Fri., Jan. 13 notes

This picture shows methane, aka natural gas and a potent greenhouse gas, coming from melting permafrost in Alaska (photo credit: Todd Paris/The Associated Press. You'll find the photo in an article that appeared in USA Today). You can learn more about methane in a short video.

A little bit of additional material, not mentioned in class, was stuck onto the end of the online lecture notes from last Wednesday. We reviewed that quickly at the start of class today.

Then it was on to the main topic of the day. The origin and evolution of our present day atmosphere.

The atmosphere we have today (mostly nitrogen, oxygen, water vapor, and argon) is very different from the earth's original atmosphere which was mostly hydrogen and helium. This original atmosphere either escaped (the earth was hot and the gases were moving around with enough speed that they could overcome the pull of the earth's gravity) or was swept into space by the solar wind (click on the link if you are interested in learning more about the solar wind, otherwise don't worry about it).

With the important exception of oxygen, most of our present atmosphere is though to have come from volcanic eruptions.

Don't worry about remembering all of the gases listed above. Volcanoes emit a lot of water vapor and carbon dioxide. As the earth began to cool the water vapor condensed and began to create and fill oceans. Carbon dioxide dissolved in the oceans and was slowly turned into rock. Smaller amounts of nitrogen (N2) are also emitted by volcanoes. Because nitrogen is relatively unreactive it remained in the air and its concentration began to built up over time. There are lots of poisonous gases such as sulfur dioxide emitted by volcanoes. We'll learn a little more about sulfur dioxide, in particular, next week when we cover air pollutants.

Two years ago, air travel to Europe was being severely disrupted by the eruption of the the Eyjafjallajökull volcano in Iceland. Here are some really amazing pictures published in the Boston Globe. Here's another set of photos also from the Boston Globe.

Volcanoes didn't add any of the oxygen that is the atmosphere. Where did that come from?

The oxygen is thought to have first come from photodissociation of water vapor and carbon dioxide by ultraviolet (UV) light (the high energy UV light is able to split the H₂0 and CO₂ molecules into pieces). The O and OH then react to form O₂ and H.

By the way I don't expect you to remember the chemical formulas in the example above. It's often easier and clearer to show what is happening in a chemical formula than to write it out in words. If I were to right the equations down, however, you should be able to interpret them. Ultraviolet is a high energy form of light and it's probably also good to remember that ultraviolet light is capable of breaking molecules apart.

Once molecular oxygen (O₂) begins to accumulate in the air UV light can split it apart to make atomic oxygen (O). The atoms of oxygen can react with molecular oxygen to form ozone (O₃).

Ozone in the atmosphere began to absorb ultraviolet light and life forms could then begin to safely move from the oceans onto land. Prior to the buildup of ozone, ocean water offered protection from UV light.

Photosynthesis is the 2nd and now the main source of atmospheric oxygen.

Photosynthesis in its most basic form is shown in the chemical equation above. Combustion is really just the opposite of photosynthesis and is shown below.

We burn fossil fuels (dead, undecayed plant material) to generate energy. Water vapor and carbon dioxide are by products. Combustion is a source of CO₂. We'll see these two equations again when we study the greenhouse effect (CO₂is a greenhouse gas ) and global warming.

The following figure is the first page in the packet of photocopied ClassNotes.

This somewhat confusing figure shows some of the important events in the history of the earth and evolution of the atmosphere. The numbered points were emphasized.

First, Point 1: the earth is thought to be between 4.5 and 4.6 billion years old. If you want to remember the earth is a few billion years old that is probably close enough. Something I didn't mention in class, it's in small type above. The formation of a molten iron core was important because it gave the earth a magnetic field. The magnetic field deflects the solar wind and keeps it from blowing away our present atmosphere.

Stromatolites (Point 2) are column-shaped structures made up of layers of sedimentary rock, that are created by microorganisms living at the top of the stromatolite (I've never actually seen a stromatolite, so this is all based on photographs and written descriptions). Fossils of the very small microbes (cyanobacteria = blue green algae) have been found in stromatolites as old as 2.7 B years and are some of the earliest records of life on earth. Much older (3.5 to 3.8 B years old) stromatolites presumably also produced by microbes, but without microbe fossils, have also been found.

Blue green algae grows at the top of the column, under water but near the ocean surface where it can absorb sunlight. Sediments begin to accumulate on top of the algae and start to block the sunlight. The cyanobacteria would then move to the top of this sediment layer and the process would repeat itself. In this way the stromatolite column would grow a layer at a time. We're learning about stromatolites because the cyanobacteria on them were one of the earliest forms of life able to produce oxygen using photosynthesis.

Living stromatolites are found in a few locations today. The two pictures above are from Coral Bay (left) and Shark's Bay (right) in Western Australia. The picture was probably taken at low tide, the stromatolites would normally be covered with ocean water. It doesn't look like a good place to go swimming, I would expect the top surfaces of these stromatolites to be slimy.

Point 3 refers to banded iron rock. These rocks are 3 billion years old (maybe older) and are evidence of oxygen being produced in the earth's oceans. Here are a couple of pictures of samples of banded iron formation rock that was passed around in class.

The main thing to notice are the alternating bands of red and black rock. The next paragraph and figure explain how these formed.

Rain would first of all wash iron ions from the earth's land surface into the ocean (at a time before there was any oxygen in the atmosphere). Oxygen from the cyanobacteria living in the ocean water reacted with the dissolved iron (the iron ions) to form hematite or magnetite. These two minerals precipitated out of the water to form a layer on the sea bed. This produced the black layers.

Periodically the oxygen production would decrease or stop (rising oxygen levels might have killed the cyanobacteria or seasonal changes in incoming sunlight might have slowed the photosynthesis). During these times of low dissolved oxygen concentrations, layers of jasper would form on the ocean bottom. Eventually the cyanobacteria would recover, begin producing oxygen again, and a new layer of hematite or magnetite would form. The rocks that resulted, containing alternating layers of black hematite or magnetite and red layers of jasper are known as the banded iron formation. In addition to the red and black layers, you see yellow layers made of fibers of quartz in the samples passed around class. The rocks are fairly heavy because they contain a lot of iron, but the most impressive thing about them in my opinion is their age - they are a few billion years old! And thanks for returning them by the way.

Eventually the oxygen in the ocean reacted with all of the iron ions and was free to move from the ocean into the atmosphere. Once in the air, the oxygen could react with iron in sediments on the earth's surface. This produced red colored (rust colored) sedimentary rock. These are called "Red Beds" (Point 4). None of these so-called red beds are older than about 2 B years old. Thus it appears that a real buildup up oxygen began around 2 B years ago. Oxygen concentrations reached levels that are about the same as today around 500 to 600 million years ago (Point 5 in the figure).

We finished with a question from last semester's final exam:

Equation (c) is the first step in the natural production of ozone. This is followed by the reaction in Eqn. (a). Together Eqns. (c) and (a) produce ozone. Equation (b) shows the natural destruction of ozone.