The Great Oxygenation Event Flashcards
Name a common sedimentary rock type in the second half of the Earth’s Hisotry and why its colour is significant
Typical ‘red-bed’ sandstone
The red-colour shows evidence of oxygen in the atmosphere
Why is ‘red-bed’ sandstone red?
When Fe(II) is in rocks, it is usually a greyey-greeny colour (ferrous)
When Fe(III) is in rocks, it is usually reddish (ferric)
Hence the ‘red-bed’ sandstone is the colour it is due to the presence of Fe(III)
The formation of ‘red-bed’ sandstone can increase
In a wamer climate
Originally oxygen was a trace element in the atmosphere until the great oxygenation. This is when ‘red-bed’ sandstone would first start to form
When was this?
2.2-2.3 million years ago
What other source can scintist use to test for the presence of oxygen is older atmosphere
The presence of oxygen in soil can provide evdience for its concentrations within the atmosphere
How did the increase in oxygen within the atmosphere affect the structure of sedimentary rock
On the surface you get ‘red-beds’ of sandstone where the iron has become oxidised
With sandstone containing Fe(II) underneath
How did the concentration of Iron, Uranium and Manganese in Oceans change before and after the Great Oxygenation event
Before (>2.4Bya): Iron was present in rocks as Fe(II) which is soluable and hence could leach into seas. However Uranium and Managnese where immobile
After (<2.2Bya): Iron becomes oxidised to Fe(III) which is now insoluable and cannot move from the rocks. Uranium and Maganese becomes oxidised and are now soluable, hence entering the Oceans
What formation starts to disappear in the Sea after the Great Oxygenation event
Around 1.8 Bya, banded iron formations disappear
Suspected because the sea has become oxygenation
How did Banded Iron Formations (BIFs) occur before the Great Oxygenation event
Volcanoes pumped out Iron, which the Microbes on the sea floor can provide oxygen from respiration to oxidise Iron (II) into Iron (III). Because Iron (III) is insoluable is would line the floor of the ocean
Oxygen is toxic within high concentrations. Mammals have adapted to an atmosphere which has around…
20% of oxygen
Microbe invented oxygenic photosynthesis sometime before the Great Oxygenation Event.
How would this affect oxygen levels in the atmosphere
Oxygen is a by-product of oxygenic photosynthesis
However equal amount of oxygen are removed from the atmosphere by respiration and decay of photosynthesisers themsleves
Hence there is no net gain of oxygen and can not explain the increase the increase in oxygen from the Great Oxygenation event
How could reduced carbon explain the changes in oxygen levels
When reduced carbon is burried is leaves some oxygen behind as it is not reoxidied (e.g. when organisms are fossilised)
However, when considering the long-term carbon cycle, buried carbon will return to the atmosphere from volcano magma when an ocean slab is subducted, and is oxidised again, leaving no net gain of oxygen
How could methane explain the increase in oxygen within the atmosphere
- One of the most common gases (2.2Bya) was methane, which is a powerful reducer, meaning it can combine with oxygen when exposed to UV
- Methanic bacteria produce methane at the sea bed, which will bubble up through the water column into the atmosphere
- Oxygen can react with methane, forming carbon dioxide and water
- However, the formation of an ozone layer, slowed the reaction between oxygen and metahne and allowed oxygen to build up
Why could the formation of Ozone, not help in the increase in oxygen levels within the atmosphere
- If oxygenic photosynthesis increases, and oxygen with it, methane production would have increase to, because the two are in a tight biological cycle
- The balance of oxygen and methane depends on which reacts more readily with water, rocks and sediments - which this would be oxygen
- The result of an increase in oxidative phosphorylation, would lead to even more methane and hence a more reducing atmosphere
Despite this: How is it though oxygen levels rose
It is though to have happened through ‘flipping the system’
(1) an ozone layer forms
(2) oxygen sinks fill up (oceans, sediments and rocks)
(3) Methane and oxygen swapped places as dominant and trace gases in the atmosphere
