Molybdenum Flashcards
When did Molybdenum first occur in records and how has its occurence differed across time
- First occured 2.7Bya
- Around 0.5Bya there was a large jump in Molybdenum concentration (this was right after snowball earth and before the Cambrian explosion)
Where is Molybedenum predominantly found
In black shales
This is fine-grained rock deposits in low-energy environments with little/no input of coarse sediment
Far-ish from land, lagoons, continental margins and bases of continental slopes
Why are black shales an indicator of low atmospheric oxygen
- They are usually an indicator of low oxygen because otherwise organic matter would have oxidised back to CO₂ and water
Describe the chemistry of Molybdenum
Atomic number: 42
A silver-grey transition metal, which doesn’t exist freely
Very high melting points
Most Mo compounds have a low solubility in water (bar MoO₄²⁻)
Why is Molybdenum important to life
Mo is an essential trace element for life in higher eukaryotes
Living things can take or leave Mo (though needed in tiny amounts)
What is Moof
In the presence of oxygen, Molybdenum forms MoO₄²⁻ - Moof
Moof is soluble in water
Hence the the large jump in black shale right before the Cambrian explosion suggests seawater became oxygenated
What is the issue with the idea of Molybdenum showing an increase in atmospheric oxygen
It has been long thought that BIFs stopped forming because the ocean became fully oxygenated
However there is a large gap in time between the last known BIF formation and the Mo jump
BUT the lost BIFs hence may not be due to oxygenation
How could BIFs formations stop without the presence of oxygen
Oxygen isn’t the only element which can oxidise Fe²⁺ into Fe³⁺ - sulphur can too
Hydrogen sulphide (H₂S) can form insoluble Fe³⁺S₂ known as pyrite/fools gold
Why would the oceans have become sulphide-rich?
- Oxygen can convert insoluble sulphide in continetal rock to soluble sulphate
- Weathering mobilised sulphate and moved it into seas
- Bacteria reduced sulphates into sulphides due to anerobic waters
- Causing the deep ocean to become sulphidic
How do oceans between 1.85 and 1.25Bya have similarities to the Black Sea today
- In the sediments/water column of the Black Sea, bacteria first use O₂ in the breakdown of phytoplankton and zooplankton
- After the oxygen is depleted, other natural oxidants like nitrates, manganese dioxide, iron oxides, sulphates are used
- Once oxygen is removed dissolved iron can be liberated through photosynthetic microbial reduction which become dissolved in water
How could biology explain the large increase in oxygen
- A small sustained increase in Carbon burial though preferential oxidation would raise O₂ levels
- Hence there would be a build up of O₂ at the Earth surface
How could the Colonisation of land by protolichens lead to a rapid increase in atmopsheric oxygen levels
- Using organic acids from Fungi in a symbiotic relationship, this broke down contiental rock
- Thic caused enhanced silicate weather which drew down more CO₂ AND mobilised nutrients like phosphorus which washed into the sea
- This increased the amount of phosphorites which increase photosynthesis, highening atmospheric oxygen levels
