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.
References:
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
