released to water) and O2 forms iron oxides that precipitate out onto sea floor. Chert forms at other times.
Why are Banded Iron Formations the answer to the delay in oxygenation?
Can only release O2 into atmosphere when Ferric iron Fe2+ reservoir is used up (or production of O2 exceeds rate it’s being used up at)
In what conditions do red beds form?
When oxygen is available
What should happen to Banded Iron Formations when more O2 is available?
They should stop and red beds should begin to form
Where can red beds be found?
Huronian Group, Canada
What are red beds?
Strata of reddish sedimentary rocks that were deposited in hot climates under oxidising conditions
What is the key point about lithology and biology?
Lithology/ lithosphere is linked to biology/biosphere and it creates a non-uniformitarian geological record
