Snowball Earth Flashcards
What is the name of the rock with poorly sorted rocks with random assortment and size
Diamictite
How is Diamictite formed
- Glaciers move very slowly
- Where the bottom of the glacier touches the rock surface, there is large pressure which erodes the rock underneath it, which pebbles/sediment then get incorporated into the ice
- Glaciers have a life span and will eventually melt and the sediment inside will just fall out
- This forms the ditinctive textures seen within the Diamictite rocks
What is a ice-stretched surface on a rock
Groves in the rock which are caused by studs in the glacier base which scratch the rock underneath it
How is a ‘dropstone’ structure formed as shown in the picture
As glaciers have moves into lakes and warmed up, ice starts to melt and the studs in the bottom of the glaciers drop out
In the photo, the lump of granite has dropped out into the clay forming this structure which has formed part of lake sediment
All these rock formations indicate what
They can be dates to show a previous ice ages, the ones previously mentioed are from glaciations in the Cryogen period
The rock formation show previous glacial sites
Why is the evidence of glaciations from dropstone structures contested as evidence of glaciations
Did dropstones drop from ice or something else? Volcanoes? - However granite is a deep sea rock and hence couldn’t have come from volcanoes
What is particuarly curious about the location of where these dropstone structures are formed
Many of them were found in Australia, where at the time the equator biceped West Australia
Hence suggesting that some glacial ice reached sea level at the equator, and there is a possibility that the earth was so cold it could have been a ‘snowball earth’
How does the idea of ‘snowball earth’ have a positive feedback loop
Ice has a high albedo - hence an ice covered earth would reflect a high amount of UV and there would be a lower warming effect
How is it suggested that ‘snowball Earth’ occured
- Carbon dioxide stripped out of the atmosphere by some mechanism
- Tipping point as ice reached 30° N and S
- Albedo positive feedback runs away
- Snowball for millions or tens of millions of years
How is it suggested the ‘snowball Earth’ came to its end
- Silicate weathering slows down as Temperature fall AND little rock exposed to weathering because of ice cover
- CO₂ from volcanic sources builds up to many time ‘normal’
- Albedo stranglehold breaks
- Rapid melting (thousands of years?)
- Super-greenhouse and intense weathering?
Why is the theory of how ‘Snowball Earth’ ended contested
- Glacial deposits are overlain by widespread and thick ‘cap carbonates’
- Suggested to come from CO₂ which was quickly stripped from the air and dumped in the sea as carbonates through silicate weathering
- Meaning the atmosphere would have not been overloaded with CO₂ for long
What is another piece of evidence which contests the occurence of snowball Earth
- BIFs - Banded Iron formations
- BIFs come back after a massive 1Bn year gap
- If the Sea is capped by ice, the oxygen in the seawater would decline BIFs would not occur
How is a decrease in δ13 Carbon in rocks evidence for the two glaciation events
- δ13 C is carbonates which precipitate from seawater
- a decrease in rock 13C suggested a relative increase in 12C, suggesting a relative increase in seawater 12C
- 12Carbon is what Rubisco favour which was as a result from dead plants being swept into seas
The Sturtian and Marinoan Glaciations, with a 10Bya interval between them, were the most severe glaciation events seen
What is suggested to have caused these two periods
- Tectonics: Movement of continents and tectonic configuration
- Lead to a supercontient clustered around the tropics , where warm and wet weather could enhance silicate-rock weathering
- Extreme weathering could outstrip rates of CO₂ return by normal mechanisms
- Cooling effect
What is some evidence against the formation of the supercontinent at the tropics for the reason of the intense glaciation
- Plate positions and timings not clear. Palaeomagnetic data not as straightforward as one would wish
- AND supercontinents have straddled the Tropics at other times without similar consequences
- The break-up of the Rodinia supercontinent (750Ma) acturally led to ice sheets extending closer to the equator from the N and S
What is the suspected more plausible theory about why the Sturtian and Marinoan Glaciations, with a 10Bya interval between them, were the most severe glaciation events seen
- Life
- ‘Protolichen’ began eating away at continetal rock
- This drew down CO₂, but even more important enhanced silicate weathering
- Moblised nutrients like phosphorus swept into the sea causing algal boom and hence more drawdown of carbon
- In turn, cooling the Earth
