Changes in the Geosphere, Atmosphere and Hydrosphere

Question 1

How did the Great Oxidation Event occur?

Answer 1

Photosynthesis (at the border between the Archaean and Proterozoic) triggered the Great Oxidation Event. The oceans became fully saturated and the remaining free oxygen was released into the atmosphere

Question 2

How are anoxic conditions caused?

Answer 2

With the presence of oxygen (anoxic ferruginous) in a phase of oxidation, iron ions bonded with the oxygen to create iron oxide (rust). This causes the anoxic/ferruginous stage.

Question 3

What were the short-term effects of the Great Oxidation Event on the biosphere?

Answer 3

Initial
* An abundance of anaerobic organisms and the evolution of aerobic organisms
* At first, there were only cyanobacteria that can tolerate oxygen, then cyanobacteria than can use the oxygen which became widespread due to biological advantages (more efficient/aerobic respiration)
Later
* Aerobic organisms continue to diversify and increase complexity
* Formation of the ozone layer protects DNA from deadly UV radiation and allows for organisms to live in surface waters and eventually land
Note: CO2 and O2 are in equilibriums, as O2 enters the atmosphere it is broker down to individual oxygen atoms which reacts with O2 to form the ozone layer (O3) and reduces the amount of UV entering Earth’s atmosphere

Question 4

What were the long-term effects of the Great Oxidation Event on the biosphere?

Answer 4

• Increasing oxygen from photosynthesis and increased nutrients from erosion run-offs from land forms the protein collagen. Collagen holds eukaryotic cells together and leads to multicellular organisms with complex bodies.
• Therefore, free oxygen is a prerequisite of multicellular life (needed for collagen formation)

Question 5

What were the effects of the Great Oxidation Event on the hydrosphere?

Answer 5

Initial
* Some metal ions (E.g. Fe 3+) precipitate in the presence of O2
* pH increases and becomes more basic as
* CO2 is removed
* Sea levels are reduced as global temperatures produce
Later
* Ongoing global glaciation reduces sea levels
* Erosion from geosphere causes various changes to ocean sediments and chemistry

Question 6

What were the effects of the Great Oxidation Event on the atmosphere?

Answer 6

Initial
* Ocean outgassing of O2 once point of saturation is reached
* Carbon dioxide decreases while oxygen increases
Later
* UV breaks O2 apart to form ozone (O3) and the ozone layer
* Oxygen and carbon dioxide reach modern-day levels

Question 7

What were the effects of the Great Oxidation Event on the geosphere?

Answer 7

Initial
* Precipitated iron oxide settles to the ocean floor contributing to the banded iron formations (BIFs) until Fe 3+ only in trace amounts
Later
* Global glaciation causes high levels of weathering and erosion

Question 8

What are banded iron formations and why are they important

Answer 8

• Consists of alternating iron-rich and iron-poor layers
• The iron came from continental weathering or hydrothermal events, and the oxygen came from photosynthesising cyanobacteria

Question 9

Why don’t we get banded iron formations anymore?

Answer 9

There is no dissolved iron left in the ocean, it was all precipitated away by oxygen

Question 10

How does the biological banded iron formation model explain their banded nature?

Answer 10

• Oxygen is very reactive when first formed
• Extinctions followed by recoveries, organisms which couldn’t tolerate oxygen
• Until cyanobacteria could tolerate oxygen, later ancestors learned how to utilise oxygen

Question 11

What are issues with the biological banded iron formation model?

Answer 11

• Older BIFS have been subject to large-scale metamorphism, and do not line up with the evidence for the timeline of the evolution of aerobic photosynthesis, except for later formations.
• Also iron oxide has been found to form without free oxygen

In exams use the details of the biological model and sidenote geomorphical model

Question 12

What is the geological banded iron formation model?

Answer 12

The original mineral for BIFs were probably greenalite from volcanic and other sources. Therefore the banded properties are correlated with periods of more and less volcanism

In exams use the details of the biological model and sidenote geomorphical model