This summer’s arctic ice cap is the smallest since we’ve started watching it from space in the 1970’s, and the summer isn’t over yet.
Over the last few years, the rate at which the ice is melting is accelerating, probably due to the ice-albedo feedback. Albedo refers to how reflective a surface is; the average of the Earth is about 31%, while snow and ice has an albedo closer to 90%.
When the albedo is high, a lot of sunlight is reflected back into space, but when it’s lowered, such as when the sea-ice melts, the surface absorbs a lot more sunlight, which heats it up. Of course, more heat melts more ice which further decreases the albedo which causes more warming which melts more ice …. And you can see the problem.
The ice albedo feedback takes a small change (melting ice) and accelerates it. That’s a positive feedback, although the effects are usually not what you want, because they take the system (the Earth’s climate in this case) away from it’s current equilibrium. This is not to say that there are no benefits; the NorthwestPassage will open up eventually, if it has not already.
Extent of Arctic sea-ice at the end of August 2012. The orange line shows the average extent (1979-2000). We're a bit on the low side at the moment. Data from the National Snow and Ice Data Center.
Crude oil is extracted from layers of sand that can be deep beneath the land surface, but it was not created there. Oil comes from organic material, dead plants and animals, that sink to the bottom of the ocean or large lakes. Since organic material is not very dense, it only reaches the bottom of ocean in calm places where there are not a lot of currents or waves that can mix it back into the water. In these calm places, other very small particles like clay can also settle down.
Figure 1. Formation of sandstone (reservoir) and shale (source bed).
Over time, millions of years, this material gets buried beneath other sediments, compressing it and heating it up. Together the organic material and the clay form a type of sedimentary rock called shale. As the shale gets buried deeper and deeper and it gets hotter and hotter, and the organic matter gets cooked which causes it to release the chemical we know as natural gas (methane) and the mixture of organic chemicals we call crude oil (see the formation of oil and natural gas).
Figure 2. The trapping of oil and natural gas by a fault.
Shale beds tend to be pretty tightly packed, and they slowly release the oil and natural gas into the layers of sediment around them. If these layers are made of sandstone, where there is much more space for fluids to move between the grains of sand, the hydrocarbons will flow along the beds until they are trapped (Figure 2).
In this exercise, we will use the wave tank to simulate the formation of the geologic layers that produce oil.
Materials
Wave tank
Play sand (10x 20kg bags)
Colored sand (2 bags)
Observations
For your observations, you will sketch what happens to the delta in the tank every time something significant changes.
Procedure
Fill the upper half of the tank with sand leaving the lower half empty.
Fill the empty part with water until it starts to overflow at the lower outlet.
Move the hose to the higher end so that it creates a stream and washes sand down to the bottom end — observe the formation of the delta.
Observe how the delta builds out (progrades) into the water.
After about 10 minutes dump the colored sand into the stream and let it be transported onto the delta.
After most of the colored sand has been transported, raise the outlet so that the water level in the tank rises to the higher level. — Note how the delta forms at a new place.
After about 10 more minutes dump another set of colored sand and allow it to be deposited on the delta.
Now lower the outlet to the original, low level and observe what happens.
After about 10 minutes, turn off the hose and drain all of the water from the tank.
When the tank is dry, use the shovel to excavate a trench down the middle of the sand tank to expose the cross-section of the delta.
Analysis
1. How did changing the water level affect the formation of the delta.
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2. When you excavated the trench, did you observe the layers of different colored sand in the delta? Draw a diagram showing what you observed. Describe what you observed here.
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3. Was this a realistic simulation of the way oil reservoirs are formed. Please describe all of the things you think are accurate, and all of the things you think are not realistic?
When organic material is buried it is compressed and “cooked” because the deeper you go beneath the surface of the earth the hotter it gets. This causes the breakdown of the organic matter and the production natural gas and oil. The stages of decomposition are:
Diagenesis:
Decomposition of biological material produces methane gas. At slightly higher temperatures and pressures the organic matter is converted to kerogen – an unorganized (amorphous) material of carbon, hydrogen, and oxygen.
Catagenesis:
At higher temperatures and pressures kerogen is altered and the majority of crude oil is formed. During this phase and the next, the larger molecules break down into simpler molecules such as octane and propane (a process called cracking).
Metagenesis:
In the final stage of alteration (at higher temperatures and pressures) of kerogen and crude oil, natural gas (mostly methane) is produced and residual carbon is left in the source rock.
