L7 - Water and Temperature

How does zircon show evidence for liquid water?

• Zircon is typically formed when granitic magma is solidified

• This is formed when basalt or mantle material is melted in the presence of water. This proves water must have been present at this time.

• Zircon = 4.4 billion years old

How does stable isotope fractionation show past climates?

• As water evaporates, the light ¹⁶O molecules evaporate more easily than the heavier ¹⁸O molecules.

• Over time, the water that is evaporating will be enriched in ¹⁶O. The water left behind will be enriched in ¹⁸O.

• There is lots of fractionation at low temperatures; less at higher temperatures because molecules in water get more energy rich so it is easier for them to escape into the water vapour phase.

Why does water not escape the atmosphere like other volatiles?

• As the water moves up through the atmosphere, you would expect UV to break that into H and O2.

• They are relatively volatile and would be lost from the atmosphere.

• This does not happen because the ozone layer reacts with UV, absorbs it, and dissipates the energy.

• UV does not reach water so is not evaporated

How does the water trap prevent water from escaping?

• Water trap exists in the lower levels of the atmosphere

• Water freezes and falls as ice and rain as it rises up through the atmosphere

• Therefore, water is very difficult to lose because water can’t get through the atmosphere: it comes out of the vapour phrase before it gets very high.

How are volatiles recycled?

• The ocean crust becomes subjected beneath the continental crust - some water will be carried with it.

• Some of it will have reacted with minerals, and these water-bearing minerals also will have been carried down.

• Water is released back to the atmosphere through volcanic eruptions.

What is the negative feedback loop that controls temperature?

Higher CO2 (solar luminosity) → higher temperature → increased weathering → reduced CO2 → lower temperature

How did the weathering of the Himalayas act as a temperature control?

• India collided with the Eurasian plate causing mountain building and the exposure of the continental crust which was weathering, consumed a lot of CO2, and cooled the planet.

What is the ‘snowball earth’ theory?

• There is evidence of glaciers at the earth’s equator and glacial sediment overlaying warm marine sediments.

• This happens when continents are located around the earth’s equator.

• The oceans at the poles freeze quickly and easily, increase the reflectivity of the planet, causing the snowball effect.

• The earth will continue to degass CO2; eventually, CO2 levels will accumulate, the greenhouse effect will be accentuated, and the ice will melt.

How is earth protected from the sun’s solar wind?

• Liquid outer core generates the earth’s magnetic field which deflects solar wind

• This prevents the stripping of volatiles from the earth’s surface

• Ozone layer absorbs UV, stopping UV reaching the earth’s surface.