Saturday, November 29, 2014
Climate Change Part 12: Where Oil Comes From Part 2
Note: This program first aired on November 29, 2014.
Last week’s show was a bit of a climate change cliff hanger. We were looking at where oil comes from, and we ended last week talking about primary productivity in the ocean, the process where by plants fix inorganic carbon from the atmosphere and incorporate it into their bodies transforming it into organic carbon. Part of this process fixes the sun’s energy and turns it into chemical energy, the energy we liberate when we eat food or burn gas in our cars.
That process is usually thought of as part of the “fast carbon cycle”, carbon travels from the atmosphere into plants and then up the food web into the bodies of other organisms, and all along the way it gets returned to the atmosphere as that chemical energy is burned and carbon dioxide is exhaled or released. It is a tight loop. Yesterday’s atmospheric carbon dioxide is today’s plant is tomorrow’s cow is next week’s atmospheric carbon dioxide, and the cycle repeats. In the ocean carbon is fixed by phytoplankton, some of which are eaten by zooplankton and small fish, and that carbon proceeds up the food web. Some of the phytoplankton dies without being eaten, and that plankton slowly sinks to the bottom, where it can be decomposed by decomposing organisms. In the ocean these are typically bacteria. On land fungus joins bacteria as one of the two primary decomposers. Decomposing is just another name for getting eaten, but by things that don’t have mouths. Chemically generally the same thing happens as when an animal eats, chemical energy is liberated from the body of the dead phytoplankton and one of the break down products is carbon dioxide.
Now knowing that there is a “fast” carbon cycle might clue you into the fact that there is a “slow” carbon cycle as well, and this is where the formation of oil comes in. In the slow carbon cycle, instead of getting eaten or decomposed, carbon moves out of the biotic realm and is sequestered into the rock cycle. The carbon can build up as calcium carbonate deposits in certain areas of the ocean (also known as lime stone), or can build up as organic deposits on the ocean bottom. This is where oil comes from.
Typically, when plankton dies without being eaten at the surface of the ocean, it slowly sinks to the bottom, where it can be decomposed, as we just learned in the fast carbon cycle. Sometimes though, things can go a bit haywire and the balance of carbon in and carbon out gets disrupted. There are places in the ocean, places like the Gulf of Mexico for example, where there are both lots of nutrients and lots of light, the two things that limit phytoplankton growth. The light comes from being at a sub tropical latitude, the nutrients come from a. being a coastal environment and b. being at the mouth of one of the largest river drainages in the world. The Mississippi River drains much of the central United States, and winds its way through much of the country’s agricultural zone. That is a lot of top soil and a lot of fertilizer that ends up in the river. With all that extra nutrition, the phytoplankton in the Gulf of Mexico go crazy and reproduce prolifically. They reproduce so much that they can’t all get eaten, there’s so many of them. So they die without being eaten and float down to the bottom on the continental shelf. There they start to be decomposed, but again, there are so many of them that the bacteria has a huge job to do, and consequently so much decomposition happens that the bacteria use up all of the oxygen it the water. Its like a fire burning in a closed space. Once it uses up all the oxygen, what happens? The fire goes out, as does the decomposition. This is called a dead zone, because anything that can’t swim or crawl away from this area of the ocean floor will die due to lack of oxygen. Dead zones form in coastal waters in many areas seasonally, and they aren’t all due to human influence, though the regularity with which this happens in the Gulf of Mexico is certainly tied to industrial agricultural practices and the subsequent nutrient load the river carries to the sea.
If dead zones happen repeatedly in the same area over time, those little phytoplankton bodies build up in the sediment without being decomposed, and all the carbon contained in their tissue is locked away. This process pumps carbon out of the atmosphere, and results in a decrease of atmospheric carbon dioxide. Over time, and it doesn’t have to be that much time, thousands to hundreds of thousands of years, and with heat and pressure that result from ongoing sedimentation on the continental shelf, that organic matter is transformed into gas and crude oil. The process goes on today, though at a much slower pace than we are using up the oil. Right now we are pulling that carbon back out of the rock reservoir and pumping it back into the atmosphere much faster than it was taken out, hence, the rising levels of atmospheric carbon dioxide.
So that is where oil comes from. Much like coal, it is photosynthetic material that is deposited in sediment without being decomposed. It’s a naturally occurring process, one we really aught to be paying more attention to.
Read the whole Climate Change series here.
About making crude oil in the lab from plankton: http://www.gizmag.com/algae-crude-oil-process-pnnl/30235/
Not really about where oil comes from, but a great graphic from the BBC showing how it is refined: http://www.bbc.co.uk/schools/gcsebitesize/science/ocr_gateway/carbon_chemistry/crude_oilrev4.shtml